The Milky Way: Our Home Galaxy in the Cosmos

Hello, I'm Jason Kendall and welcome to the next of my introductory astronomy lectures. We finally left all the stars behind and now we're going to go out and look at the entire universe as a whole, so this begins the lecture series on the largest galaxies. the largest universe and the big bang, the origin and destiny of the universe, so let's start with a cosmic direction and first we are going to define what we mean by the concept of cosmology, which is what we are going to talk about. the next group of things, cosmology is an area of ​​physics, it's an area of ​​astronomy, but it's something independent and it's the study of the entire universe as a whole, so we're looking at all the physics of the entire universe, how things are distributed. on all lateral scales how things move within the universe, such as galaxies, space and time itself and how the universe itself evolves over time and then finally, as we move forward in this long series of lectures, we will talk of the origin of age. and the fate of the universe and that's what cosmology is a study of most of the things that we're going to talk about that are called big bang cosmology because the big bang is the current paradigm of the way we think about how is the universe organized and what the origin of the universe is like and there are four big pieces that are associated with the big bang and I'll start with the number zero, which is an underlying assumption and the underlying assumption of everything is that the universe in its largest form size scales are homogeneous and isotropic and homogeneous means that no matter where you go, anywhere in the universe you will have approximately the same things around you and isotropic means that no matter where you go in the universe, I see more or less the same things in all directions and that's kind of like they're two different terms that have different meanings when you think about it, but we'll talk about that in more depth later, the next thing that is the cornerstone of the big bang is the cosmic redshift, which is the fact that all galaxies seem to be moving away from us, the further away they are, the faster they move away.

Next is the cosmic microwave background, which is a prediction of a hot early universe and the third. It is the nucleosynthesis of the big bang where elements are created within the first moments of the

cosmos

and the last is galactic evolution, it is simply seeing how things change because if the universe had its birthday then it was different a while ago from how it is today and, so we should wait to see that if we had observational results that show us that galaxies actually change over time and then these four five things, these four things with an underlying assumption give us what we call big bang cosmology, So our first important thing is that we are going to look for that version that the number zero is called the cosmological principle and that there is really nothing special anywhere in our location in the universe, except for the fact that it is our place and our

home

, the really special unique. and basically, if you went to any other place in another

galaxy

, you could be in a place where there are many more galaxies or many more stars or something, but the underlying idea of ​​the cosmological principle is that the laws of physics are the same everywhere. parts of the world. entire universe, that's what the cosmological principle really means, so it's a very fascinating assumption and it's a principle that is confirmed by observations, and the cosmological principle implies that isotropy means looking the same in any direction you look around. any point. in the universe combined with this cosmological principle means it is isotropic everywhere isotropic everywhere means homogeneous everywhere means it is the same no matter where you go and so homogeneous everywhere would mean it doesn't matter where you were in the entire universe you would see approximately what No matter where you look so that we do not live in a special and advantageous place, we do not live somewhere that is on the left side, facing the ballroom, where everyone on the wall is blooming and we are not.

We live in a great universe in the center of the city where everything was born and leaves from there. No, we live in a typical rural

galaxy

in the center of the city, somewhere in space, and we happen to be where we are, that's all, it's a cosmological principle. let's take this idea and give our cosmic map and see where we get that from and to be completely honest, I took this image from wikipedia, so hey, have a great time, but let's take a look, here's the earth, the earth. It is our starting point, our first starting point in the cosmic direction and the earth is a planet that orbits the sun and yes, the earth is actually round, don't worry about those people who say no, they are very, very stupid and that's why the earth is one of the nine planets, I'm going with nine because I met Clyde Tumbao, who was the discoverer of Pluto and I dedicate myself to those things and I thought that the New Horizons mission was really something so I mean Pluto, so I'm going with nine, but there are many other objects orbiting the sun besides Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune and Pluto There are several dwarf planets There are millions of asteroids and comets and things like the approximate size of our system Solar diameter is about 50 times the distance between the Earth and the Sun or 50 astronomical units and that is roughly equivalent to what we call the Kuiper belt.

The Kuiper Belt is roughly where most short-period comets are found, that is, some comets that are. a few hundred years or less in terms of their orbit around the sun, that's where they come from, and Neptune, Uranus and other large bodies like Eris and Haumea would actually disturb these things and cause them to fall into our solar system, so that's our first time. line from the earth to the solar system and then we zoom in and look at the sun in the center of this diagram and we are looking at the nearby stars one of the nearby stars is sirius another is epsilon iridani then tausceti but the closest one to ours is alpha centauri , actually proxima centauri, which we don't see in this diagram because well, it will be right next to alpha centauri but a little closer and it might look like taulcetti or the others are different, same thing, but we are trying to pretend that this is a three-dimensional map, so many of these stars that we see here are actually bright stars in our night sky and our solar system is part of the solar interstellar neighborhood that spans roughly 50 light years.

It looks like something big and you see that these dots that are stars fill up, they don't fill this place, they look like fireflies in a jar and that's a very good way to analogize it, however, the problem with this image is that the stars themselves are extraordinarily small relative to their distances, so if we were really honest, we wouldn't have any of these stars larger than a pixel in this image and we would have to get very close to see them, they would just be bright dots, not disks like we see, but let's take some liberties and go with this because you know, hey, Wikipedia had its thing, so we're going to live with it, so that's fine, but what we can see is that there's a space around it.

We are not densely populated with stars, so our local solar neighborhood is one of many such neighborhoods throughout the Milky Way. Now, the Milky Way is approximately one hundred thousand light years across and is made up of more than 200 billion stars, so when we move away from something that is 50 light years or 100 light years away through two hundred thousand or a thousand times that size or ten thousand times that size, we see that our Milky Way is a large collection of stars where the spaces between the stars are incredible, but nevertheless, no matter how we look at this size scale, it looks like a large object and blurred and that blurring arises as a result of the softening effect of seeing so many things from such a great distance that they cannot be resolved individually. things like that the center of the

milky

way is not a big ball, it's a series, it's a large number of stars and those stars are actually so dense in that region that, seen from very far away, they merge into one thing, but they don't merge.

Remember that the distances between the stars are incredibly large, so this Milky Way is made up of a huge number of stars, gas and dust, so next we say: let's go further in our cosmic direction and see the Milky Way. We go much further away and look, there are other small galaxies around it, like the large and small Magellanic cloud, some other dwarf galaxies, all these small dwarf galaxies in the local galactic group, which is about 10 million light years in diameter , is approximately 3 million light years from the Andromeda galaxy. the closest large galaxy to ours with the triangle m33 and its small spiral galaxy nearby, but now we are not looking at individual stars, but rather each of these galaxies specifically m31 and the

milky

way are composed of billions of stars, tens of hundreds of billions of stars and the triangle galaxy, being somewhat smaller, does not have as many, but we can see that there are incredible chasms of space between the galaxies and the milky way and andromeda are falling towards each other and will collide in about five billion years or so. long after the sun has gone out, so we zoom out even further and look at the center of this image as the local galactic group and the local galactic group is a series of spent galaxies that are part of the Virgo super cluster of galaxies and now we see groups of galaxies, many large groups with the virgo cluster on the right which is made up of thousands of galaxies, the fornax cluster on the bottom left is also made up of thousands of galaxies, the virgo cluster has about 65 millions of light years. far from

home

and therefore the light that we see from these distant galaxies left at the time when the dinosaurs were being wiped out by meteorites on Earth, so we are looking at a huge size scale where the diameter of this area exceeds 200 million light. years and it takes 200 million years for light to cross this vast, vast, vast gulf, but notice that there are also enormous amounts of space, there may be a few galaxies scattered throughout, but generally the spaces between superclusters and independent clusters are filled of enormous amounts of empty space that we now call voids, we zoom out even further and find that the Virgo supercluster is just the spur of another very large supercluster of galaxies, so the local superclusters can be mapped this way, which has a diameter about 2 billion light years and these are now super clusters of galaxies that give us hundreds of billions of galaxies, that is, galaxies of stars, and there is a huge, huge, vast area, but notice that what we are Seeing now it's starting to look a little cloudy, the super clusters themselves are grouped into Filaments, sheets and lines and those sheets of filaments, lines and nodes form the edges of a foam-like structure that are tendrils that connect together and the reason for this appearance is the result of what we call dark matter and these super clusters.

They are the places where most of the matter in the

cosmos

has grown and joined together and we see that there are enormous voids that can span up to 500 million light years in diameter, such as the Bolotes void, the Capricorn void and the from the sculptor in the Aliens exploit all these other big voids where there are simply no galaxies anywhere and if we look down into the clock cluster we see part of another huge supercluster next door which takes us to what we call the local supercluster group that we know. We saw this in depth and now we have moved so far away that even the superclusters themselves look like tiny specks, so if we imagine the diameter of an area of ​​a volume over 27.5 billion light years in diameter, we would which means the travel time of light through such a diameter would be 27.5 billion light years, we see that now it is on this size scale, at this distance, the universe itself appears smooth, which means that all we've smoothed it out so that when you take a cube of the universe and that cube of the universe might be, say, a billion light years across now, the universe looks really smooth like static on an old television screen that's turned around. with rabbit ears towards some long-lost distant channel, but in reality it is filled with hundreds of billions, if not trillions, of galaxies. in this entire observable universe and the question arises what is outside this cylindrical area and the answer is more galaxies because remember that the cosmological principle says that it does not matter where you are if we are in the center of this particular group, let's say we go all time.

I walk up to the edge to what we would see as the edge and if we went there well we would see exactly the same thing that we see around us now the same type of things not the exact same clusters not the exact same galaxies not exactly thesame local groups but the same kind of things other galaxies groups of galaxy clusters just in a different arrangement with different stars different pretty things in the night sky for people who live in that far away distant galaxy but would see the same things in general , what we do on that edge, so what about the people's edge for that side on the right?

Well, it just goes on, so we really think the observable universe is better. We know him well. The observable is what we can see and what we can see. We can possibly see, but the extent of the universe there is every reason to think that far beyond the observable universe there is a place that is much, much, much wider and, for all intents and purposes, is essentially infinite in extent, so that is our cosmic direction and our great cosmic map is now what are we going to talk about because we started talking about the solar neighborhood, the distances to the stars and how they live in their diet and how they cross the milky way and form and now we are going to talk about the levels lower and we're talking about how the universe is organized on the largest time scales, today we're going to talk about the cosmic distance debate or, more specifically, how we know the distances to the distant galaxies we've been to. looking at distances to stars using parallax and now we're going to look at distances to extraordinarily distant objects, star systems like the Milky Way, but far, far, far beyond and this debate was one of the most important things that happened in the beginning of the 20th century and we will see some of its history and we will see how it is resolved, but the really important thing is that when we look at these stars we have no idea what their distances are a priori, we only see them as points of light in the sky and if Whether or not they were distant objects has been the subject of wonder for thousands of years and only in the early 20th century did we finally get a glimpse of how far away they are.

The distance problem is incredible because getting distances will always be the biggest technical problem in astronomy and, frankly, the most important thing to relate astronomy with astrophysics because if you do not have distances you cannot know the size of the luminosity, the mass or the distribution space of objects and without this you cannot understand their life expectancy how they were born how they live how they die what they do how they evolve or anything so distance is the key in astronomy and it's also one of the most difficult things well let's see how difficult at first in previous lectures we talked about the astronomical unit and the astronomical unit is incredibly important because it forms the basis for trigonometric parallax.

Copernicus used the geometry of the orbits of the planets to determine the relative distances between the planets and that was something that could be done, but it was necessary until radar was invented to obtain very solid planetary distances using the speed of light and power. reflect radar off objects like Venus when it is square to the sun. so it has a phase of a quarter phase nucleus as we see it and that allows us to measure the distance to Venus in relation to the sun and give us the astronomical unit of simple to simple ninth degree geometry and therefore we can use parallax trigonometric, which is the measurement of distant stars, but trigonometric parallax, which is the apparent motion of a foreground star with respect to much more distant stars as a result of the Earth's orbit around the Sun and the apparent change of That location has a distance limit. on the ground, the best you're going to get is 100 arcseconds and that's going to be used with adaptive optics, but the Hipparcus satellite in 1998 was incredibly good because it hit about a thousand parsecs and that.

It's an important distance measurement because it got accurate distances to about 100,000 stars or so at that stage, although the Gaia satellite greatly improved on Hipparchus in 2016 with its first data release that came in at one percent of one percent of a second of arc reaching about ten,000 parsecs and having at least a billion stars within that area, so the Gaia satellite has radically improved the distances using parallax, meaning the distances are much better known to many more objects, allowing physics to be better known. Okay, so let's define a new type of distance that is secondary to parallax distance because parallax is the only geometric way of knowing things, it's the only direct measure of distance and that's why Gaia was a mission as important, but the luminosity distance is just as important.

The way you do it is you measure the brightness of some object in some filters, for example, and then you have some way of knowing that you're estimating, guessing, posing or vg in the air, the luminosity of the object and then. you solve for its luminosity distance by applying the inverse square law of brightness and that is what we see here d sub l is the luminosity distance is equal to the known luminosity of an object divided by the brightness that you observe which is multiplied by 4pi and take the square root, so you have to know the luminosity first, so it helps to get the luminosity of, say, the parallax of an object, but that tells you something you need to know what is the nature of a standard candle, a candle standard is something where you already know the luminosity a priori and if you know the luminosity before measuring the distance or the brightness, then you have what is called a standard candle and you can calibrate standard candles in many ways, but I just clarified that before taking the luminosities of nearby objects where you can get the parallax and then you find similar objects using some kind of distance independent property that they share maybe the spectrum maybe some variability maybe something that knows you know maybe it has a smiley face and you assume that the distant objects have the same luminosity as the nearby objects for which you have these distances, the good distances, that's interesting because steps two to three assume that the laws of physics are the same everywhere and that's what which we will always assume at the beginning.

The laws of physics that are close to us and that we would measure in the laboratory apply to distant stars, so we can use the ideas of physics to reach the cosmos, because we really can't do it any other way, so once that we made it, that calibration set of standard sails, then you can measure them for things that are too far away for parallax, but the trick is this distance-independent property and every distance-independent property depends on physics, so We must assume that the laws of physics are the same there as they are here and in this small diagram it is shown well if they have a candle of known brightness, you put it further away, it looks dimmer and it is also another property independent of the distance the physical size of something, so the further away the smaller it appears, but there are very, very few standard criteria, but there are some very critical new standard candles, so one of the most important is spectroscopic parallax because you can get easily the temperature of a star by looking at its lumina, its spectral type o is uh or is bv uh bv color co is b minus v color or some other color variation and if you have a well calibrated hr diagram then you simply have to say oh, I know it is this type of star because the luminosity class tells me that it is a super bright giant, so I know the difference that it is not an a81 type star, not an a8 main sequence star, it is a bright giant, no a dwarf, a dwarf would be a faint giant, it would be bright and then you can say I have the spectral type and the luminosity class and that's for stars that are incredibly bright objects that you can see from a distance.

Likewise, if you calibrate the hr diagram, then you could say using an hr diagram known as the Hyades. The Hyades become the basis for all other hour diagrams and therefore the only reason the Pleiades are see different is that its main sequence looks dimmer than the Hyades, which means a magnitude v that is larger but with the same colors for the same stars, it is because the Pleiades is further away and therefore the distant ones they are just fainter and therefore have larger v magnitudes for exactly the same type of star, so you calibrate all these things, you get distances to pleiades, you get distances and sahids and a lot of other things, and the hyads themselves form the basis for all this calibration, so it is the most critical of all the nearby clusters and therefore we can construct the cosmic distance ladder from extraordinarily close objects, so what we have is a power of 10 and a distance each time we go up.

The number at the bottom means that something is about 10 times more distant than the other in terms of parsecs and parallaxes. It allows us to say now much further than we had thought, but originally the parallax brought us out with a park that is about to arrive. Let's say about a hundred parsecs or so, but really good parsecs, really good, we might have to push this thing, the little purple thing to the right, a little bit further, to the number four, because that's as far as we can see now with the parallax. up to ten,000 parsecs, so I have to update this slide and we can use the proper motions of the stars to get us there, but then we can use parallax to get us to distant, distant clusters with main sequence adjustment and that's where our spectroscopic parallax works , but in the next one we are going to talk about the rr library and the cepheid variables are incredibly critical because studying the lmc or the large magellanic cloud and then getting the distance to m31 is the topic of our next conversation which takes us to a megaparsec or 10 to the 6th parsex, so there were no ways to get proper parallax or motion or even a sp on a group of stars that we would call the cluster in m31, so exactly how far away is the Andromeda galaxy? m31 you have to use our libraries and cepheids to get there so let's see how that was done historically and the details are coming because to the left of this thing with this ctio image we see the large and small Magellanic clouds in the center of the milky way on the right, but the lmc is at the bottom and the smc is at the top and those clouds are actually galaxies and they are orbiting the milky way and that's what we know now but we didn't know. their distances and what those objects were and that was the study of the topic we're about to talk about, okay, so let's go back in time.

The distant, the concept of distance and how big the universe is took a big step forward. in 1610, so after thousands of years of looking at the stars and simply knowing that they are far away, but thinking that they were tasted in a celestial sphere, galileo galilei published the starry messenger in 1610 and what he showed was that the galaxy itself is a many stars so he took his little telescope and looked at the milky way and these drawings that he made of the stars showed many faint stars that he couldn't see with the naked eye and so if we look at the milky way on an extraordinarily dark night where there is no light pollution which they didn't have in 1610, I mean it was actually a dark sky, unlike today where you can barely count 50 stars in 1610, the night sky in the city was really dark and you could see many, many stars. but through the telescope you could see more, so I knew that the galaxy, that is, the milky way, was simply many faint stars that were just mixed together and that could be resolved with a telescope and that was the first indication that the milky way was much more.

Much much larger in 1750, a theologian named Thomas Wright created a model of the Milky Way based on theological ideas and his goal was a really strange concept, but his goals basically said, "Well, the sun is in a big shell around the center of everything and So if we looked along the edges of the shell, we would see more stars, which is similar to what we see in the sky. We see a shell or a ring in the sky, so if we were in the middle. of that ring, we would see stars. in that ring around us and it would diminish, but looking above and below that shell we would see fewer stars, so his model explains the appearance of this guy, but you know this was his real model. , so he was looking at the concept of theology behind the star work and we were on one of many stars inside this big shell and so his conceptualization was that somewhere in the center of some big shell that we couldn't perceive was god and god looked at the rest of the stars around him in the firmament and all those stars had countless planets and you can see him alluding to it in this thing, so his concept was theological and if you look in the work of Thomas Wright you will find something really mind-blowing . things and it always has these little stars with eyes in the center, it's very Illuminati, but anyway that was in 1750.

Well, people like William and Caroline Herschel in 1785 were a couple ofbrothers and William and Caroline Herschel were arguably one of the most important astronomers. In the story they told, they decided after building a four foot diameter telescope together, uh, where William crushed the mayor's floor while Caroline fed him bananas over the course of many days and kept him going so he could maintain his moving arms so that the glass could take the shape they wanted, but what they did was finally, after many years of building this telescope, they finally built everything and started looking up into the sky and looked in many directions, around 690 addresses, 683 addresses. with their great range and they assumed that all the stars were the same brightness and if you really don't know any better you can also assume that and therefore if the stars have certain brightnesses and they all have the same brightness, how far away? what they are is simply their distance is their relative brightness and this starts with the concept which we now know is not true but in 1785 it was a good assumption to say that all stars have the same brightness and then assume that their distance is based on So, they assume that all the stars were standard candles, which we know they are not, but still in 1785, they did pretty well and this gave them the first map of the universe, the first ever created, and that's why the sun was on one side we can see the effect of the edge of the galaxy effect in that little one where the nodules are with these two little arms like an amoeba shape towards the left and then we have this huge huge shape that would be called maybe called a grindstone , but the sun is not in the center, there seem to be many, many. more stars on the left than on the right and for them this was the map of the universe now there may well have been things beyond this but for them this was it these were the stars in the sky so we seem to be one of many in this kind of patch and then in 1755, which is before this, so let's jump around in time a little bit.

Emanuel Kant, a philosopher in 1755, thought that he had looked at Thomas Wright's model of a disk and that and I thought, ah, this seems to be right and emmanuel khan said, well, maybe there would be a disk-shaped lens, stars that all these that say that the universe was a disk-shaped lens, a lens-shaped disk, that rotated around the center and the sun. There really wasn't any special star there and all these other nebulae that were being discovered were rotating milky ways similar to our own and what was being discovered was uh william parsons, the third earl of ross, and in 1845, about 100 years later , some built a large telescope.

In Parsons Town this was in the United Kingdom, well, Britain at this point and Parsons built this huge telescope called the Parson Town Leviathan and discovered these things called spiral nebulae. Now the spiral nebulae looked like disks. but they had a sort of spiral pattern, some of them looked edge on but they had dark bands cutting through the middle but he couldn't resolve anything, none of them into stars, I mean his telescope, although huge, wasn't very, it didn't have the resolution that we think of today for high resolution telescopes, so this was a big problem and it has the structure that I don't think is maintained today, but this huge telescope couldn't be pointed very far to the left or to the right. right, but it certainly could be pointed up and down and using this he visually recorded the appearance of the star of the galaxies that we now call that he called spiral nebulae or cloudy nebulous spiral-shaped objects.

Okay, Kant's idea about an idea from 1755 came up uh and Alexander Humboldt. He looked at the Top Parson spiral nebulae and said those are other milky ways, they're just made of stars and therefore they should be very, very, very far away, so he thought those disk-shaped things look the same than the Milky Way, but only seen on the edge and very far away, so Alexander Humboldt said that the Milky Way is an island universe with many stars and there are many, many other galaxies out there and we are just one of them and this idea dates 1845.

However, the very famous mathematician Pierre Simon Laplace in 1796, before we will be bouncing back in time, of course, I am sure that these, but the spiral nebulae that were being seen, were actually swirling clouds of gas that are in the process of forming planets. and solar systems so gas clouds should collapse under gravity and as they collapse they will form disks if they have any rotation and this is what Laplace postulated and the solar system is disk shaped and there are numerous examples of gaseous phenomena and These were seen in telescopes and in the Orion nebula, so people could say well if there is a disk shape and if they are spiral-shaped structures, then they must be spiral formations of new sun rays.

Systems are being found now if they are new solar systems and they are close maybe rotation can be discovered and Laplace's set of ideas took Herschel's idea more and more seriously and said that the only things we see in the sky They are nearby stars. and the nearby objects, so the milky way is the universe and this is the nebular hypothesis for this spiral for the spiral nebulae, well, it wasn't really understood what the heck is which way to go and until about 1906, the captain of Jacob's camp said , actually do something with this and then he started saying, let's say, let's do photographic counts of stars in many different directions and then he took the idea from Hershel, but then he said, let's photograph stars and measure the apparent magnitude, the spectral type radial velocity and the proper motion of many stars in a lot of different zones, more than 200 zones and this huge project had many different observatories, 40 different locations and its process was enormous, it took almost 20 years to complete approximately 16 years to do everything and then , in 1922, published his concept of the map of the cosmos that the universe or where the milky way was or whatever the galaxy itself was the sun was not in the center there were many stars and many of them were very far away was that and the The diameter of this disk-shaped structure was about 17 kiloparsecs or 17,000 parsecs or 34,000 or 40,000 a part of light years and about 12,000 light years thick, the thing is that his problem was that he did not understand what it was. interstellar reddening. and you grossly underestimated it, in fact, no one really had a good handle on it at the time, but other things were being discovered and we're seeing here a Kepler space telescope view of the messiest object number four in the globular cluster and as you can see there, there, there, there, some of the stars appear to be twinkling and this is from Hartman and Stanic of the Harvard Center for Astrophysics and this is using the Kepler emission in its k2 mode as it does.

It only has two reaction wheels to help it stay stable, so it can only focus on one area of ​​the sky for a short period of time before it must move, which is why one of the objects Kepler studied in its K2 mode was this globular cluster and we can see that there are particular types of variable stars called rr libraries that literally get brighter and dimmer over time and you can certainly take a look at these things and look at the Kepler k2 data. It's an incredible idea. Take a look and there's actually a YouTube video showing this and just very simple stars getting brighter and dimmer and this is a globular cluster so these rr library stars have a particular brightness profile and how they get brighter and dimmer, what is the pattern of brightness and darkness? and they're called our libraries, so if we measure our brightness profile, how they get brighter and dimmer over time in our libraries, they have this particular shark-tooth pattern where they quickly increase in brightness and slowly dim and then they get brighter. quickly. and it dims slowly and this is characteristic of the type of variability that you find with an rr library and the reason they are called son our lyric type stars is because if you find something with this type of variation it is called son our library star, so we attribute a distance-independent property to variability, so there is variability that seems to be characteristic of some class of objects and we call them our lying stars, so maybe there is something we can use with this, in fact, if we look at our library.

In itself, this is the data from the American Association of Variable Star Observers and this data is actually, you can derive it, you can just go to their website and say please show me the dot data for this and download it and this is in numerous filters and This has been contributed by many, many, many amateur observers as well as professional observers who are showing the different brightnesses, the variation of brightness at different wavelengths and that is what different colors indicate in different bands of light of wavelength. Alright, our lyrical stars are actually. physically pulsating stars and they are found in old clusters, such as even in the galactic bulge or in the halo, and they are globular type, since they will be found in globular clusters, so they are old stars that are dying and physically getting older larger and smaller and they have this particular pattern that we saw, that pattern of shark teeth and that particular pattern is a distance independent property and so maybe there is a relationship between that and it turns out that our libraries vary between about a few and a half hours to maybe a day or so to get brighter and dimmer and at their brightest they become as bright as about a hundred times the luminosity of the sun, so all you have to do is find the variability and find out if that's how it is. it's this kind of variable that has that particular pattern and then you see what the brightness is, you compare it and you say oh, all these stars have about the same luminosities at the peak and then it's a star of the type from our library, so there we go.

This is a period luminosity ratio for the variability period of a liar rr to the brightness it reaches at its maximum and this is something important and pure, so other variable stars were looked for and this is incredibly important, so Harlow Shapley. He used this concept in nineteen for six years to study distances to globular clusters, so he was working with them and he discovered that we use our libraries as a distance indicator because you can calculate their luminosities with their luminosity you can get the distance. because you can measure the brightness and we noticed that they were evenly low distributed above and below the Milky Way concentrated in the direction of Sagittarius and they appear to be physically or at least dynamically oriented towards that as their center and using that he said, well, let's suppose that the center of the globular cluster distribution is the center of the Milky Way and when he made that positive position he said, wow, that makes the sun about 18 kiloparsecs from the center of the Milky Way, it's actually quite distant, but Again he had trouble with interstellar reddening, so he overestimated the distance, but that still gave us a good distance to the center of a distribution of globular clusters.

Now you have your results, so if you take everything into account, it makes the Milky Way appear to be about 300,000 light years in diameter or 100 kiloparsecs wide the sun about 16 kiloparsecs or 40,000 light years from the 50,000 light year center. of the center didn't know the full extent of interstellar absorption so he overestimated this and cemented everything he said was bigger was too big and too far away so what is this interstellar reddening we talk about? Well, we talked about it before when we talked about the stars that are there and exactly how they form and the interstellar gas, but we know that interstellar space is full of this dust and gas and we know that the dust absorbs and scatters the blue light of stars and since it does so, it makes distant stars look red and faint because it absorbs or emits them, the stars become or their parents look redder because the blue light has scattered leaving red light, so if you really don't know, if you can't take this into account, then you'll say, wow, that thing is very faint, but it's this kind of object, so it's really very far.

As far as you can do it, if you say that the light of our library passes through a cloud like the one you see in these two images below, it will look fainter and you will think it is further away because you are saying how bright it looks and We are assuming that there is a luminosity period relationship, not a bright period, which means that Shapley and Captain thought that everything that was was much further away than it was because they underestimated the effect of interstellar dust and only in 1930, long after this debate that we will talk about was resolved , we discovered that the absorption of interstellar dust was extraordinarily significant and that is one of the most important elements of 20th century astronomy: actually measuringthese things well, but let's go back to the 20 years how big is the milky way how distant are these spiral nebulae the earl of Ross discovered these spiral shaped nebulae that he saw visually and there were two competing concepts about what the milky way, one of them was that the universe is the milky way and then the spiral nebulae are just in uh or just uh our um well, the island universe said that we are, that no, that we are one of many islands in the entire universe and our galaxy is just one of many, making the cosmos extraordinarily large. and we're just a piece, we're just a little thing inside of it which seemed a little difficult for astronomers at the time and most of them preferred the nebular hypothesis and the nebular hypothesis said that spiral nebulae are just gaseous clouds and they knew a lot about clouds. gaseous and similar nebulae, it turns out that they are spiral-shaped and if we are spiral-shaped and could be forming planets, then planets existed around stars and stars had to come from somewhere and Laplace was right that gas formed them. clouds should collapse under gravity and then rotate and if they rotate they will form a disk, if they form a disk they could form a spiral, so the nebular hypothesis at that time was the most convincing, however the island universe was also convincing because of the physical characteristics of spiral nebulae, okay, so let's see how this worked, and we have several people at Harvard who were working on classifying stars and Annie Jump Cannon and the rest of the group were busy working on the star system. on the nature of stars and the cataloging of stars and they were the Harvard and Piccol computers and one of them was Henrietta Swan Levin and Henrietta 11 um was working to find variable stars in the Magellanic clouds and in 1912 she discovered that these variables more brighter there were these longer periods, so she was the one who first discovered the period luminosity relationship of Cepheid variables, a new type of variable star, but at the time she had no way of obtaining the luminosity calibration, so which he didn't, but let's call it the levitt relationship because that's what cepheids really are and our letters have this brightness that varies regularly with the distinctive pattern, so delta cepheids and rr lirias discovered that the period was directly related to the brightness and that was his discovery in 1908 and he worked until 1912 and this was what he was working on when he was studying the Magellanic clouds to find their distance because he found these variable stars inside the Magellanic clouds while he was trying to find their distance and again he let them Cepheids are similar to our libraries because they are pulsating supergiant stars, not just giant stars, but supergiant stars, and they are found in clusters of young stars, so you could associate them with star-forming regions, and they are really, really, really bright objects. and they have luminosities between a thousand and ten thousand times more than the sun, but their periods are much longer between a day and even like uh like a month or two and then he discovered that they have a particular uh profile and that differs from the libraries rr and so The luminosity period relationship of the delta Cepheids could be calibrated and in fact they can be distinguished individually due to their brightness, they can be distinguished up to almost 120 million light years using the Hubble Space Telescope, but the problem is that Cepheid variables are strange and, like, strange things.

There are very few Cepheids nearby so it's very difficult to get their parallax, but some Gaia data should be able to include them and that's something that if you're going hunting you should be able to find them anyway and there are even two different types of Cepheids too. uh the star delta cephei and w virginia so they're both the same kind of thing but you should have a slightly different period luminosity diagram so you have to be careful which type you found so deltacef still because of their intrinsic luminosity at their maximum are incredibly important standard candles, so we can see that here are some of the images of their variability.

This is from the ogil group and we can see that the Levitt relationship she saw was actually that of the large Magellanic cloud. or mlmc is that the time between the peaks the longer the time between the peaks of the variation the brighter they are at the peak so this is the relationship that Henrietta Levitt found and that was used to try to obtain the distances to the great Magellanic cloud and so also, so we can make an example where you say if they gave a well-calibrated Cepheid relation or specifically the Levitt relation, the Levitt Cepheid period luminosity relation, if you find one that has 10 days long to know how it gets bright and dim, bright and dim, so we know it's about 5,000 times the luminosity of the sun and that's incredibly important because now all you have to do is find the periodicity and make sure it's the same type of character as a Cepheid, not an rr library but a Cepheid variable and then the meaning of their light curves, how they get brighter and dimmer over time is different between our letters and the Cepheids, so So just read it if you can find one that has a 10 day period and if we go back to the original time. diagram but the lifespan of stars we find that the Cepheid variables are supergiant stars that tend to be much larger and the young stars perhaps are o and a and ob and asrs that are in the peri at the time of leaving the main sequence or are dying and so their giant horizontal branches of stars that are arriving, their variability is the result of physical oscillations of the star that actually get smaller and smaller over time and any star that is within this band of instability shows some kind of variability, whether Cepheid variability or our own.

Lie variability and these are stars evolved off the main sequence anyway let's take a step back, the problem was that her boss stopped her from continuing the relationship, however, in 1913, just a year later, after Levitz published her ideas or at least I tried to publish them, she calibrated, uh, he calibrated the Cepheid, their relationship and took some credit and then I said, well, these are Cepheid variables, oh my God, let's kill, let's do the calibration of necessary luminosity, it did. 1915 Shapley came in and refined the calibration. for the globular clusters in our libraries that allowed him to begin his work, so basically Henry Henrietta Levitt started all of this by discovering them and determining that there is a period luminosity relationship there anyway in 1920, about five years after Shapley did his job.

The National Academy of Sciences had had enough of these things and decided to discover the universe of scientific skills. Let's all get together in a room and we'll have a big fight and it's called the Curtis Shapley debate which happened in 1920. Shapley from Harvard defended his model for the galaxy and the very conventional one based on a lot of well understood physics, the nebular hypothesis which said that The Milky Way is the collection of all the stars in the universe and outside of that there is a great void and the spiral nebulae are local to the cosmos, however, there was an upstart astronomer called Hebrew Curtis at the Lick Observatory who defended Captain's model. and that the universe was very, very large and he considered the concept that the island universe hypothesis was correct. which means that the Milky Way was one of many and that is why Shapley used physics to enter into physics and the postulate of the formation of planets and stars and the Hebrew Curtis used the idea that that star, spiral galaxies or spiral nebulae, seemed very similar in appearance. to our milky way and that's why things must be the way I think things must be like objects, so the big questions on the battlefield of their debate were how big is the galaxy, what was more reliable, the counts of stars, the photographic counts or the Shapley clusters and, in addition, what is the distance of the Andromeda Galaxy and the curious thing is that there was a real anova event that occurred in the Andromeda Nebula that was called the most disordered object 31. and it was still called the andromeda nebula and it was called the largest spiral nebula at the time and so we look at this and say how far away is it and a nova was discovered and that nova had some regular outbursts and I thought, well, let's see if we can calibrate and find the distance and finally what is the motion of the spiral nebulae themselves is there a rotation in which they are actually spinning and what are their radial velocities and what is the most important, here is what shapley said shapley said that the galaxy has about 300,000 light years in diameter and there was a nova in 1885 that gave a distance due to a luminosity of only ten thousand kilopasca ten thousand parsecs which is smaller than their idea for the size of the galaxy, therefore m31 had to be internal and interestingly adrian von monen just discovered a proper rotation for the most disordered object 101, that was his observation of the rotation of a galaxy, so he thought it had a very large rotation that was actually detectable and Shapley used that and he said well if m101 was really distant and so we could actually see that rotation. then it would have to spin faster than the speed of light, so combining conventional physics about star formation or at least about how stars could form and how planets could form and all sorts of other things, Shapley had some Pretty decent arguments. against all four, the concept of the nebular hypothesis for spiral nebulae and therefore the tiny, tiny, tiny milky way with respect to the universe, curtis, however, said well, a typical nova in Andromeda gave him a much, much, much greater distance, so that's really strange.

A single nova that Shapley was talking about brought it closer, but if there was a typical nova out there, it would be very far away, so the distance is about 10 kiloparsecs and that gave it about the size of the Captain's Milky Way at that distance , and so. therefore it is an external object and, until now, also widely argued that the original dark bands of the earl of ross that he saw with the leviathan and that we all are and were seen with later telescopes for a long time are simply edge spirals and are like you would see in the milky way, so the edge feature in spirals would be to take a spiral galaxy, a spiral nebula and look at the edge, you have this dark disk shaped structure and you have a line. through it, which is exactly the Milky Way and that's why you have these incredible objects, they must be similar objects and finally he said, well, spiral nebulae have really big radial velocities, so they should be able to escape the Milky Way problem. , there are a lot of shoulds in their arguments now there was no winner for the big debate and that's the problem and basically it was getting everyone in the room and airing the grievances so the grievances were aired in this debate um shapley had the observational evidence more compelling because there were like he did, but the evidence was flawed, so his observational evidence was compelling since it was right, but it wasn't right, it was flawed and Curtis ended up being right, but his observational evidence was weak because he said, just look at this thing, it's on the edge. and if it's in john, it could be that a lot of things can be marginal, so it just didn't, it didn't hold up because it wasn't compelling and exclusive, so the observational evidence that Curtis chose didn't exclude others. ideas, so there were many things in the debate and it was still not conclusive, but the big problem was that no one had an Andromeda nebula at a good distance and no one could reproduce the fen man's own motion of m101, no one could hint at it because it is very far away, no one ever.

So it was a wrong observation, damn it, and it didn't take long. Three years later, Edwin Powell Hubble ends that debate. He used the 100-inch telescope on Mount Wilson in California and looked for Cepheid variables in the Andromeda Nebula. Shapley's 0-0 version of the Levitt period luminosity ratio product for the Cepheid variables and got a really big distance at two um to m31 of 300 kiloparsecs and that's much bigger than the Milky Way according to Shapley, so whatever happens. m31 is outside the Milky Way and since he decided to keep the distance in 1925 he got better data and got even more Cepheid variables from Andromeda and discovered that even a distance greater than a thousand kiloparsec so he thought it was an incredibly distant object and so so much must be much more distant, it is outside our galaxy and must be at least as big as the Milky Way and he ended the debate and reign of distance and ushered in a new era because it is beginning right now.

We now knew that the universe was extraordinarily large and not just the realm of galaxies and, in fact, this is what Einstein even said when he thought about thegeneral relativity and he thought, wow, his original work before that 20 years earlier assumed that the stars were the universe and now Hubble comes along and says well, the universe is actually much bigger, so the universe grew in size radically, no one He knew how big it really was, but in 1925 the concept of how big the cosmos was changed forever and this is what he looked for. Here is some of the actual data from Hubble.

He has a particular look for the Cepheid variables in m33 and m31 and these here are the ones he marked and looked for specific Cepheid variables and showed that both objects might be too far away. Okay, and here's one of the photographic plates of him where he actually marked variable stars. In fact he discovered that there was a nut, that there was a variable star, so here's something really interesting: there's a photograph from 1923 that he took with the 100-inch telescope. mouse wilson and here is another set of photographs that he took in 1926 or at least they were published by an astrophysical magazine in 1926 where he noticed Cepheid novae and variables throughout the triangle galaxy m33 and this led him to understand that these things were incredibly In fact , distant m 100, which is another spiral galaxy, also has Cepheid variables and we can also see this observation from the Hubble Space Telescope with the Wide Field Planetary Camera and finally, Dave Soderbold of the Space Telescope Science Institute in 2011, used o published in 2011. his observations of the exact field of Hubble and that's why he wanted to take an image of what Hubble was looking at and that's why the images from the Hubble Space Telescope confirmed using the wide-field w wide-field planetary camera 3 and the and the uv uh inter on the integrated spectrograph to discover uh discover what its variability looks like using the Hubble Space Telescope so literally Hubble looks at the original Hubble data and it showed the variability of this star that it was on in the first Hubble diagram in its first discovery image from 1923.

So the light curve of the Cepheid variable star can be seen here and this is all out of the Hubble site archive from March 2000 to May 2011. And I will post the links for this on the YouTube channel. So we can see here little stars showing the four observations that they got with Hubble space with Hubble time and if that is the light curve of the Hubble data, then we can see that the brightness closely matches the curve of expected light, which is fantastic anyway. Other galaxies have them, such as m100, and there is a close-up image of a Cepheid variable in m100 and another variable star.

Other Cepheids exist in other spiral galaxies, such as nzc3021. This is another review by Adam Reese of Spa who also uses it at Johns Hopkins. looking for Cepheid variables and stars and his goal was to get Cepheid variable distances because he was looking for supernovae in 1995. So you can see he's looking for another standard candle shape, which is the 1995 ai supernova, which is what he's actually looking for and that was his goal and what he had to do once he found that supernova. I clearly looked for Cepheid variables within that interior of that galaxy to get conclusive distances to that galaxy so I could calibrate the distance to that super using the light from that supernova.

We'll see that become really important later. Okay, in 1936, eh, Edwin. Hubble published his book called The Kingdom of Nebulae and it is one of the most important illustrated books ever published in astronomy. He actually showed a fantastic fantastic set of images and discovered that the spiral nebulae were actually now we call them spiral galaxies. and the milky way is now just the galaxy and all the spiral nebulae and alexey galaxy are now called spiral galaxies and they are like the milky way, their sizes are about tens of kiloparsecs and their distances are megaparsecs or millions of parsecs and their The research is still basically, he's asked some of the fundamental questions and they still are, they're still relevant questions today, so if you go and find that image, it's kind of amazing, I'm sure it's like a few thousand Dollars.

On Amazon, if you want to get a used copy, in fact, I doubt you can buy a copy and most of them are in research libraries. Now our current view of the Milky Way is that the Sun has approximately 24,000 or 30,000 lights. years from the center and the center is in the direction of sagittarius, the disk is about thirty thousand or one hundred thousand light years wide or thirty thousand parsecs wide and is about one thousand parsecs thick or about three thousand light years thick where it is Sun. and the middle left image comes from the two micron all-sky survey and the other image is a kind of illustration that has been made by the Spitzer space telescope group that shows where the sun is and the approximate layout of the galaxy, so this is what we meant by Earl of Ross' idea of ​​what the Milky Way would look like from a distance and this is a very nice detailed image of ngc 891 showing a dark band of dust running through the center cutting through of it, and therefore, if we go millions of light years away from the Milky Way and look at it from the edge, we will probably see a very similar band of dust, a kind of bright lump in the middle in a halo of stars. around it some bright orange glowing pinkish glows coming from star forming regions and bright stars dotting its lanes, which would be o and b type stars, so we would call ngc 891 is an analogue of the milky way o something we could consider the milky way. way and then we would say that this is the galactic center and if this is what the Milky Way would be, the sun would be about 8,000 parsecs from the center, the galactic bulge would be that bulge that we call in the center and the disk would be the galactic disk , so this is what the Milky Way would look like from a distance if we could zoom out millions of light years, so Andromeda milky spiral galaxies and that gives us a rough idea, there is an s and where globular clusters fit well globular clusters they form a halo or spheroid of stars and gas, mostly stars that form the halo around the center of the milky way, so the globular shapley clusters are actually centered in the center of the milky way and that's what we know today and that is our approximate outlook.

Type o and b stars are in the disk. Globular clusters are above and below the disk. There is a huge, huge, huge lump of stars. A galactic bulge and the gas and dust are mainly confined to the disk. from the milky way we saw pink emission nebulae which are like Orion star forming regions or trifid nebula star forming regions which are in the disk so the gas and dust are in the disk the hot stars are in the star-forming disk. it's in the disk and the globular clusters are up and down and this is our image of the milky way that we have today and this comes from the space telescope science institute as well as from the sorry nasa jpl this is hertz from ssc and this is part from the uh glimpse team from caltech anyway, if the sun rotates at about 200 kilometers per second around the orbit, it will take the solar system more than 226 million years to orbit the center of the milky.

So when did the dinosaur appear? If we are at six o'clock now in this place here, then the dinosaurs are extinct. If we think of it as a clockwise rotation, then if we are at six o'clock, the dinosaurs. They became extinct about 60 million years ago and if it takes about 226 million years to go around, then the dinosaurs that became extinct about between two and three o'clock in this diagram, which is really interesting, the age of the Earth is only about four and a half billion years old, so 4.5 billion divided by 226 million, so four times each, so you have four times four to get a billion, so four orbits times four is 16 17 18 orbits, so the sun and the solar system has only orbited the center of the milky way 18 times, so I guess it could be selected by the US select service, but it's not old enough to drink yet, that's another three more orbits , that's in 700 million years, okay, anyway, that's what we call it the cosmic distance ladder and we start from Earth, where we use radar to get to the different planets and that's the astronomical unit that we use , stellar parallax, to take us to the stars, once we calibrate stellar parallax with spectroscopic parallax. then we can observe distant star clusters, we use distant star clusters to get variable stars.

Variable stars take us to distant galaxies and this is how we know the distance in the Andromeda galaxy and beyond, so this is our image of the local group of galaxies. The Milky Way is one galaxy, the Andromeda Galaxy M31 is another, they are separated by a huge and vast gulf, more than three million light years away, and that gulf is enormous. The size of the Milky Way is one hundred thousand light years wide. and that's Andromeda, the triangle galaxy is there, but in the middle there are little dwarf galaxies and a vast void of space and that's what we call the local group of galaxies, this group of galaxies and that's how we know how far away it is Andromeda when searching for Cepheid variables. and that was the great great great debate of 1920 and the result is that the universe became actually quite large.

This time we are going to see the appearance of the Milky Way and the Milky Way is one of the most spectacular. places that you will never see in your life and they are being stolen from us due to the nature of light pollution around the world, but it is part of our human history and as such, it is something that you should go out and see if it is possible that at some point time in your life there are too many lights and cities, there are too many lights and towns for you to be able to see the Milky Way in its great splendor and glory, but if you go to dark places where there is no light and you can really experience the night. time as it was meant to be experienced, then you too can, if you can, see the Milky Way as the ancients did and as people until recently, in the 1960s, 1970s and 1980s, saw the Milky Way in the sky , but the Milky Way itself is our home galaxy. so let's see what that is and this is of course a background image of the milky way in the sky, so I was in Colorado for the recent total solar eclipse in August 2017 and at a friend's house, I'm looking outside.

From the outside of the rock I looked out onto the porch and could see the Milky Way, so I took a photo of it. This is a 60 second exposure of the night sky with a standard DSLR camera looking into space and we see these types of clouds. of structure and that is the Milky Way, it is a diffuse band of light that is in the sky that has dark spots everywhere and those are dust and gas that are dusty lanes within the Milky Way, but it is that kind of cloudy appearance which is It's not smoke from a distant fire, this is in the sky, so this is a slightly different bitter iso above and you can see the appearance of the Milky Way as a bright, cloudy cloud in the sky punctuated by dark bands and here it is.

Another view of Maine a few years ago in 2016 by Tony Sharpman with the AAI group and you can see that there is the dust, there is a band of bright light in the sky and that is the Milky Way, and then there is another better view of the center of the Milky Way as seen by another amateur astronomer and here is a view of the Trifid Nebula and the Lagoon Nebula embedded inside near the center of the Milky Way and here is a view that was taken by Stan Honda of the Milky Way looking outside. about some darkness over the horizon with some brightness in the sky from distant cities distant distant but there it is we can see the dart, the band of light in the sky that never changes from generation to generation for humans, so this is what that Galileo saw in 1610 and This is what you'll see if you go somewhere extraordinarily dark, so the Milky Way is our home galaxy and this is a picture by Stan Honda, of course, and our home galaxy is where we are now , I'm not talking about it. the horizon of clouds or the small one or the platform there I am talking about the band of light that looks like a diffuse thing that seems to have smoke in front that is not smoke that is dust in the milky way itself and there is another view This was an astronomical photograph of the day taken from Australia of the center of the Milky Way on August 23, 2010 at Lockhart Gorge in Victoria, Australia, so look at that astronomical photograph of the day, but what we see is the galaxy itself where we are. it has a band of dust that is hidden in the brightness and there are stars embedded around it, in front of it and behind it and there is a brighter area towards what we could call the center of the milky way, so the milky way is that diffuse band of light crossing the night sky and has been seen throughout human culture and only people under the age of 35 today have not seen it.

It is surprising that the majority of people alive today born in the last 30 years have notseen, but all humans before their birth have and have had many names the celestial river the celestial path a path the spine of the night in our words galaxy and milky way are derived from Greek and Latin derivations and galaxies means milky band and lactia is milky and via means so via lactia is the way of milk, so a milky way or a milky band or a milky way has been something that has been seen for thousands and thousands of years and was part of our education, It's part of our evolution and when you really go and see you will be connected to people who lived thousands of years ago, tens of thousands of years ago, who looked at the sky and made up stories about the nature of that Milky Way.

So what are galaxies? Galaxies themselves are huge collections of stars and gas. and dust and they are all held together by gravity and the largest ones have about a trillion i.e. a quadrillion galaxies or stars or more and the smallest ones are really small with only tens of millions of stars and our nearest neighbor to the milky way is the andromeda galaxy or messiah object 31 we both have around our galaxies around 200 billion stars for around 400 billion stars between the two galaxies that make up the local group and this is the pinwheel galaxy or m101 and this is an image taken with the palomar digital sky studio okay so let's take some full sky views of the milky way to see what the milky way looks like in different wavelengths of light.

We started this whole course by looking at different wavelengths and noticing that light comes in different wavelengths, so these are both full views of the sky, one is a full image of the globe, so I'm trying to allude to the fact that we're looking at the entire sky in the lower right comparing the same type of projection that the Earth's sphere has. to the projection of the entire sky and what we are seeing on the bottom right is a complete image of the sky of the entire Milky Way and the Milky Way has a number of things that are evident everywhere there are dust clouds, there are clouds of glass. bright regions and this is an image of the entire sky in optical light, so it was assembled as a mosaic of hundreds and hundreds of photographs taken from many observatories around the world because the Hubble Space Telescope, although it takes photographs in optical light, It's your field.

The field of view is too small to create this image so it was created as a result of a massive mosaic of many different observatories that were around the world to do this and if you look closely at this image you can see that there are some irregular things when there are slightly different resolutions and slightly different color schemes, but overall what we see is a dusty band of bright light crossing the galactic equator and that's why we'll call it the galactic equator, we have a coordinate system centered on the center of the Milky Way , there are stars above and below and there is a dusty material that somehow permeates the center or the galactic equator, a little further down, to the right of the center of the galactic equator, we see the large and small Magellanic Clouds and far to the left , just below the left, there is a small band that is the Andromeda galaxy and there are pink glows and those pink glows are star forming regions or h2 regions or regions of the sky where there are stars. are forming and it's hydrogen gas, so this is the view where we see yellowish stars towards the center bluish stars and pink stars pink glows on all the edges of this end and towards the right side there is a kind of shape of Pink glow and that is the Barnard loop around the Orion Nebula, so if we then look at a specific wavelength of light in visible light h alpha, which is six five six three angstroms or six thousand five hundred sixty-three angstroms angstroms, we see the due brightness. to hot stars that make the light glow, so h alpha is a wavelength of red light and it's what makes that pink glow that we've seen in other places and it's a place where it's due to hot stars, stars hot type o and b that illuminates this gas and as it illuminates this gas it heats up and as it heats up it emits at characteristic frequencies of h alpha, so these are star forming regions in the sky that emit at a specific wavelength which is due to excited neutral hydrogen atoms and which This is the wavelength that is called h alpha.

Those other points are point sources, but really what we are seeing are the bright highlights, so where it is brightest is where there is more hydrogen gas h and on the right side you can see the c of the Barnard loop around the nebula of Orion and moving away from the galactic plane, observe how the galactic plane is somewhat swollen and lumpy with respect to h alpha, there are like bubbles of h alpha and that is because the gas is being excited by heat or and b-type stars that excite that gas , so now we switch to much longer wavelengths and this is a longer wave, it's much longer than h alpha, it's outside the optical range h alpha is still visible light and you can get it from using filters, but this is an infrared view from, I believe, the Spitzer Space Telescope and it shows a different view of the sky.

We're always going to do a view of the galactic equator, but notice that we have this very tight band with kind of swelling. There is cloudiness around it, but that tight band is much narrower than the starlight we saw in the original optics and this is because the infrared view is of warm dust, so this is a wavelength of much longer light, so we are seeing colder light. The interesting processes and things that we would look at with infrared would be dust, so dust is made up of carbon atoms, small nitrogen molecules that come together to form larger objects, larger molecular objects, or dusty objects that are not larger. than cigarette smoke. tiny tiny tiny cigarette most pits can be thousands of atoms in size at most, but what they do is they can absorb and emit infrared light and in doing so we found that this dust is very tightly confined to the plane of the track dairy. and if we look at the infrared camera of the Japanese space agency, we still see another slightly different version of this with many infrared point sources above and below, but in our milky way this band of dust is predominant, glowing warmly in the dust. and then away from the center we see that it gets fluffier, but we can imagine that it's a little bit fluffy towards the center, but we just see this big thing that might be in the distance, but far from the center of the milky way it becomes more spongy and diffuse as the gas itself becomes more diffuse.

All right, another view of infrared is, by the way, the Wide Field Infrared Survey Explorer, which is a NASA mission and looks at dust, and the green and red are due to dust and starlight. This is due to the blue and the blue green in the center is due to very, very, very cold stars and the dusty component is to the right and left and is green, so we should see a huge amount of old, cold stars, as well as dust. and gas, so the infrared light is dominated by two components, the first one is very, very, very small, dwarf type stars or even smaller dwarf stars, the smallest mass stars and they are confined to the plane, but you see some kind of something like a bulge in the center to elucidate the center of the Milky Way, but then far from the center the light is less dominated by that and far from the center of the Milky Way we only see the dusty regions of the areas green. dominated by dust, so those are the two components that create infrared light, the cloudy, fluffy things are clouds of gas and dust and the blue clarifies the tiny light of tiny stars by stars that emit infrared now, if we go further and more, with longer wavelengths up to 21 centimeters.

Wavelength about six or eight inches wavelength of light. This is due to neutral hydrogen gas which is very, very cold, extraordinarily diffuse, and almost never interacts with any of us. Any other hydrogen gas in its environment is also confined to the plane of the Milky Way in the same way that the dusty structure and these and those cold stars are, so this is a place where they are hardly clustered or very clustered, so What this is is hydrogen gas which is extremely diffuse and cold. and it is because the electron changes from one state to another, changes the orientation of its spin from up to down and that emits that long wavelength.

This is a radio emission light and so deep into that radio emission light that we can also search. carbon monoxide or co in microwaves and microwave frequencies are longer wavelengths of light and this was seen by Europeans. This is a map of the carbon monoxide seen by the European space agency's Planck satellite as it attempts to map the cosmos and one of its important mission is to map what the heck is the distribution of the microwave emission car due to the galaxy, so we see that carbon monoxide is a bright emission towards the center but becomes very spongy towards the also has these. tall fluffy cirrus clouds so the dust or gas component can be extremely fluffy like clouds and they are called interstellar serous clouds for that reason and these are places with gas and dust clouds, the gas clouds congregate but we think that if we merge the concept of where carbon monoxide or co is with neutral hydrogen and say that co, carbon monoxide, which has a very, very low abundance compared to hydrogen, it's actually clearer to see that Carbon monoxide is much easier to see than neutral hydrogen.

However, the most important thing is a brighter signal, it is a much brighter signal and the carbon monoxide tracks the hydrogen, so where there is carbon monoxide there is also cold hydrogen gas in great abundance, so the image of The all sky Planck spacecraft actually combines multiple bands and frequencies, that was one of their main goals because they were looking for the cosmic microwave background, so they are trying to eliminate the milky way effect, so they looked for numerous wavelength bands in the microwave emission in order to eliminate it. to see the background emission and we can see the defined kind of interstellar serous structure in the cloud that is made up of the microwave emission of it and that comes from a very, very cold gas, which is a molecular gas like carbon monoxide. carbon, methane, ammonia or others. diatomic molecules like nitrogen, etc., and these other things that can emit in microwaves and the dusty component can actually be very hot and that can also contribute to microwaves, which is why the sky seen through a plug is the nine nine of the wave, the nine wavelengths that see and you can see the components of the dust and the gas as seen by plonk.

Now if we go to much longer wavelengths, we see what's called synchrotron radiation, so this is at 408 megahertz and this is an extraordinarily long exposure, but this is for the parks at the park observatory, together with an Australian observatory, so a couple of radio observatories made this frequent map of synchrotron radiation, which is electrons that spiral in the magnetic field of the Milky Way's global magnetic field and travel almost at the speed of light and in doing so they emit these long wavelengths of light as they spiral into this magnetic field, so the magnetic fields are not very strong and the electrons go extraordinarily fast, so they cover a extraordinary distance as they travel and we see that the magnetic fields that permeate the galaxy follow the serosal structure in the outer areas, but there are some interesting things. happening in the center now, if instead we go to much shorter wavelengths and move to X-rays that are much shorter wavelengths than what we were looking at, we see the sky dominated by point sources and those point sources are black holes or neutron stars. or quasars that are very far away, but the point sources that we see in the Milky Way, which is again the band that crosses the center, will be neutron stars and black holes in the smell along the Milky Way disk above and Below are not shown the types of objects they could be, but point sources of X-rays are really difficult to know exactly their distances, but there they are, many of them would be extragalactic in nature, but we can see the contribution of galaxies to throughout the planet. center and then if we look at the roc the rosat study, we can see again what we saw at the long radio wavelength, which is kind of a weird bubble shape at the top and bottom with some very, very bright X-ray sources .

Thethat strange serous cloud we saw. at the radio light synchrotron as well and if we go to even shorter wavelengths, the higher energy wavelengths of light we see gamma rays and the gamma rays are almost completely confined to the mag at the center of the milky way, but we see that there are point sources all over the In the sky in the blue area around it there are some point sources everywhere, but there are some extraordinarily bright gamma ray sources, one from the center of the Milky Way, another and three from pulsars which are hyperdense neutron stars that rotate rapidly and we see that gamma rays are emitted by them from the crab that is very close and the play pulsar and these are from very young stars, very young dead stars, which means that they died recently in the last thousand or ten thousand years and are therefore still capable of creating gamma rays. of the extraordinary magnetic fields that they have when they rotate, but the most interesting thing is that blazar that is there, nothing, it is millions of light years away, it is hundreds of millions of light years away, but it is one of the most bright from the sky. sail pulsar, the game pulsar and the crab pulsar are all in the milky way and very close, which means you know within a few thousand light years, but that blazar is brighter than them and, in fact, is much, much, much further away, hundreds of millions if not billions of light years away and that means we are looking directly down the barrel of a jet of material from a jet of a supermassive black hole, as When material falls into that black hole, it accelerates to almost the speed of light and creates gamma.

The rays and those gamma rays are directed almost completely towards us and considering that there are hundreds of billions of galaxies throughout the cosmos and we see this one in gamma rays, which shows how strange it is to be looking down the barrel of a jet of material coming out of a blazar and we'll talk about blazars and active galactic nuclei later, so if we look, this image was actually created by an amateur astronomer who was looking by an amateur photographer who was looking at the Fermi data that we were looking at before, we narrowed them down and found that there are these bubbles, so not only are there these bubbles that arise from radial emission and rosette X-ray emission, but there are also these huge gamma bubbles.

Gas bubbles that emit rays that no one is really sure what is causing them, but they are seen centered in the center of the Milky Way and those gamma ray bubbles are some type of shock-induced material where, when the material hits, It excites when one atom hits another almost at the speed of light, that excites the material to emit gamma rays and perhaps even when they emit gamma rays we see them as bubbles because it is not that there is a bubble of gamma rays, it is that there is material that is being impacted. it heats up to the point where it emits gamma rays, so there is an energetic process that is actually happening down there that is due to the center of the Milky Way.

Well, our Milky Way is just one of the many spiral galaxies we see at the top left. the andromeda galaxy with a gapalactic bulge there is a disk and a halo surrounding it we see m101 the pinwheel and we see another galaxy probably ngc 891 and these are the typical types of spiral galaxies and the milky way is one of them the andromeda galaxy m31 is the The nearest bright galaxy to us is about 778 parsecs away if multiplied by 3, about 2.3 million light years away, so all the light we see coming from there is about two minutes, two and a quarter million long. years and also remember that the disk of this thing is about a hundred thousand light years in diameter, so at the near edge of the disk the light is actually younger than at the far one, so we actually have a time limit for a time machine through the galaxy because the near galaxy is the near side of the galaxy 100,000 light years closer than the far side, so we see the far side of the galaxy further back in the past than the far side close, that's just wild, but the nice thing about the Andromeda galaxy is that it's pretty much the same type of stars, similar gas and dust. to the Milky Way from internal studies and we get a rough view of what the Milky Way would look like from the outside and this is an image taken from the astronomical image of May 10, 2009 very well, so the infrared space telescope spitzer gives us another view of the andromeda galaxy and we see at 24 microns, which is long wavelength infrared, then it has this hot dust structure.

There is a visible wavelength view that we see at the bottom left, but if we compare it to the hot dust view, we see that it is dominated by dust and gas. by mainly warm dust and that's the basic vision of Spitzer is to be a dust observing object and also some point sources which would be bright infrared stars, but notice that it's a ring of dusty material that's emitted from this thing, so it's It doesn't have so much a spiral structure as a ring structure, which is an interesting way to think about, but we still think of m31 as a spiral galaxy even though it is more ring-shaped in this appearance in ultraviolet light than the Andromeda galaxy. also disproves that ring-shaped structure and ultraviolet light like we see here, the blue light is due to hot o and b type stars and the longer wavelength ultraviolet light is shown in yellow, so the brighter areas hot are shown in blue and we see that the dusty areas that occurred before, that is the place where star formation occurs and the blue is also ultraviolet, which also tracks star formation and the Swift space telescope in September 2009 published This incredible survey of the Andromeda Galaxy, which has extraordinarily high resolution if you check it out showing the hottest stars in the entire Andromeda Galaxy and where they are forming so we can see the most active star-forming sites in the Andromeda galaxy in ultraviolet and we noticed that we didn't. we have an all-sky ultraviolet survey because ultraviolet light is very difficult to focus and therefore it is much more difficult to create a smaller telescope that can be put into orbit for this and the galaxy probe was the closest and it we saw, but galax didn't do a complete study of the sky, he made a lot of patches, but no one put it together.

I was in full sky yet before what they did, so if we don't get out of the milky way, let's say we have a spaceship and go millions of light years away and look back at the milky way and we look at ngc 891 This is what the Milky Way will look like from a great distance, it has a galactic bulge in the center, there is a dust disk that is very confined and then there is a starry disk that is a little wider, so the stars They are not so confined to the disk, but the dust and all the star-forming regions, which are those pink clouds that are embedded in it, are. and the galactic halo that is in the galactic center, the galactic bulge has many older stars that would be tiny stars, but there are so many there that they resolve into a yellow cloud, so the galactic center would be where the old stars would be and the Earth from there from this analogue of the Milky Way.

This is what the Milky Way would look like, so we're using it as an example. The Earth will be about 8,000 parsecs from the center. The galactic bulge is where there is what we call the central area and it is mostly red and older stars and the disk is made up of dust and gas and we see the pink glow of hydrogen as well as the dusty light-blocking material that looks like . as dark areas, as well as some bright blue spots where young stars are forming, so we can observe another analogue of the Milky Way which is ngc 737331, which is about 50 million light years away in the constellation of Pegasus and It's just a very, very, very distant galaxy and the little galaxies behind it are even further away, about 10 times farther than this one, so we see all these little spiral nebulae around it and if we get closer and look Very close up of this galaxy, what we see is ignoring the three small galaxies above it, those are much more distant objects and all the stars that you see in this image, the points and the circles, are all part of the Milky Way, so which is the spiral nebula that we talked about in previous lectures, this kind of spiral.

That is what we are talking about and this is what the Milky Way would look like from a distance and this object is 50 million light years away and as such it means that its diameter is approximately one hundred thousand light years across and it will be composed of hundreds of billions of stars by itself, so this is a nebula not of gas but of gas, stars and dust, and the reason it looks nebulous and cloudy to us is because of its extraordinary distance, but it is composed of individual point sources of stars hundreds of billions of them and the gas clouds are emitting light in microwaves and infrared and it would be and the hot blue areas would be emitting ultraviolet light and the pink areas would be places where stars are forming and They are emitting that pink glow of alpha hydrogen that we have seen in the past and that is why star formation seems to be happening in these spiral arms and in the center is where the older, dead yellow stars are where there is not much gas. and dust, so this is one of our possible views of the Milky Way.

If we turned around and went to that galaxy 50 million light years away and turned around and looked back at our Milky Way, we would see something pretty similar to this. in the sky okay we continue with the milky way and the milky way is our big description so let's start with what we call the analog milky way or something that looks like the milky way in the sky this is ngc 891 a spiral. galaxy that has a similar appearance to our milky way, so all spiral galaxies in general have a disk in a spheroidal structure and the spheroidal structure starts at the beginning, from the center to the galactic center, there is a bulge of the disk around and then there are a series of halos. of globular clusters around it and then there is a disk, then the disk is like a thin extended disk of stars, gas and dust and we see the dust there, it says dark band, the gas is illuminated by the red flares that are seen in the dust.

They're like Orion nebulae, they glow because of the hot young stars shining inside them, and the spiral arms themselves are a little hard to see in this one simply because they're seen edge on, but we can postulate that there's probably something very similar to them. a spiral arm because spiral arms are composed of stars, young stars, gas and dust, so the spheroid is central, thick and central, and a bunch of stars with almost no gas or dust, so all the gas and dust are very far out on the disc. Look closely at the disk in the spheroid, the disk in the spheroid, which would also be called a halo, is an extended group and is centrally concentrated, almost without gas or dust, and I looked specifically when we say that we are talking specifically about the bulge and in the bulge you find a lot of rr lyrae type stars because they are very evolved stars that tend to be a little bit older, which means they would be g type stars that were finally evolving into giants and there would be clusters of old old star clusters, like globular clusters , but there is almost no gas or dust in the bulge, that's fine, but the halo is a much lower density region of the spheroidal component of this, in fact, you don't see it in this image. but they are there, we would have old metal poor globular star clusters and then there are lilies inside those globulars that help us determine the distance of those things, but in a galaxy of this distance it is a little difficult to use rr libraries in order to get them In any case, we can detect these globular clusters by looking at deep, deep, deep images that overprocess the galaxy itself, well, the structure of the disk itself, although there is a thick disk of stars and all spiral galaxies are this thick. disk and it's about a thousand parsecs thick by about three thousand light years thick and it's made up of stars and they're young stars and they're old stars and the young stars are in open clusters and loose associations or we'll call them ob associations with young young stars and then the Cepheid variables appear in young clusters because they are massive stars that are going through their horizontal giant branch phase which is a very short phase in their life but they are extraordinarily luminous stars that can be seen so in any case we see them open clusters and associations of stars within the disk the structure of the disk also has a thin layer of gas and dust now the height of the scale is much shallower than that of the stars it is about 10 times less thick perhaps only 300 light years thick or 100 parsecs and It is embedded within the stellar disk nowwhat makes it up is mainly cold atomic hydrogen gas and that is what we are seeing on the right, on the right side we are seeing the emission of radiation from 21 centimeters and that is the cold atomic hydrogen gas which is what on the right in the reddish type of diagram and that happens because the atom that forms hydrogen simply has a high proton and an electron and they both have a spin and if their spins are anti-per, if their spins are parallel, that is a high and their directions of spin are parallel, then we would be in a state of higher energy and the electron can rotate randomly so that they are antiparallel and, suddenly, that loses a little energy and goes to a lower energy. configuration and it emits radiation of 21 centimeters, that's what that red glow on the left is about, it has a lot of molecular hydrogen that barely emits light, so we track it in the blue on the left with um with carbon monoxide carbon. and this is a map of carbon monoxide made by the Planck probe and you can see that it has a completely thinner focus than what we observe with stars, so gas and dust are the raw materials for star formation and it comes from an extraordinarily thin area. but as they form, they crush and inflate and that's why you see that kind of strange and very similar structure on the left side, so another possible analogue of the milky way is ngc 7331, which is the spiral galaxy in the upper right, not the little neighbors next door but what is it but it is that spiral that lights up don't look at the ones on the bottom left that is Stefan's quintet a group of galaxies that is also very interesting but we will talk about that later so ngc 7331 is a galaxy that is very similar to our Milky Way and it looks almost edge on about 49 million light years away and if you look at the little galaxies above, they are about 10 times smaller so which are approximately 10 times farther away. is not part of the system that is this particular galaxy, so if we were to look at our Milky Way up close, this is probably what we would see with dusty material, dusty spiral arms punctuated by little pink glows from clouds of hydrogen gas that are being illuminated by the stars. that are around it and there is also a certain shape that I want you to look at, if you look very carefully at this thing, you will see that the dust seems to be on one side of the bluish glows, that's interesting.

The dust seems, if you think about it, to follow the bluish glows, that's interesting, and it's not like it's following the bluish glows if there's something else going on, but let's be like that, as the dust itself may be, we'll get to that very soon. . but the dust itself is where the stars are forming and so is the bluish glow that comes from the young, hot stars around it and there is some kind of pinkish glow and those are regions of young star formation, so If we look at an analogue of the Milky Way, we see that our Sun would be rotating around a system that is approximately one hundred thousand light years in diameter and our Sun takes about two and a quarter million years to go around the spiral galaxy that It is our Milky Way.

So I described the spherical and halo components a moment ago, but the main researcher who introduced this was Walter Botta and Walter Botta was a German astronomer and because he was a German immigrant in the '50s, '40s and '50s during World War II World, he couldn't do anything in the war effort, so he's an astronomer, so he used the 100-inch telescope on Mount Wilson that Hubble had used to discover the distance m31 and he was and would watch when the angels the lights they shut down because they shut down because they thought the Japanese were going to come and bomb Los Angeles and so on, and that's why they had blackouts and because of the blackouts throughout the city of Los Angeles that actually allowed them to make this deep sky. observation and at the end of the war the angels began to become so bright that now the 100 inch telescope that still exists at matt wilson is completely useless for tasks like this, but at that time the blackouts allowed him to take some extraordinarily deep images of photographs deep of the Andromeda galaxy of m31 and what he found was that the disk looks blue like we saw in the other examples, the spheroid looks a little reddish and has mostly old stars and he was able to easily detect individual stars, so that It's what he could. find with his photographic study just like Hubble did, but now below he made hour diagrams based on the colors blue and red, just like we said, the brightness in the blue minus the brightness of the ray and the red compared with the brightness and let's say the red or the blue. it doesn't matter which one and those hr diagrams show that the hr diagrams of the disk were very similar to open clusters and if you take the hr diagrams of the spheroid they are very similar to globular clusters, so he thought that led to the idea. which, overall, were completely different stellar populations, so let's go back and look at that again because we looked at stellar life spans a while ago, but here we go again, massive stars live short lives, so if you're a star main sequence mass, they have to be young they live only a few million years, so if you see an ob or ace type star they must be less than 100 million years old. o stars and b stars are only tens of millions of years old, but if you find low mass main sequence stars, they may be young or old, they may have formed 10 million years ago or 10 billion years ago because you really can't say it. they don't change much, so the time diagrams of star clusters, if they are young, have blue main sequence stars, but old clusters don't have blue main sequence stars and by old we mean up to billions of years, so old star clusters can be up to billions of years old, but young star clusters are on the order of tens of millions or, at most, 100 million years old, so Bada took this idea and divided all the stars in Andromeda into two different populations of stars that we call one of them population one and those were disk and open cluster stars and population two were globular cluster stars and spheroids and the two populations were distinguished by where were in the galaxies, how old they were and their chemical composition, so each of those things are quite important, so let's go over each population, one type of stars are always located in the disk and in open clusters, the ages of population may be a mix of young and old and their compositions are what we call metal-rich and What we mean by metals again is that this is a funny way for astronomers to say that there is hydrogen, there is helium and then there are the metals, which is kind of funny because everything that happens with stars is basically hydrogen, helium and that little bit of everything else basically gives a star electrons and some extra mass and does some weird things inside, but actually what it does, providing electron opacity for stellar atmospheres, is really what it ends up doing, but in any case the compositions are rich in metals, which means they are pretty close to what we find in the sun and the sun. it's about 70 percent hydrogen about 28 percent healing about 2 percent other stuff and they're filed, then you have the population of 1 stars where there's a lot of gas and a spa and your especially young stars are in that region, so if you find a population of one, you'll probably find some young stars like some O and B, and they have specific types of orbits.

Population stars of a type orbit in the disk and orbit in nice ordered patterns. and roughly circular things around it and that's why they make the little blue arrows on the blue disk of this thing and that's what we call population one. They probably have the same orbitals, they orbit in roughly the same general direction and for a given radius from the center they go at roughly the same speed, so they are pretty regular in that sense and that's what we would call a semi-system. relaxed or an organized system, then we divide the second group, which is population two, and they are located in the spheroid and the bulge and those are the globular clusters and the spheroid component and they are extremely old stars and the oldest ones have at least 10 billion years old and its composition has almost no metals in general, they are a single store.

They are at most one percent outside of solar energy and at least 10 percent serve one thousandth of that of the sun. They have a lot more hydrogen, about five percent more, a lot less helium in comparison and very few metals, and they are found in places where there is no gas or dust where there is no star formation, so it looks like they were made and they were not made. nothing else next to them and they were the only generation of their only cell, so the stellar orbits. for population two, population two is a mess, they are in the halo, they have a lot of random orbits, or they are in the lump, mixed up, doing all kinds of crazy things, or they are in the halo just doing their thing and flying around they orbit the center of mass just like the disk stars do, but they orbit in incredibly random orbits some go with the rotation some go against the rotation some go up some go down some are elliptical others are circular it's kind of random and now, if we compare the contrast and compare the two sets of stars, we found that there are two populations that bought a bada found in our galaxy and in m31 in population one, they are in the disk and they are mainly in open clusters they are young and old stars, they are rich in metals and if there are blue main sequence stars, they are there, in ordered circular orbits in the galactic plane and they are present in a gas-rich environment and there is already star formation with them in the population two stars however they are in the spheroid they are in the halo are in globular clusters they are the oldest stars they are extraordinarily metal poor there are absolutely no blue main sequence stars in their group it is mostly uh gk and m type stars and mostly k and m type stars and they have random elliptical orbits in all kinds of directions and they have no dust or gas there and they never coincide with the stars. formation, so what this means is that there is a chemical evolution of the entire galaxy, so what is happening is that metals are formed because metals, astronomy, I am talking about astral metals, carbon for the purpose of astronomy is a metal so I should really put that into quotes, metals form because there is fusion inside massive stars so when there is fusion you get things heavier than hydrogen and helium and therefore when there are explosions of supernova, enrich the interstellar medium with the metals that were produced in the core and the next generation of stars.

It's formed from this new gas that's been around and has all sorts of things like iron and carbon and oxygen and magnesium and neon, all sorts of other things, phosphorus, whatever, so that each successive generation of stars gets richer and richer. with heavier elements we call metals so the population of two stars is from a previous generation of stars compared to the population one they had and they haven't changed since then so really what we have is that this is an interesting little diagram that was created by the team at the chandra planetary nebulae or explode and the material that has formed on the right side goes out into the interstellar medium and you could actually think of it as the right side of this image, then it turns to the left side and more protostars form and so on, and so on, but if you have very, very, very young, very small mass stars, like brown dwarfs and red dwarfs, and things less massive than the Sun, then they're not going to participate in this because they don't participate or they're not part of it. from all of this.

So if they are in a region where there is no gas, no dust, and no star formation, then you won't see their friends, companions, or nearby stars getting enriched by the material around them. Now this begs the question: can a star? change its spots, can a leopard change its spots so that the galaxy as a whole evolves over time, but chemical evolution affects only broad populations of stars? This is because fusion occurs deep in the star, in the core of the star and only the carbon and nitrogen-type elements oxygen ever reach the surface due to churning through convection zones.

The surface composition essentially of a star. The composition of the star's surface basically remains unchanged throughout the life of the star, as there is some convective upheaval thatcan cause some problems. are the ones best treated at the research level, but for our purposes what we can really say is that the surface composition remains essentially unchanged throughout the life of the star. Of course, there are many things that are current areas of research that should go against that, but for an introductory purpose it's more important to think about how we break things down into simple blocks, then we'll come back to that later if you decide to be a researcher and a time the star forms, therefore, the chemical composition you have.

It's pretty much fixed for its entire life, so the surface, the composition of the surface that you see on the star, gives you a clue as to how the star formed, so if you find that a star has almost no metals in its atmosphere, it is probably a much older generation of stars and that is confirmed by the fact that the chemical composition is different for the spheroid and halo components as opposed to the disk components and they are completely different in that sense, so in that sense we have all kinds There are a lot of interesting review questions for you to talk about and think about, and what I would really like to think about is that you remember that these two groups of stars are quite divided between what they have, what is their chemical composition, what is their location and where. meet, so the most important thing we're seeing here is that population one and population 2 stars are radically different groups of stars, they have different origins and they've gone through the cycle, well, we've been going through the Milky Way and talking From it, we have gone over many properties of the appearance and naming of aspects of the Milky Way, but now let's see how the Milky Way evolves and forms over time, so galaxies have to come from somewhere, so let's see what we have about these things and here is a little summary of the things that we consider properties of the Milky Way that must be part of it and that we can remember the Milky Way itself.

It is a spiral galaxy It has a disk It has a halo and it has a galactic bulge and the disk is very flat It has young and old stars It is gas and dust There is star formation The stars move in circular orbits and there are spiral arms and in general it is You have arms blue spirals and the colors of the stars are a little off-white because you have a mix of red and blue stars. Now in the galactic halo the halo is quite spherical, maybe it could be a little flattened just because you know how to shoot.

At the top there are actually just old stars contained in globular clusters and those are the only things in them - there is literally no dust or gas in the galactic halo and there has been no star formation for the last 10 billion years. As we have seen in the types of stars that we find in the population of one type of stars, there is actually a population of two types of stars, and neither has happened in the last ten billion years. The stars have all kinds of random orbits in the halo and there are almost no disturbances, no substructure. there are globular clusters and maybe some tidal currents and things like that, ripped stars which we'll talk about in a second and generally the halo looks quite red, the bulge itself is a little bit different, it's kind of a mix of the halo. and the disk is quite flat, but it is flattened along the path of the disk and contains young and old stars, but there are many more old stars and the more stars there are, the further they are from the center, the older they are.

There is a lot of gas and dust, but in the inner regions, but that is only in the central region of the Milky Way, which we will talk about in the future, and there is star formation going on in the inner regions just because of gravity. because there is gas and dust, but star formation only occurs in the very central regions of the galactic bulge. However, all stars have fairly random orbits in the bulge, but there is a net rotation around the galactic center and it is always nearby. a ring of gas and dust near the center in the galactic core which we haven't talked about yet but will soon and the galactic bulge tends to look whitish yellow due to the presence of older stars, so this is kind. from our general image of the stars in our image of the milky way we have old stars in globular halo clusters that are clearly old only type o and b stars are only found in the disk there are emission nebulae which are those clouds of hydrogen gas that they are pink, the sun is about eight kiloparsecs, about 30,000 light years from the center, the galactic bulge is a mix of old and young and in the disk there are only young stars, but you can find that there are some, there are also many stars old but they tend to you'll only find the youngest stars in the disk in the gala in the disk area and there's gas and dust predominantly in the disk and open clusters, there's blue and white types, things like the hyads and the pleiades. and the double cluster and Hercules those things and not in Hercules in Perseus that is what is found in the disk globular clusters like m3 and m13 are found in the halo, so how did the milky way form and create this structure?

In a very, very, very general sense, taking small beams of gas and dust in small dwarf galaxies and then smashing them together. On average, when they collide and form a larger ball of gas and dust, they will generally have some sort of generic rotation. and as they collapse together, they will have an orientation that will have an overall rotation across the entire assembly, which is the average of all their angular momenta, which is averaged across the entire assembly and will flatten out along the axis of that average rotation to form a disk, once it does, matter can then gas and dust can fall into the disk because that is not where most of the matter is and, as the mercury, the mass begins to rotate because The material begins to fall. on the plane and eventually you get something where you have a bunch of stuff passing through the disk and those are the old old stars and the halo and the disk then sort themselves out because that's where the gas came from so the rotation initially was kind of of a large balloon.

It was kind of spherical, but it flattened out and then the gas just fell down, while the stars that formed from the original group were already formed, so they'll just keep their random motion, so that's what the dots are. yellows. the yellow dots are the old stars that formed before the formation of the galaxy or during the formation of the galaxy and before the gas formed and in fact not before the gas formed but from that gas and the gas that was formed. there was almost all hydrogen and helium, so those stars were formed from gas that was just hydrogen and helium, that's what we mean by metal poor, so this concept was devised by Laplace in terms of his idea on the flat formation. of the solar system, as well as the formation of the galaxy, so we get that the disk is a gaseous component because the gas will collapse along the axis of rotation, but the halo that is the stars will not do that type of collapse.

They will just randomly hum around each other and they will stay humming for a long time and they have been humming for billions of years so they will keep going so generally we take small dwarf galaxies that have the stars in them say 10 billion years ago More than 10 billion years ago those young stars became the globular clusters that we see in frame a and those globular clusters were made of stars that had almost no gas, almost no dust, almost no carbon. nitrogen without oxygen without iron almost nothing almost made purely of hydrogen and helium and that first generation of stars was created in the globular clusters and that is what the yellow dots indicate and those stars some of them were type o and b stars, of course They were, they dyed them and seeded the first sets of stars that became the next generation and that material came out into the gaseous component that then collapsed along the axis of rotation to keep the gas going and the gas.

It wasn't completely exhausted in the first sets of stars formed when small dwarf galaxies merged, which is probably how the Milky Way formed in the most general sense and why we still have small galaxies around like the dwarf Ursa Minor. the sextants outshine the dwarf corina the large and small Magellanic clouds all these small dwarf galaxies are still floating around the milky way and that's why they are orbiting the milky way they are still there but they could be absorbed and in fact gravity does they work to that they orbit, which means they will eventually lose some of their angular momentum and merge with the Milky Way, but wait a second, there's that Andromeda galaxy, it's about the same mass as the Milky Way, maybe it's bigger and that it means that the milky way and andromeda are probably falling towards each other, that's really interesting, but let's talk about that in a moment so we can have evidence for this concept of dwarf galaxies being dismantled by our galaxy because there seem to be streams of stars that If we look at certain areas of the sky, we find that there are streams of old population, older population, uh, population of a type, stars that mean metal-poor population, type one or at the metal-poor end of population one, and these Stars are small galaxies that have been stripped away by the tides and as they are stripped away there, they stretch to where they can't even be seen anymore, so the Spitzer Space Telescope was able to use infrared vision to find these stellar streams that orbit our Milky Way and these stellar streams are believed to be remnants of dwarf galaxies that have collided with our Milky Way and, in fact, large and small Magellanic clouds are in the process of merging with the Milky Way and will dynamically merge with the milky way in the future and We have evidence of these streams of stars and specifically this is an amazing image of an astronomical image of the day of 2008, specifically June 19, 2008, and it comes from Dr.

J's Blackbird observatory of Dr. Gabbani and also collaborated or collaborated. with several people to show the titles around this particular galaxy, these titles clearly demonstrate that small dwarf galaxies are torn apart by larger galaxies and essentially lose their cohesion because the larger galaxy simply stretches these small galaxies and the stars become streams rather than clots like small dwarf galaxies or spirals or something and eventually the stellar streams merge with the rest of the galaxy or become part of the halo component of that galaxy, but they don't have enough gravitation. uh influence to tear apart the spiral structure of that larger galaxy so that they just orbit in their streams and are like stellar streams that are similar to the meteor showers that the Earth encounters when a comet breaks up. up, it just orbits through the galactic plane, so then we can make a pizza, do one thing, we can think that if we look at extremely deep photographic studies of looking very, very, very far away and therefore very, very, very back in time.

We see a gallery of galaxies like the Milky Way from a long time ago until today and if we think about the oldest things, we would say well, if we have small dwarf galaxies, those small dwarf galaxies will be very, very blue because, well, they are mainly hydrogen and helium, so they will produce a lot of blue stars and that's what we thought 11.3 billion years ago, they would be two small galaxies merging, so they would be blue, tiny, misshapen and bright, a billion years later, those things merge. they come together to form something a little bit bigger and if another half a billion years later they merge with something else and then another billion years later they merge with more things and they get more dust and gas and then they spend another three billion years years and merges with even more, so a continuous merger of small dwarf galaxies appears to be the dynamic way that the Milky Ways form as large spirals, where they continue to be replenished with more gas and more dust as they collide with more small galaxies. dwarfs that are mainly gas and dust to enrich the spiral structure with more material to create more stars and so we can see that perhaps three billion years ago the milky way was a little smaller and another spiral dwarf collided with it to form more stars of the milky way, so this is kind of photographic support for the idea of ​​the formation of the milky way by looking at the oldest galaxies that are very, very deep in space and very, very far back in time, because light needs time. get here from there so we can see things like before if we look further back in time and then it really just begs the question of how the hell do we know it's so far back in time and that we are We won't be afraid to get to that , so what we do is discover that the things that are further away are misshapen, small and shiny, and as we get closer and closer and almost withoutspiral structure, it just takes until later, much later. that you can get a structure big enough that, where enough stars have gone through enough generations, enough dust has been created to get things that look like bands of dust, like we see in the bottom right image, where there are little bands of dark dust that are just barely visible and this is a NASA image, and they believe it is also a Hubble type image, so yes, we will definitely provide the links on the YouTube channel for any event, this is a good little simulation, uh.

I think it was put together by Dr. Summers at the Space Telescope Science Institute to try to show what would happen when the Milky Way finally collides with the Andromeda galaxy, so about five billion years from now today, the Milky Way and the Andromeda Galaxy will collide, they will merge because they are the two big bad boys on the block, and eventually they will meet and form a big galaxy that we call the milk army, because the Milky Way and the Andromeda Galaxy , so Milk Ahmeda will be the neck, it will be the name. of the great galaxy notice that when they merge, the tidal interruption occurs and the stars are thrown out and the colors go from blue to red and the reason why the colors go from blue to red as the nuclei merge is that star formation is occurring as gas.

The clouds merge in wild bursts of star formation and get rid of all the gas and form all these stars in massive quantities at the same time and then the spiral structures are destroyed and you're left with just a bunch of old yellow stars and no gas or gas. dust as it burned up in a huge, huge, huge event, and that's what will happen about 8 billion years from now. 8 billion, not in a hundred years, not in a thousand, not ten thousand, not a hundred thousand, not a million, not ten million, not a hundred million but not even a billion eight billion years that is in more time in the future That's twice as long in the future as Earth has been alive, so what would it look like from Earth?

What would all that simulation look like? So this is what It looks like if we look towards the Milky Way, uh, towards the Andromeda galaxy, today we see the Milky Way in the foreground. It's like we pretend the Earth will survive this long and you can set up a camera and you can get through this. generation after generation and we take the same image over and over again, but hey, we're just doing a simulation here, so this is all simulated at this point, so the milky way in the sky, Andromeda, there and then we jump forward about two billion years. the andromeda galaxy is now so big in the sky that it is something that everyone will always notice and be part of their lives if there are people on earth two billion years from now, maybe the giant cockroaches that are going to be the survivors will enjoy it, the view is fine, so we go to approximately three and a half billion years, the Andromeda galaxy is now extraordinarily large in the field of view, it occupies about half of the sky, more than half of the sky in our view and then about very soon after the stars because they collide they are starting to collide there are all kinds of pink star forming regions as the stars form in furious quantities there will be supernovae in the sky tons and tons of Orion nebulae super large and trifid nebulae the sky will glow pink at night due to the glow of hydrogen in the sky and multiple stars will become supernovas in our nights in the night sky and then if we go a little further the star formation continues in an even more robust and sick way and large superclusters of stars will form.

This will occur and there will be huge, massive amounts of star formation that will disrupt and destroy the remaining gas in the Andromeda system, the Milky Way, as it is a race between consuming all the gas and expelling it into space and the enormous stellar winds that we saw from the orion nebula, stars this massive can cause enormous stellar winds that will blow out the gas, and in fact, as these supernova explosions occur, star formation will quickly dissipate because the gas is light and doesn't hold up well to shocks. massive, so perhaps in about four billion years, as they pass by each other, the Andromeda galaxy will be tidally stretched, most of its gas used up into red stars, then there will be no o and b type stars, but it will be an interesting and beautiful sight. very strange and red in the sky and then in about five billion years the core of the Milky Way and Andromeda will appear as a couple of bright lobes in the sky in some direction that will be just the remains of the core of that and there will be some some residual star formation, but in reality it will be these two types of wild glows in the sky and then after about seven billion years there will be a massive elliptical galaxy, the bright core dominates the night sky, in fact it could be so bright that there would be absolutely nothing to see at night there would be very few stars unless the sun set and the stars but in seven billion years the stars the sun won't even be here and the earth will probably be burned once ash as it passes the sun becoming a red giant, but hey, maybe this was taken on one of the distant moons surrounding Saturn that survived the sun's growth into a red giant, so this sequence in particular it was done by the NASA group on space telescope science with uh levate and van der merle and ha and hollis and mellinger and they basically put this together to show what the sky would look like from that wonderful image that we now know when The Milky Way will collide in about five billion years with the Andromeda Galaxy, so I have some questions for you to go over and think about, and we can ask what the hell is going on, but we have all this interesting evidence that the Milky Way galaxy is evolving from an early state from photographic evidence by looking at ancient galaxies in the past, that most galaxies were small, blue, misshapen and bright and then as time progresses, those misshapen blue galaxies that were just tall, deep , deep, in the past they were made mainly of hydrogen and helium and then, as time progressed, the stars, generation after generation, made more and more dusty material such as carbon, nitrogen, oxygen, iron, magnesium and all the rest elements that form the dark bands of dust that would eventually become planets and other stars, so we have a theory of sorts about how the Milky Way formed and where it came from.

Right now we're going to talk about the spiral arms of the Milky Way and how they formed and what they look like, so the most important thing we have in this section right now is to understand how we know where they come from and, in fact, how we can measure them. because we see in the past things before that people had thought that the Milky Way was just like the other spiral arms, except that we are inside other spiral nebulae or their spiral galaxies, except that we are inside it, so we can't see the arms very easily and since the size scale of the Milky Way is So you have to go over 3,000 light years to get above the Milky Way, no one is ever going to do that, I mean, ever, which means you know people don't do these things and you know humans aren't going to travel. thousands of light years as an individual and see them know that the Star Wars movie is just not going to happen in any case, so what we want to do is find a way to measure the position in the movement of the gas clouds because they plot the size of the milky in a spiral shape, well, again we know that there must be massive clouds of gas and massive clouds of dust.

We know it because we see it in the sky, so not just plaster on the side of the sky, we want to see if this cloud is in front of that cloud and what direction they are and then hopefully we can map that, so that's our goal, so what are spiral arms and spiral arms are these spiral-shaped patterns that are dotted with hot stars, o and b type stars, young star clusters? and gas and dust, o and b type stars form in these regions in the spiral arms and illuminate them and we know that o and b type stars have very, very, very short lives, so the spiral structure pattern should change because well they are illuminated by type o and b stars, therefore, that pattern must change and we see giant molecular clouds that can be seen in radio light and hydrogen gas and dust clouds can be seen in the infrared, but these objects are almost never found outside of the spiral arms, i.e. up and down or specifically between those things, so the gas clouds must be there, but they clump together, so what does clumping have to do with it?

What are spiral arms? Well, they can't rotate along with them. the galaxy along with the stars because the spiral arms were fixed objects and rotated and the stars played a role in their rotation then they would remain illuminated and as such the things that are illuminated are the stars and then we would perceive them as spiraling up, so we never see anything like the latter, there is almost nothing like that in the sky, there are things like the first one there and things like the middle one, but nothing like the last one, hence the spiral arms They are not simply formed.

They form and stretch like spaghetti or something, that shouldn't be how they work, but we do know that spiral arms are where the formation of massive stars occurs and the sun takes about a quarter of that in almost 200 million years. , approximately 260 million years to take an orbit and will therefore make a total of 50 orbits around the galaxy before dying and becoming a white torque. Now type o and b stars have only been fighting live for about a million years, so a million to 10 million years, so let's call it a million or 10 million just for fun and that means they move at most an hour in the galactic clock so you can look at the galaxy as a disk and we can think that if you move 200 million years or 240 million years over the course of one revolution around the radius of the sun, then if that's the case and let's say that An object lives for only, say, two million years, so that's one twelfth of the orbit, so it's about an hour.

We can think of the star moving very, very, very short distances on the clock face of the spiral galaxy long before it dies, so basically, where you see a star or is where it was born, is where it lived and where it is going to die, and that is. More or less what we think, that's what we know, so we see the o and b type stars and the h2 regions that light up and are in the spiral arms and that's what Walter Bottas said, they were the beads of such a galactic string . So the spiral arms are probably not fixed objects, but rather density waves or waves of or places where there are more things, that's a better way to think about, waves of things where things enter this traffic jam and then leave , and we can see that in this little diagram that there seems to be a flow because yes, if you do Doppler, if you do some Doppler checks and to see the rotation, you see that the galaxies are rotating, but what exact pieces are rotating, they are not objects fixed, so a galaxy is not.

It spins like a disk, it is a swirling mass of stars and particles, so what is happening we can think of as the warning that the diagram that we see on the left is a kind of diagram par excellence, we have this cloud of dark dust and at the same time front of the dust cloud in terms of rotation there are a lot of hot o and b type stars, so we can even think that they are in front of the dust cloud or, more specifically, that that is where there is a lot of things, the spiral arm it's where an excess of stuff is so gas clouds come into the place where there's a lot of stuff, they compress and then they form stars and then the stars move from their inherent motion around the galaxy, so basically the gas comes in in the thing and the stars come out. is another way of thinking about it, so the spiral arms themselves, if we think of them as density waves, then they can look very similar to this, so everything has an orbit on the sun, has an orbit, the nebula of Orion has an orbit, uh, Betelgeuse has an orbit every star in the sky has an orbit around the milky way and because the orbits of each orbit are an ellipse, which are Kepler's laws, then we can say that we can nest the ellipses, so let's say we nest a bunch of elliptical orbits and they're offset maybe they're rotated a little bit and offset a little bit you'll see that in this nested rotated version of everything we find that there are some darker areas where the lines line up and they tend to be uh and when they line up that's where a lot of orbits intersect and so if there's a star or a gas cloudspecifically, let's say, say, a long trail of gas cloud that runs all the way around the orbit, so a stream of gas clouds, then, like those streams of gas clouds, let's assume that all of these nested orbits are streams of gas clouds. , then they get to a place where they can press each other and when they press there is more gas, if there is more gas then they can form the stars, so the area of ​​compression of the gas streams is where things happen, so that's what we mean by nested orbit and in fact if you look closely we can see kind of a spiral pattern and if that's where the action is where they group together then the orbits are still there. contained with a lot of gas, all these dark lines are where the orbit of the gas streams is, it's right where all the orbits are grouped together, it's where all the activity happens and that's the idea of ​​a density wave, like this which we can also think of this way, it's a bit of a strange way to think about it, there's a kind of place where things have to be compressed and put together and that's just what's happening and they can be pressed together.

They have to slow down and when they do there are interactions or we can say that they actually stay at the same speed. Another way to think about it is that they all stay at the same speed, they just pass each other, so they're nested. elliptical orbits create the stellar jams, those stellar jams where the gas clouds are compressed and that's pretty much the same thing, that's exactly how I think, but now, if you take this whole system and then spin it around, the whole system too It rotates as a group and that is because the elliptical orbits are not fixed, so the whole system rotates and then you have a rotating spiral galaxy, so again the clouds of gas and dust move towards those places where the density is higher and because the gas moves towards that place where the density is higher where there is other gas moving, then they are compressed, which forms stars of type o and b and there are remnants of that gas, then they are illuminated by the hot stars of type o and b as they go through star formation happens vigorously dust forms gases form and you get things like if you look to the right now it's not known exactly what starts it how does this process actually start?

It is an area of ​​active research, but we can see that this explanation has a very good explanation in terms of the image on the right where we see the gas clouds which are dark gas clouds that have excessive density in general and those are places where the gas is accumulating and potentially starting the star formation process, so the gas that's where we have like the string, so the dark lines are like a string and the star forming areas are like the beads on a string , like the ones that Walter Botta described, so looking closer at this diagram we see that the red arrows indicate that the gas enters one of those density areas, that's where it arrives, that's where the gas is compressed, stars are formed inside that, the gas and dust compress together, they form those stars and they come out the other side because they don't stop flowing just because it becomes dense, you get associations of type o and b stars where there are groups of stars where there is no longer gas around them, but they are simply associated, they eventually explode, but they have used up all their gas. but then you have h2 regions or regions of ionized hydrogen around o and b type stars if the gas was not completely depleted in the process, so you can know and associate if the gas was depleted and the h2 if not, and some of the remnant gas somehow made it through so that's another way of looking at it and here's a really fantastic example of exactly what I'm talking about what Walter Body was doing and this is a visual view of the Hubble heritage of the galaxy whirlpool m51. so let's see what we mean by the order of things, so we have a general rotation that, according to this image, would be counterclockwise;

However, as the gas enters, it will encounter density in the dense area of ​​the streams' nested elliptical orbits. of gas and you can almost see how the gas looks nested like there are currents going in and out, but still there is an overall dominant feature which is the dark, dark, dark areas which are the string-like things that the beads have along. So we're talking about two big arms here and the gas flows into those dark areas which is where it's dense and on the other side stars flow that are brighter, so this is what we mean: we see a dark lane where the stars are. . where the gas accumulates and in front of that dark lane, after the process has occurred, we compress the gas and it becomes these hot stars and the pink glow is the pink glow of hydrogen and we notice the pink glow of hydrogen and the glow the stars are on one side of the spiral arms, that's because the flux enters the density wave and through the density wave and leaves the density wave and notice that you don't have too many bright hot stars far from the density wave because the o and b type stars practically detonate before they have a chance to get very far because they live a very short period of time, so this tells us a lot about the evolution of stars and now we're going to say, well, Do we see evidence? for that, that's m51, an outer galaxy, we see it here and this is an image provided by the Spitzer space telescope of a very familiar area of ​​the sky actually and this is in the concept, it's close to the constellation of sagittarius and m17 It's one of the large nebulae, emission nebulae in the sky, but now we're looking at infrared light and we're actually looking down a cross section of a spiral arm, so this is like taking one of the spiral arms by cutting it lengthwise and Looking down, we will start with the direction of the flow going to the left, it goes from right to left in this image where we have the dark dust clouds that are forming young stars, then they leave the star forming region and become a region h2 which, like for example m17 and that is where we have this bright star forming nebula where many stars are forming and we have type O stars that illuminate the nebula and the gas has not been completely used up, but it is present in quantities copious even to the left. and to the right, but the m, but m17 makes it glow and then we go further and this older area where there is like a supernova remnant like a bubble shape where something detonated and formed a bubble, so let's look at each of these areas in Here there is an area of ​​dark dust clouds where tiny little stars are forming and now that we are looking in the infrared we can see through the dark bands of dust and see the young stars that are forming within that, for what each one of these tiny little dots and this is from the Spencer Space Telescope at Cal Tech and they call it the dragon and the swan, which is actually very fancy, it looks like a little dragon, so go watch their video, turn a bit, so that's what I did when I removed that twist, so I didn't like that twist, so anyway, this is his work and it's an amazing job anyway, we see the dark, dusty bands, we see the little ones baby stars forming inside the prototype. dark area and then we look towards the middle where we again have a dusty region, there is a lot of dust in this region and that is what this infrared light shows, however it is very hot dust and this is a loop, it is part of this because it is an h2 region and this is the omega nebula because it looks like an inverted omega, it's really hard to see in this image, but whatever we see, this dusty region is made to shine because of the hot o and b type stars in that area and then We move to the left and we see this kind of faint outline of some kind of bubbly shell and it's a certain diameter and you really have to squint and stare to see it, but on average you can imagine that it has at least formed a cavity and at least there is a shock front on the left side around nine, 12 and seven. o'clock and maybe even there's some kind of central thing in the center, on the right, where it looks like it's been cleaned up, so a shock wave appears to have formed in this area and that's a natural consequence of a massive explosion of one or type of star and this would be something very, very, very old and it is sending and sending shock waves into the cosmos, so it would show the many areas of star formation of the era by looking down a spiral arm of star and this work was a result of povich at penn state university using the nasa jpl spitzer space telescope with the iraq mips instrument so fascinating fascinating study of what it looks like inside a spiral harp, so what's another way in that we can see it because it's kind of complicated and the infrared light may not be able to give us the map that we want, but there is a very good map that we know comes from the colder gas and that cold gas will show us the source material of the stars so high that they remember the stars.

They are made up of between 75 and 8 70 and 75 and 80 hydrogen, so if we can map that hydrogen, we will map where stars will form and if there is no hydrogen, then we should not see any missions, remember this is the picture of everything the sky of the milky way at 21 centimeters, which is radio light and is due to the emission of hydrogen and we see it continue, it is right along the band of the milky way, just below the galactic disk, we have some cirrus clouds high up and down. but that's concentrated almost exclusively along the galactic equator and it's a very, very, very narrow band, so where does this come from?

Well, what happens is that you have an electron and a proton and if you have an anti, their spins are aligned, that is, the electron to the proton. they have a quantum mechanical number or a state that we call spin that is very closely related to angular momentum and is actually the angular momentum of it, but they don't spin like little tops but they still have angular momentum, it's kind of a strange thing in any case, um, that's not really what we're talking about, but we'll pretend for a while that the electron and proton are like little balls and they spin, so if they have spins, they are and, in fact, that's not true, But we.

We're just going to pretend a little just for the sake of your argument and if the electron and proton spin in the same direction that the electron orbits the proton in the ground state, this is always, of course, the ground state, so It is at a higher energy. state and it can randomly, you know, just go okay, I'm done and relax and spin randomly and that will emit light with a wavelength of 21 centimeters and it's very, it's 1400 megahertz, so this is definitely on the radio and it's a long wavelength. light that must be seen with a support in large scale radio telescopes and this can only happen when the temperature of the gas is very, very cold, between 15 and 100 degrees kelvin, that is, just above absolute zero and the density has to be extraordinarily low with just in comparison, a hundred to a thousand of these atoms in a cubic centimeter, there are like 10 to a billion billion billion billion atoms in a cubic centimeter of, say, salt, so these are extraordinarily diffuse clouds and this is an extremely rare process.

If hydrogen makes up most of the galaxy, but it's still a random process that happens surprisingly infrequently, then this will underestimate the total amount of hydrogen there, so you have to say what the probability is of it happening and then multiply. okay, so this gives us a path because it radiates in a specific labeling, so our path is to look for redshift changes within the spiral arms, so if you have a small redshift that means it's okay, okay, so we have an arm. rotating and part of the rotation of that arm is away from us, so we can think of it as following the blue line, but it doesn't directly cross the blue line, it's moving away from us quite sharply and it's basically to the left, but a good rise to the left which is the a arm and it will have a large redshift because it is moving away abruptly now the arm of b will have a less pronounced redshift and therefore will be very, very close to whatever it is.

Although it will have less redshift, the c arm is almost entirely along the line of sight, so we would expect it to have almost no redshift associated with it, and likewise there may also be a blueshift if the arm is approaching us and So we can use this kind of concept to say where the spiral arms are. You can say: well, this cloud has a high redshift. This cloud has a low redshift. This cloud has a lot of redshift. So we can say it must be moving toward us moving away from toward us and toward us and we can map the redshift, the Doppler shift of the line from those 21 centimeters to say oh, this is it and map it into redshift space, so whatwe get is an image that looks a lot like this and this is the image of the Milky Way in 21 centimeter radiation and we get the distance to these various clouds by hook and by crook with the brightness so that it has a brightness associated with the clouds and that gives both the redshift and the 21 centimeter distribution. in space, so there is a kind of pancake-like structure and this contradicts the fact that we do not see a kind of spiral structure, but mostly we see a kind of rings and this is indicative of the fact that these are the places where the hydrogen clouds are, and not necessarily, attached exactly to a spiral structure.

Remember that they go in and out of the spiral structure, so our sun is at the top and that kind of wedge at the bottom is because we can't see through the structure. center of the galaxy with 21 centimeter radiation because the center of the galaxy blocks that light, it's so incredibly dense that the 21 centimeter radiation is absorbed on its way here, so you can see that was a fascinating little thing to pass through the sun and there's like a spiral arm that we're going through that gives us all the very pretty things that we see in the sky, like the Orion Nebula and all the other forms of star formation. regions that we can see and are not that far away, so we can see all kinds of pretty things, that's what the result of that little hook around us means.

Well, here comes the most recent edition of the neutral hydrogen map. Levine and Nadal published this in the journal Science in 2006 and they had a much more robust study of neutral hydrogen and they were able to trace spiral arms, so their tracing showed that they saw what looks like three or four spiral arms depending on how you map it so that let's have these spiral arm structures where the neutral hydrogen map gets closer to the spiral structure once you get a really good map of where things are and the sun is, of course, in the central central region, a the right. at the tip between one and b right in that kind of wedge and we're not in the center uh the wedge is the center is the center of the galaxy, okay, so now we have if we look into space and compare this concept.

We see that it is very familiar, this is the triangle galaxy, a small galaxy very, very close, almost the same distance as Andromeda and m33 has molec as neutral hydrogen, so neutral hydrogen, if it is brighter, it is more dense and molecular clouds are molecular. the clouds trace that and those are where the little ones, so we see that there are all kinds of tracers, so you can see that the molecular clouds are tracers of things and they indicate how many masses of material are in the green dots. Basically, hydrogen maps out how much hydrogen there is and so the more yellow it is, the more blue it is, the less there is and then there are different concentrations in the green dots, so we can say, wow, this also maps out exactly where it is. carbon". monoxide lives so we can use carbon monoxide as a tracer, we can use h1 radiation as a tracer and the sources are superimposed so we have two different images with their image on paper so there is a really fascinating study to show that molecular clouds are Hydrogen can be traced and so if we really look closely at a lot of things and do an extended study with, say, the vla and through nrao, the national radio astronomy observatory and this was the study of things or the study of nearby galaxies h1, are they right?

Aren't they fantastic? So this group led by Walter Brinksblock of Bigell did a study of nearby galaxies and looked at their h1 or their neutral hydrogen at 21 centimeters and we can definitely see the spiral structures, but there are also some confusing things that they just like we saw with the via milky way so the milky way is definitely a spiral galaxy like many of the nearby ones on the top left is m101 in the uh that's the whirlpool galaxy which is very interesting. Notice that in all of these, the nuclei or the centers themselves are heavily depleted in neutral hydrogen, the centers are basically excited by things that are happening in the nucleus, there are some that are actually a bit disordered as well, as you can see as above to the left, right under m101. on the top left there is one that looks all fidget and basically the spiral structure can be bounded by these neutral hydrogen and therefore the neutral hydrogen bounds the spiral arms as well as the carbon monoxide emissions okay , so there are a lot of little review questions left to ask. especially about things like how we know that the spiral arms of the Milky Way, etc., and the spiral arms are part of it, and we have ways of measuring the size scale of the Milky Way, what its spiral structure looks like, and of course In fact, infrared maps show it.

Also and to a certain extent, but it is the radio maps with 21 centimeter radiation, as well as the Doppler shifts of carbon monoxide and other molecular tracers that show us the spiral arms. This time we will continue our exploration of the Milky Way. and we observed the rotation of the milky way through the spiral arms, we saw that they were made up of stars, dust and gas, but there were some, there is something interesting if we study more deeply the nature of the milky way, one of What you might ask is how fast the Milky Way is spinning and that's a very good question, so let's start way back and go all the way back to Newton and the apple tree and that big old object, but let's go way back.

Go back and remember the old thing where Ptolemy said that the Earth was at the center of the universe and that the Earth was everywhere and that all the stars and stuff orbited in spheres and so on and people finally discovered that that didn't work, Mr. Galileo. appeared and showed that Venus had to be orbiting the sun and that the orbit of Mars could only be explained by elliptical orbits, and yet, although Copernicus tried to put things in a new way in 1543, he put the sun at the center, but He still kept almost everything else. He also did this posthumously so he wouldn't be hanged, so that was a big deal, so Copernicus came up with this heliocentric idea mainly because it was easier to do in a sense, you have to admire that, but there's no physical reason, There are many arguments against the movement of the earth, so Copernicus said this in 1543 with very little justification. tico bra came along very shortly after and also noted that things were a little different but created his own version combining the earth in the center but everything else orbiting the sun creates some really interesting orbital paths, maybe the sun and Mars could collide, you know, according to this, but you bro was trying to figure out some things, but finally Kepler said this doesn't work and that's why it stays the way it was.

Using Tico Bras data on the motion of Mars, he discovered that the only way to make everything work was with ellipses and this is what an ellipse is: you have the sun, it gained the focal point, say f1 and it travels along this, huh. this curve is called ellipse and if it is a circle then the two points f1 and f2 do not overlap at all and we call it zero ellipticity if it is a very large ellipticity say 0.4 the foci of the ellipse are far apart and the elliptical path of a planet around the sun is due only to the sun at one of the focuses and Kepler said that this should be the case in 1609, this was one of his laws and then he also determined that when it was close towards the sun it moved more faster than when it was far from the sun and that led him again in 1609 to declare that the period of a planet's orbit squared is equal to its average distance cubed for all planets orbiting the sun and that works very well where The period is measured in years and a is the relative distance between the planets compared to the earth from the sun.

Well, then Newton came along and in 1687 he published the principles and in the principles he detailed exactly how motions work uh uh and What is in the nature of the laws of motion and what was derived using his law of gravity is that it is actually you can completely derive the law from Kepler's laws in a new way where we now say the period squared again, but this time in seconds. is equal to a cube where this time a is in, say, meters and then you have a derivation that is based on the sum of the two masses that attract each other due to gravity m1 and m2 could be the sun and the earth or the sun on Jupiter or whatever and g is Newton'sThe gravitational constant and the 4 pi r squared arise as a result of the geometry of the situation, but in any case we have g as the link to gravity now, if you measure this in years p and years and a and a u, so interestingly, because of these solar units, then the four pi squared g m one or m two uh become one, especially if you measure m in solar units, so it's very interesting that you can declare that that thing It is one in the middle and even this is the true derivation. of it and shows how the orbit of a planet behaves with respect to Newton's laws of gravity, so Newton's laws of gravity were incredibly important and we did not underestimate them because they solved many problems that arose since ancient times, i.e. , how the planets move in the sky, also solves the problems about how everything moves and we will use this to great effect now, so now let's say we use each other.

One of Newton's important results in court is that if you have a central force, something pulls and is mu and what orbits follows a circular path. It's the same as taking a rope and putting something heavy on the end, like a rock, a baseball, or a weight of some kind. You know, you just go to the gym, you tie your lock to the end of a rope and you swing it, people stay away from you, but you can do that, but then the length of the rope holding the lock that you're swinging above your head like some kind of crazy weapon um it's, it would be r would be the swing radius of the strange thing or over your head and then the mass of the thing is the mass of the lock or the object that you have on the end of the rope says which is a rock look, you're going to break the rock with a can of beans or something or you know you're trying to open a can of beans with a rock and then you're swinging it you're swinging the rock over your head you're going to hit it against the can of beans to open it because you want beans, then m would be the mass of the rock at the end of the rope whose length is r and v is the speed with which it rotates in a circle and you square it, so if you take the mass of the rock multiplied by the speed with which it rotates around the circle, you divide it squared by the radius of that circle and there has to be some force pulling on that rope. so it spins around in a circle and that's any force no matter what is doing it some force is causing it to spin in a circle now we could pause it for a second that instead of a rope and an arm then it's actually gravity and so if it's the gravitational force and then we equate the gravitational force with the circular force, we derive this very interesting little equation when we derive this based on if we do if we look at the energies uh we have a factor of two there, but this says that if we have a series of things, then two arise because we are looking at the energy of an incredibly large system of objects humming around, not just one thing humming at a central force, but a series of objects with total mass m humming around a large radius with an average radius r and their velocities are again denoted by v squared, but that also means you're checking the average over a large set of bodies, but if we're just thinking about a thing whizzing by or an orbit, we can get rid of that too , so we have a relationship between the mass pulling on something, the force of gravity, and the speed with which it moves. going around and how far it is from that center of gravity, then again we would find that as Newton's laws dictate, the further it is from a given source of mass, let's say all the mass is located at the center and g is Newton's gravitational constant, so it doesn't change and 2 is just a number that is the average, says the average, so the speed at which you spin if you are orbiting very far from the center of mass is the big m, so Your speed will fall as the distance from it over the square root of the distance, so that's what would be expected if something was orbiting another large body of objects.

Well, Newton's discovery and Kepler's discovery of Newton originally in 1609 were based on data from Tico Brass and then Newton's reform. His ideas mean that things orbit in close circles around the center of mass of the object and the farther you get from the center of mass, all the mass you orbit, the slower you go, so gravity attracts things because of mass, so gravity The force says that every object attracts every other object and mass is what does that and we can also have another very important thing that we are going to touch on now is that light interacts with normal matter if it has any kind . of electrical charge, so if you shakean electron, as we talked about from the beginning, will emit light.

If you shake a proton, it does the same thing. If electrons and protons can absorb light because they have an electrical charge, they can also scatter. I mean, there are interactions that light can have with matter, so if it has mass it reacts with gravity, if it has charge, it will interact with light and we're going to use that now, so it's one of the great things about that we want to talk. We mentioned that the galaxy is rotating, we talked about that extensively when we talked about spiral arms, so what do you see if you measure the rotation of the Milky Way disk using the Doppler effect on the gas side globe? find out that there are motions, you can get the stellar motions from their absorption lines and the ionized gas has different emission lines and those things come from, for example, ionized hydrogen or h2 regions and there are other gases there too, like ionized oxygen , ionized carbon nitrogen. specifically oxygen, neon and carbon are pretty good, but in any case the largest maps we can get are of neutral atomic hydrogen at 21 centimeters or h1 regions or neutral atomic hydrogen neutral hydrogen and therefore if this rotating object, the approaching side will be blue shifted and the receiving side will be red shifted for a spinning bright thing that has specific wavelengths of light that it is emitting at, so if it wasn't spinning everything would be emitting at a wavelength of wave, but because it is rotating, the approaching side will have to rotate the rotating port, the bright part of the root coming towards you will be blueshifted and the receding part of the emitting light will be redshifted , so it will be expanded.

Now we saw earlier that the orbital speed of any object around the The Milky Way and the Milky Way are assembled as a vast collection of stars, gas and dust, but the orbital speed of any object, including the Sun, depends only on the amount of mass between the galactic center. Now there are disturbances due to things that are outside the Sun. but that can be mostly ignored because of the enormous amount of mass that is inside the orbit of the sun and actually the sun orbits the galactic center and the average and if it were smooth and like a disk, then we could, then We can ignore the things outside because it is an average, there are a lot of things on the far side but it is very far away, there are not so many things on the near side, but it is close, so that balances out in this diagram, so we really just care. about things within the orbit of the organ that do not balance, so we can measure what are called rotation curves and then we will plot the rotation speed of the Milky Way as a function of the distance from the center of the galaxy, etc.

It's quite interesting what we're going to find, since it's not a mystery novel, since you know the disk, if the Milky Way rotates around the center of the Milky Way and acts as a central thanks, it's not that fun. Let that happen while you're in the middle of something. These people are fantastic, so in any case I'll leave it alone for fun and enjoyment. The disk rotates around the center of the Milky Way and the inner parts of the galaxy. the disk spins like it's a solid body, but it's not, you know, it's the Milky Way, it's not a solid body at all under any circumstances, so we have that, uh, but then the speed increase for that part , the increase in speed increases with the radius and the period of the orbit is approximately constant because everything rotates at the same rate like a disk, but the external ports rotate differentially, which means that as you go further and further, their speeds are its orbital speed is approximately constant. but the orbital period therefore increases, so the fragments get one thing lagging behind another, so this is a graph of what we mean by what we are describing in the inner parts, uh, everything goes at speed. is not constant because it rotates like a disk, so the outer portions of the inner disk are going faster than the inner portions of the inner disk, so a very, very small increase, that sharp increase right in the center of the galaxy a an approximate distance. near zero at the center of the galaxy is very, very narrow, but then you'll notice that once we get roughly to the sun, it moves up and down a little bit and stays more or less constant all the way, this is interesting because look at that little line that says Keplerian motion let's get into what the heck that is so the outer parts of the speed go about 100 to 200 kilometers per second the sun goes about 220 kilometers per second around the track Milky Way, but this is interesting, there's something about those right diverging curves that we're going to talk about that's really fascinating, so what we can do is look at this particular graph and determine the mass of the Milky Way by doing all the equations that we looked at before, so once the entire galaxy is within one orbit, the velocity should decrease with distance, so if you see the entire galaxy or the entire Milky Way is within one of the orbits, it which means Well basically everything is inside this orbit and there's nothing outside of that orbit so the speed should just slow down because then you're orbiting all the mass and if you're orbiting all the mass it's like a planet orbiting the sun and If it's like a planet orbiting the sun, the farther away you get, the slower you should be orbiting and that's not what you see, that's called Keplerian motion and Keplerian motion is the motion around a body that is much more massive. than the object it is orbiting.

Capillary movement is not seen. which means it's constant and it actually rises at the edge, so let's take a look at this again and then a star or a gas cloud is held in its orbit by the mass inside its orbit, that's the real trick. , so the outside matter only forms a small contribution and at best perturbs the orbit, so we use Newt's law of gravity and the circular orbit to derive the following equation and notice that we got rid of that number two and that's because now we're just looking at a single object orbiting a single mass and that's basically Kepler and Newton coming together for two objects, one object orbiting another object with a circle and that's the root, that's what we're saying here, so the inner mass of the orbit m sub r is equal to the speed at which the circular orbit rotates squared multiplied by the envelope radius divided by Newton's gravitational constant g, so the orbit capillary would be what we see on the right side of the equation we saw before if all the mass of the galaxy was included within the orbit then it should fall as the inverse square root of the square root of the radius of the orbit but it does not, so it should fall but it doesn't the reason it should fall is because if we are looking at all the mass if we know about all the mass then it should fall but it doesn't that is very interesting and shows a rotation curve flat in an extraordinarily large radius about twice the distance of the sun from the center of the Milky Way that it maintains. going up and up and up it stays flat, the only way for that to happen is for the amount of mass in the disk to grow at the same rate as this radius grows, so there's so much mass, there's more mass as you go out. more and more and more and that's the only way for this to happen, so you have to have more mass so that at a given radius there has to be more and more mass for the speed to stay constant, that's what this means so g is a constant gravitational constant, two is that number that comes from averaging a large group of particles m is the total interior mass of the orbit and r is the radius of the orbit, so for v to be constant the mass must be proportional to the radius and therefore if the radius grows, the mass must grow, as an example for the Milky Way and specifically for the Milky Way at eight kiloparsecs.

Hot is where the sun is, the rotation speed is 220 kilometers per second and the interior mass is around 10 billion or around almost 100 billion solar masses, which has a large amount of solar masses. Now there are measurements of gas clouds in the outer disk, twice the distance between the Earth and the Sun and its speed is only actually a little higher, it is 275 kilometers per second and that means that in the inner mass there are almost three or 300 billion solar masses, which is a lot, it is not 10 billion or 100 billion solar masses, but 300 billion solar masses. It's a huge amount 30 actually about 10 30 billion solar masses, wow, I'm reading things wrong, right?

It's 30 billion solar masses 300 million, so 300 billion solar masses wow, I really shouldn't have a Montauk beer before doing everything. These things should not be like this and in any case we measure the rotation speeds in spiral galaxies and they provide us with an excellent way to measure the masses of these galaxies. The point is that you see more mass as you go out, no matter what happens, it's okay. So this brings us to the next three images of the Milky Way, the interior is the rotation of a solid body, which means that the speed is proportional to the distance, remember that it is not actually a solid body, but when you have a lot of mass packed in a very very very small dense area can interact so that it is a solid body the area inside the milky way is not solid but it can behave that way however we should expect that once you are outside you will have rotation Keplerian, which means you have all the mass inside, so it should fall as the inverse of the radius squared, but what you really observe is the galactic rotation at the bottom, which is the speed, it's approximately constant , so there is invisible mass and what do we mean by invisible mass?

Be very careful with this because it is important to clarify what we mean and test it with real data because that is what we do in science: we actually take data and support our concepts with data more specifically to make a measurement than to try to explain it and this . It's the data from Clemens, an astrophysical journal from 1985, and he made a graph of the darkness of the rotation curve found in the carbon monoxide h2 regions, h2 regions, neutral hydrogen regions, and globular clusters, and we found that way beyond the eight kiloparsec radius of the sun that the error that the data points don't go down, they're certainly spread out, but on average they seem pretty constant as we go out, you really have to average it out and then you smooth it out and it definitely doesn't go down. steel. it certainly doesn't fall in Keplerian terms, it certainly goes to the right and stays constant and that's what we mean, this is dark matter, this is evidence for the existence of matter that doesn't emit light and what we mean by that, here's an image. of a galaxy that is very similar to the Milky Way, so let's assume this is the Milky Way for a second position outside of it and we get about eight kiloparsecs, which is about two-thirds of the output, and we find that by looking at the emission radius of h1 neutral region uh regions of neutral hydrogen that still rotates far beyond the bright spots the bright spots are where the stars illuminate the gas and dust and they are stars, they are very bright but then there is neutral hydrogen which is very cold , very faint and emits only in radio light, so that is what is measured in what appears to be the dark region.

However, if the stars follow the light, if it were just the stars following the light, then you should get the red band, but because and even it doesn't matter. However, what should be should be like this, because it is a constant or stays constant, so the measured speed of things that are very, very, very far away is constant, which means that there is more mass that is not seen and which is not in the form of hydrogen, which is really interesting, then the average of those three things together indicates that there must be some kind of invisible mass deep down there.

We start with a very, very dense region which is the rotation of the solid body that is very dense near the core of the galaxy and then it tries to fall a little bit, but then there is something else that starts to act and it is dark matter, something prevents For it to fall as a Keplerian, there is more invisible mass that causes it. spin go at the same speed when it should when it should be slowing down, so if we look at other galaxy rotation curves, not just the Milky Way, we see that this is a very common feature that each of these graphs come from.

In 1991 there are a number of studies of other rotation curves and we see that there are three different curves plotted together and if you add them up, it actually shows you what the dark matter component should be. The dark matter halo is the dotted line. and that dark matter halo apparently gets bigger as it gets further and further away from the signal; in fact, what we find is that it is mainly stars, gas and dust in the center and the luminous components or the discontinuous componentsand dotted components are points. So you have a dash that's like star gas, which is the points and stuff that contributes a certain amount of rotational speed, but then the total rotation is the curve across the top and that's really interesting that you can separate. these things and find out what should be due to the gas, what should be done with the stars and you have to have some component that is neither gas nor stars in order to replicate the real data that is discovered by looking at the rotation curves of other spiral galaxies and here there's another set, this was by vera rubin in 1978 and this is one of the most important areas of his work and it wasn't like that and he also labeled the different types of spiral galaxies as sb sc and types like that.

No matter the type of spiral galaxy, they all have these flat rotation curves, which means that every spiral galaxy has enormous amounts of dark matter and even if we look at the cosmology, that is, the entire universe, we had discovered that approximately of all the energy density of the universe about 23 of all the energy density of the universe is dark matter and this can be seen by many, many different methods and many different methods, like saying how much mass there is compared to light, it's a mass- light, that's what they do. We're measuring and no matter what the mass-to-light ratio is, it says well, how much mass is there compared to how much light there is and every measurement shows that no matter what there is, there is a significant amount of dark matter, so what? what is this darkness? does it matter if it's made of normal things like normal elementary particles which are like protons and neutrons which would be like brown dwarfs jupiter planets black holes neutron stars white dwarfs primordial black holes maybe even just frozen balls of hydrogen that got stuck in space who Even I know there's a lot of things that could be, but this would just be a normal thing and all of these things would be called massive compact halo objects or males, which is a lot of fun to call them, but massive compact halo objects are amazing, they're interesting. possibility, so let's go look for them, so how would you actually look for them if a male passes between Earth and some distant star and as it does so, light that would be going in one direction is gravitationally deflected by that object and redirected towards you?

And then you get what's called gravitational microlensing that briefly magnifies and illuminates the star, so you have, I mean, the background star, so something faint that you can't see passes in front of a star by pure chance and has a perfect alignment so that the light coming from the distant star gets microlensed from the dark nearby object, so people went out looking for these things and they actually found some and this is kind of a better view of what you see, which suddenly the star gets. Brighter in the background and these things have been seen, but the problem is that it takes a huge amount of time and you have to look at a lot of things because it's extremely rare because stars are really small objects and they can be bright. but they are extremely small and you have to get exactly the perfect alignment to do it, so for six years, one particular group studying for a study looking for males looked at millions of stars over half a decade and only found a dozen or so. halo microlensing objects looking directly at the large Magellanic cloud from the small Magellanic cloud and worse than that, because you can say it got brighter by this amount, you can measure the mass and that's a question of general relativity and That allows you to say that it is there, they are small, they are approximately the size of Jupiter up to perhaps the typical white dwarfs, however, it cannot constitute the majority of the mass of the galaxy because if it were forming, if that was forming.

For the most part, there should be a lot of these things, but they just don't occur in large quantities, so males can't be the amount of things, which means that normal baryonic matter is pretty much excluded, so what? what do we think they are? If you go hunting and check out what the best things are and there you have it, now you're starting to talk a little funny, okay? What is dark matter right now? Internet wisdom says as of summer 2018. there are basically five main things and none of them are normal matter, they are all like these crazy things, one of them are weak or weakly interacting massive particles, so they would be normal matter that doesn't interact with light and doesn't interact with the weak nuclear force, they just don't do anything like a neutrino, but maybe something else, but no one really knows what they are, so what are they?

They are weak, no one knows and then, well, the axions are even weaker. It's fun to talk about it because quantum mechanics has some very strange things about quarks, because exactly how do quarks form to come together to form protons and neutrons, so you come up with a theoretical particle created from a thin shell . air to solve an unsolved problem related to quarks, then maybe axions exist and that's kind of like, well, we don't know, but someone has to keep them, then the protons together and then the quarks together and in groups of three , that's what quarks do, so what's an axion well, it's useful for that thing, but let's go look for it, maybe it's dark matter, ooh, so people are looking for axions.

Next up is ultralight scalar dark matter, which is a really strange thing if it were similar to axions, except that its mass is so incredibly small, so incredibly small that its de Broglie wavelength would be thousands of light years and then you have another thing that's like we know three versions of neutrinos and we talk about the sun, there were neutrinos that came in the sun and and uh and exploding supernovae that neutrinos come out of the sun quickly and they're produced by nuclear reactions and as they travel through the space oscillate between the three different flavors maybe there are more flavors maybe there are more flavors maybe there is an alternative group of neutrinos that are much more massive than the normal triple number of neutrinos and would only interact with matter when they change flavor and not while they are actually a particular type of nutrient neutrino, uh sterile, so you might have sterile nitro neutrinos, uh, big bear. bear and mama bear papa bear baby bear right, whatever they are, they don't interact with anything until they wonder where the porridge is and then they interact like crazy, neutrinos so sterile, then you have dark matter that interacts on its own and currently that's the one that is.

The most attractive thing for researchers now is to take all the normal matter that we have in the universe and turn it into dark matter and you have many different dark matter particles that simply don't interact with any normal matter except through gravity, and that's how it is. kind of like Star Trek's mirror world, but with dark matter it's a strange thing, this is all extraordinarily speculative and this is a result of the fact that dark matter is an observational thing and many ad hoc explanations are created in order. to explain something is absolutely necessary because that's why a lot of people like to say well, let's modify Newton's law of gravity because that's a lot simpler than trying to find things that sound weird because a lot of people look at this and say this is garbage, just Let's modify the gravity, it will be easier to make a new equation, but there are many things that say that the modified Newtonian gra theories of Newtonian gravity do not work, in fact, there are many predictions of modified mond things that do work. it doesn't work so that's a throwaway thing and that's very negative, it was quite popular about a decade ago but it's not cool anymore so dark matter halos what is the source of this extra mass if it's not stars or gas , galaxies have these extraordinarily extended Halos: they could contain up to 90 percent of the galaxy map and are even more extended than the starlight component;

They can even reach 200 kiloparsecs or more, so these incredible dark matter halos surround individual galaxies and dark matter. just because it doesn't interact with anything it can't collapse, it can't lose its energy and fall under the influence of gravity, so since it's hot in a way, it must stay hot and by hot I mean it has big orbits where moves quickly. and it can't collapse into clumps, so dark matter, light and normal matter can emit their energy and collapse and fall into structures like galaxies, but dark matter can't, so it stays spread out, so We find that the Celtic astronomer Fritz Wiki in 1933 also went to look at galaxy clusters and discovered that they were moving extraordinarily fast, perhaps thousands of kilometers per second relative to the center of the cluster, and that is much greater than the escape velocity.

If you add up all the stars and all the galaxy clusters and I just looked at the clusters and he said there must be some dark matter component and in fact Fritz Wiki was the one who created what he called dark matter, but actually in German it is material dunkles and that's great. What needs to be said is that there must be extra gravity, the x, and therefore, if in the galaxy clusters there must be almost one hundred percent of it is dark matter, which is really surprising, okay, however , we know that around 10 millionThere is an enormous amount of X-ray emitting gas in galaxy clusters and, in fact, this gas is so hot that it moves so fast that without dark matter the gas would dissipate extraordinarily quickly and we can see the evidence of this dark matter due to gravitational lensing. of background galaxies by the clusters, so this with the galaxy rotation curves means that there is a lot that we don't see, so let's look at that X-ray emitting gas and here's something from the Chandra X-ray observatory on the right , is an optical image.

The image of the Perseus galaxy cluster taken from the Blackbird observatory on the left is a Chandra observation from the Chandra X-ray observatory for the observation of the core of the Perseus cluster and we see this enormous amount of X-ray emission on the left, which is the The center of it is now a lot of gas, in fact it makes up more hydrogen than all the galaxies combined, but it's still too hot and moving too fast to stay in this group. There are two. It's either moving too hot for its light mass level or it's not massive enough to hold together, so there must be dark matter and then we're going to look at galaxy clusters and gravitational lensing and if you look closely at these images you see these arc-shaped structures and these arc-shaped structures are formed as a result of the gravitational lensing of dark mass when light from distant galaxies passes through the gravitational field of the galaxy cluster and this is a cluster belt of galaxies in 1689 and was taken with the advanced cameras of the Hubble Space Telescope for studies and there is a version from the Chandra X-ray observatory that looks at Abell 383, another galaxy cluster and we can even see X-ray versions of these things, for so the purple glow is the outline of X-ray emission, so we have an X-ray emission on top of what appears to be an image taken by non which are that the images of the galaxy are from the digital study of the palomar sky of the rest, so there is a huge amount of hot gas between clusters, but it is too hot and does not have enough mass to stay together at its temperature and there is also no enough mass to hold the galaxies in place and this is called the fat one, which is another galaxy cluster that emits X-rays and we have other galaxy emitting clusters like ga 351.03.

Well, very interesting names in X-rays, but we also have this. I think a lot of the gas is primordial, meaning it dates back to the early days of the universe, 13.6 billion years ago, 13.4 billion years ago, but there's not enough in it to become the clumps themselves. galaxies, so how do we know? map this either way and yes we can and this is how we do it when you go to see an ophthalmologist, let's say an ophthalmologist or even an optometrist and you want to buy new glasses, what they do is they put in a series of lenses and they do images and they look at images on the surface of their retina and they focus images on their retina or they basically specifically focus their view of their retina in an infinite review and they are also very close because they want to see what that image will do.

It seems like they want to measure the distortion caused by the lenses in your eye, so what you can do is do it in reverse and by the simple process of you know, an optometrist would say, "Oh, so we have a distortion of the background galaxies ". They're distorted into little arcs, so what do you have to have to distort them? What is the shape of the lens? Which means you have something that is causing a change in the lens. So you can think of this as gravitational lensing and in fact we think of ray tracing and we canWe trace the light back to its source and then attribute a certain amount of gravitational pull to what a dark matter map demands, so this purple glow here no longer exists.

The map is due to the the cluster, so this is a huge rich galaxy cluster as you can see abell 1689, which is this huge group of galaxies, our milky way is not even close to such a group, it would be nice, we would have a very different view of the sky nocturnal if we had it, but in any case all these galaxies are in this big group and something holds it together that something is a huge amount of dark matter and that dark matter is shown in purple but it is derived through computer simulations and then the derivation of that computer simulation is superimposed on top of This image from the Hubble Space Telescope is the same for this image, which is the classic bullet cluster and what we are going to have is that there are three separate images superimposed, we have one that is an image visible which is the type of stars and galaxies. type of image so we have the red image and that is the Chandra X-ray observatory which shows where the hot X-ray gas is from the X-ray observer and then we have the dark matter map which is blue so X gas when these two galaxy clusters intersect, the even as the galaxy clusters pass by the x-ray gas is left behind and then the clusters move forward without the x-ray gas and here is a better image of that version so once again this is a light image visible overlaid with red.

Chandra one through the other and this is a similar situation to the musket ball cluster where the red is again the gas that emits a series of series of ball-shaped structures and I guess that works for anyone and we see where the gas was left when the two galaxy clusters collided and moved forward and we have another one where the dark matter is mapped in blue, the X-ray emission. is mapped in red and is similar to the bullet group, but has now been mixed up a bit as the two groups have moved left and right as they crossed each other billions of years ago or took billions of years.

It takes years for them to cross that vast gulf of space and the galaxy and the dark matter just keep them together, they don't mix, but the gas between clusters stays behind because it interacts with itself and gravitates and collides like any gas cloud would. So gravitational lensing happens everywhere and we can see that this gravitational lensing also provides another indirect measure and here are examples of gravitational lensing here and here with very high redshift galaxies and in fact we see them in many things different. and here's an extraordinarily striking view of a galaxy cluster, where we see galaxies lensed in the distance that look like arcs and that's one of the most striking appearances we've seen a couple of times, but this is around 1689. once again and let's zoom in on some of them and you know part of the reason for showing these images is that, wow, you don't get to see these things very often, I'm sure, and this is part of the reason. is that these images need to be seen more publicly and most of them I really like this one a galaxy cluster a bell 370 almost everything in this the blurry objects are all galaxies but on the right in the upper right corner there is a very distorted image image of the galaxy and that image of the galaxy is projected across the sky in four different places in two different places and we see that the top left and the top left and the bottom right are exactly the same galaxy but through the galaxy and in fact we see the The top left is to me the most interesting of the group because the image of the galaxy that is behind all the other galaxy clusters, the light from that distant galaxy, which is very, very bright, was redirected perfectly to our eyes because it is such that and Well, not perfectly because everything was blurred and everything, so we have multiple images of that distant galaxy being blurred by the foreground galaxy and here is the last one that moves away and this one is at 370 um within uh, so this galaxy cluster is about 5 billion light years away and we're looking at something extraordinarily distant that's much further away than the galaxy cluster.

This is somewhat far from the galaxy cluster. This image, but the spotted tadpole is much further away than the galaxy cluster. It could be 5 or 10 billion light years away even further, which means we're looking at something that's very, very distant. If it's 10 billion light years away, then we're looking at something that's 5 billion years older than all the other globes or five. billion years younger than all the galaxy clusters in the foreground cluster and we can see some evidence of the earliest nature of a galaxy in this that they are very strange, small and bright in their lines.

We see huge bright starry features in this galaxy, but it has multiple images that have all been tadpole-shaped due to the distribution of dark matter in the galaxy cluster altering the path of light, so it's a huge huge overview of the nature of dark matter in our Milky Way and beyond and you know I could have left a lot of that stuff for much later in the course, but I thought dark matter is a really interesting topic that I take some time with. extra time. and as a result I have a lot more review questions for you to review and take a look at, there are a ton, but you know you can make them however you want, but what we really think is that dark matter really exists and the hard thing about matter dark matter is that all the things that could be part of dark matter are very strange things, they are not normal matter in any sense of the word, they cannot be necessary, most of the matter cannot be normal. atoms or molecules or planets or dwight dwarfs or whatever because they just aren't numerous enough and people have been looking for them, so they must be some weird weird subatomic particle predicted by the standard model or the extended model of particle physics that They could be it's there to solve some crazy problem in quantum mechanics or something, so there are a number of active searches for dark matter candidates and I'll post some of those links around the web on the YouTube channel, links below, so we really believe that most of the matter in the universe is actually dark matter and not normal matter that's the real trick with this so we'll see what that means when we get to cosmology we'll continue with the milky way to our last part on the nature of the Milky Way itself, but we'll start by first looking at the Andromeda Galaxy, which is the closest large, bright galaxy to the Milky Way and let's say we take an optical view like the one we see to the left of the entire Milky Way. galaxy and instead we focus only on the central regions and look in the X-ray area, so we use the Chandra X-ray observatory and if we zoom in and look with a combined X-ray optical view of the center, we find that The blue glow that is due to X-ray emission has a number of very, very bright point sources, as well as a lot of diffuse emissions coming from the center.

So what exactly is this bright X-ray source at the center of Andromeda? galaxy, what is it? Well, we can ask ourselves the same thing. What is in the heart of the Milky Way? And this is laser tracking. An artificial star created by an observatory in the VLT by the European Southern Observatory. Interesting photos taken by people under the deep night sky and that laser is pointed at the sky to make an artificial star to use adaptive optics so you can look at the deepest center of the Milky Way, so adaptive optics are essential to do that.

We talked in a much, much, much earlier episode and that allows us to see that view to get a better view of something obscured by clouds, so remember that dust is completely blocking our optical view of the center of the Milky Way, so We have to use infrared or radio or . so that doesn't count, we want to see what's in our backyard if we combine a series of observatory images of a large area of ​​the sky using NASA's large observatories, specifically the Spitzer infrared telescope, Hubble in the near infrared, it's say, just a little. a little bit longer than red, about 10,000 angstroms or so and the Chandra lightning and dust and gas going at full speed, so it's a turbulent and violent downtown, but let's see what we can determine what the heck is really going on there if we look specifically at the X-rays from the Chandra X-ray observatory and do.

If we look at the inner 400 by 900 light years, we see that there are a lot of point sources of X-rays and they are probably neutron stars and black holes, but there are also dead centers, some bright, large, very, very diffuse, especially one of the most intense. It's actually towards the center and that's really what we're going to be looking for, we can actually also look at the same approximate size scale but tilted at an angle at wavelengths of 90 centimeters almost a meter long in radio emission and we also see extraordinarily bright things happening, we also see circular spherical features.

Also, those are radiolobes due to the expanding gases of supernovae. We also see snake-like structures that are very similar to the prominences or the sun, except that they now extend for many, many, many light years, if we then get even closer. more with higher resolution using the Chandra X-ray observatory inside one hundred per 100 light years we see diffuse the center but it is again that incredibly hot diffuse gas that is millions of kelvins that is somehow heating up to extraordinary temperatures near the center of the Milky Way and this shows again that where Sagittarius, a star, is actually the center of where this bright radio emission comes from.

From that we saw in the other image, so this X-ray emission is in diffuse cloud forms as well as in many point sources, and those cloud forms are superheated gas that has been heated to millions of degrees and is emitting x. -rays and the inner region has many very interesting bits, there are non-thermal filaments and radial arcs and the radio image shows that there is a huge magnetic field that is causing the electrons and protons to spiral at a very high speed, almost the speed of light. all along all through these magnetic fields that cause these filamentary structures there are also supernova remnants labeled s and r and those supernova remnants are the result of a star that exploded, but there seems to be a large amount of them, so for For some strange reason there are a lot of supernovas that happen there and the dimensional scale is about a thousand light years across that dashed line from the top left to the bottom right in any event we have, we can zoom in on the radio views, we can take the VLA image, we can get closer and we look closer and closer and as we get closer and closer to these images we see finer and finer details of the radio interruption, but each of these has a wavelength different, that's the real trick is that we don't really have.

We can't do this with the same wavelength because you get different resolutions at different wavelengths, so we have to change our observing wavelength to get closer and closer, the last being the shortest wavelength as infrared. , so this is another is another radio image also vla that demonstrates that there are huge radial arcs that are the magnetic field of that region that accelerates the material and has synchrotron emission. I mean that heArc-like glow is the result of electrons spiraling at nearly the speed of light. in a very strong magnetic field and as they do so they emit radio light and there is again another radio arc image just to show you that and it looks very interesting, it has a kind of tenuous structure as the magnetic field extends for hundreds of years light. from the center, but we have that bright source that is deep in the center and what exactly is happening deep in the center.

Well, let's take an even closer look at about three 3.6 centimeter radius six lights. The interior of 20 by 20 light years shows gaseous material. that's actually spiraling around at extraordinary speeds of hundreds of kilometers per second or thousands of kilometers per second, but there's still an incredibly bright point source right in the center and there are these arms of gas that are spinning around it, but look at that bright source and we. We're going to get even closer to 3.6 centimeters and as we get closer we see the arms of gas moving extraordinarily fast, but there's still that incredibly bright point source right in the center.

What is happening there, well, this did not stop. this was the result of a massive study done by andrea getz who i put in the picture in the bottom left frame she heads a group called the galactic center group at ucla and they went and used the keck observatory in adaptive optics mode for De fact we visualize the center of the Milky Way in the near infrared and we can see the difference at the Keck observatory with the adaptive optics on and the adaptive optics off and we can definitely see that you get a much higher resolution with the adaptive optics on and the advantage It is the center of the dynamic structure of all these stars now all these objects are stars that are seen to be orbiting somewhat downwards, we see an incredible number of stars within the center of the milky way, so inside two light years for two light years, this is one of the images of the guest group, uh, in infrared we see that there are thousands of stars within a volume of space where near the sun there are only two or three stars because even if you were to go diagonally through this image to get from the sun to alpha centauri which is within our reach, but down at the center of the milky way there are literally thousands of stars packed into the inner two light years around the dynamic center of it all, but still there is no distinctive bright object directly in the center there is some bright gas and bright dust that is heating there are some bright star-like structures but there is nothing directly bright deep in the center in infrared so now I What we can do is this is what gets his group he's been doing for the last two 20 years, the last two decades, he's been leading a team at Ucla and it's called the gets group and that's the galactic center study group and they have actually been mapping, using the keck observatory and other observatories in order. to discover the actual movements of the stars in the center and what we see is that these things are spinning over the course of about 15 or 20 years and to have them move in this exact way according to Newton's law of gravity, which is down there must be 4 million times the mass of the sun for them to move that fast on the size scales we're looking at, which correspond to huge orbital parameters, so there's something big there and this is a map of what we actually saw in the previous uh little video that they did, so here is your work with your team at ucla and we see that one of the most important is that there star s02 that has that reddish pink ellipse in the center that is very, very small one, which has an orbital period of about 20 years and so every 20 years it gets closer to whatever is in the center and they can actually map these things and see where they can actually track the movements, so here's another interesting piece of work the group have done and what's incredibly important is what's going to happen and get us a little bit closer, so now that it's zoomed in, I can see a specific star that's actually moving.

Sorry, this could be the work of the Max Planck Institute. On the other hand, we will also see some of his work, so there is another star, the one that moves very strongly. and that has an orbit of 20 years, so they are going to bring the box closer and the box is going to come closer and we see that it is tracing the orbit of a star that revolves around that plus and that plus is the dynamic center of the milky way and For that star to move that way, something has to have four million times the mass of the sun according to Kepler's laws.

Now the surprising thing is that as of July 2018, Andrea gets her group after 20 years of studying this. been their race is that they are finally waiting for this star 0 number 2 to pass within a thousand times the size of whatever this thing is and I just revealed that there is a supermassive black hole in the center of the milky way and what they do is to check how its orbit changes and there are predictions about what the orbital appearance will be if gravity is governed by Newton's laws or if it is governed by Einstein's theory of relative general relativity and they are completely different and can actually have a degree of measurement so precise that they can distinguish between the two theories because the orbit will be a different shape, the speeds will be different, they will accelerate and decelerate differently there, so you can really distinguish between the two and in September 2018 they will publish something interesting where they analyze exactly how, uh, how this thing has changed, so they made their May observations, they made their April observation. and now that it's coming out of the gravity well with it being so close to that supermassive black hole that you're going to be able to make out, you're going to be able to say something important about the nature of space-time near a supermassive black hole, so stay tuned for that. future and this comes from their website just go to the ucla galactic center group with andrea getz g-h-a-z okay so this is also a study done by Max Plankton led by Dr.

Genzel and assume that something is about 3 .7 or 4 million solar masses, let's just call it four because what the heck and we can see that this is the orbit of that particular star, the dimension of this orbit is a few light days, so this thing rotates in 20 years where remember that the Voyager spacecraft, if we look at how far Voyager is currently from the Sun, it's about 17 or 18 light hours away, almost a light day away, so about 1 8 of the length of the long axis of this ellipse you see This is about how far Voyager has gotten from Earth, but it has taken Voyager since 1975 to get there, taking 40 years to get there. distance between Earth and Voyager, which is 100 astronomical units, so it would be a thousand astronomical units away, so this is a very long orbit, but it makes that orbit instead of taking hundreds or thousands of years to do it. 20.

So something is pulling on it very strongly and that something is a supermassive black hole in the center of the Milky Way that is approximately four million solar masses and this is what they found between 1992 and 2013, their observations were modeled by uh could be modeled. according to general relativity, which is which is which is the curve on the right side, then the orbit is what has been done, the ucl group, the ucla group by andrea goetz has in the advantages of the red star and the group of max planck is in the blue, but what we see is that they can both fit into the general relativistic group, but now they're actually going to measure the acceleration and deceleration over the course of the next few weeks and months as it passes by the supermassive black hole. called sagittarius, a star or sgr. a star, so here it is, this is what those orbits look like and we can definitely see that it's a nifty little view that, and the nice thing about the so2 star is that the orbit itself is almost expensive Look, there are other stars that do it dice3 or that other one that launches into that really tight orbit on the green line, which is its orbit, it's not so frontal that we can't see a full orbit, however, so2 is almost a face on an ellipse. and its orientation is perfect for making these types of measurements and knowing the mass because we can see the exact size of the ellipse, the orientation can be easily derived due to the displacement of the center from the tap from the focus from the apparent appearance of that so that the andrea getz group can make a very precise measurement of the exact size and also the maximum watch group and this is what they have been doing for the last 20 years so what we have learned over time and it is something really Fascinating is that the stars move very quickly near the black near something that does not emit light there is a huge amount of radio emission but still it is not visible there is no visible object there is no bright star and there is nothing if there were one 4 million solar mass star there or a 4 million solar mass star cluster, it would be quite bright, it would be incredibly bright and it's not there, we see a bunch of stars in the center of the central galactic center. but we don't see a bright object that overwhelms everything, so there is something there that weighs 4 million solar masses that emits almost no light but affects its environment in surprising ways and that thing is a supermassive black hole and we already talked about black holes before , but this is the source of all those very interesting phenomena the stars that move very quickly the magnetic fields the emission of It is actually physically very small but it has a huge influence on its surroundings in the area, so the galactic center has a stellar density, there are more stars there, millions of times more stars closer in volume than there are near the Earth , there is a huge ring of molecular gas approximately 400 parsecs wide. emits in uh in radio light as well as infrared are incredibly strong magnetic fields that permeate the entire area there is a huge rotating ring of gas and dust that you can see that is a few parsecs wide around the center and there is an incredibly strong source of X-rays in the center, that means there is a supermassive black hole in the center of our Milky Way and that's about 3.7 solar masses, but what we saw at the beginning or at the beginning was something in the center of Andromeda and the Andromeda galaxy appears to have a supermassive black hole that is a hundred million solar masses, this is really something surprising, so the surprising thing is that there are many ways to look for supermassive black holes or just black holes in general, but it is one of the best evidence . for the existence of a black hole or even that it exists it is in the center of the milky way and we can search for cygnus x1 we can search for many other things, but just the fact that we can see so many details in this object that is very, very, very clear in comparison allows us to really understand the nature of black holes and general relativity in our own backyard, so we will understand it now that we have seen that we are We will venture into the rest of the cosmos and see what the rest of the others look like. galaxies and that's what's coming next, so see you soon.