The Origin of the Elements

Tonight we have Ed Murphy from UVA with us. He's an astronomy professor out there. He is also in charge of the evening events at McCormick Observatory and, if you visit his website, he will notice that two nights a month they have public nights at McCormick Observatory at UVA and it is a good opportunity. So please join me in welcoming Ed Murphy. Thank you so much. And I would really like to thank JLab for inviting me here tonight. It's great that the Hampton area has such a fantastic science education and outreach group like you have here at JLab because they just do wonderful things.

I was here last summer working with them on a teacher workshop, so there's a lot going on here that's really cool. But I thought what we would talk about tonight is the

origin

of the

elements

and the fact that everything around us is made of atoms. We all learn in elementary school that the atom is a kind of building block of matter. You are made of atoms. I am made of atoms. This table up here is made of atoms. And the question is "Where do these atoms come from?" And in particular, there are a couple of atoms that I think are especially important to talk about.

And one of them is gold. We all carry a little piece of gold wherever we go. Or, rather, the vast majority of us. So for many of us, it's our rings that we wear. If you're wearing a pair of gold-framed glasses, they probably have a bit of gold plating. You may be wearing an earring that has some gold in it. If you are not convinced about wearing gold jewelry, you may want to wear gold if you carry a cell phone because gold is an excellent conductor of electricity and is found in almost all modern electronic devices.

And then you carry some gold there. If you are not carrying jewelry or a cell phone, you may not be carrying gold, but you are likely carrying some mercury. If you breathed air last day, and I see everyone here has, then you have some mercury in you. It comes, in many cases, from coal-fired power plants further west from us. And, as we burn that coal, it releases mercury into the air, which ends up landing in our food and our water. So where do these heavy

elements

come from on the periodic table? So what we're going to talk about tonight is the

origin

of gold, element number 79.

Right next to it is element number 80, mercury, on the periodic table. But I also want to talk about some elements that are a little more interesting to us personally. And those are the elements you are made of. And the first question I have for the audience tonight is: "What are you primarily made of?" Water. Most people are made primarily of water. The vast majority of our body mass is water. What element is water mainly composed of? You think of hydrogen. Then you will remember that the chemical formula of water is H2O. They are two hydrogen atoms and one oxygen atom.

So for most people, we would say that we are made primarily of hydrogen, because, if we are primarily water and there are two hydrogen atoms and one oxygen atom in water, we would say that we are made primarily of hydrogen. But the thing is that astronomers, and indeed most scientists, think a little differently because we think about the amount of mass it takes to form an element and not the number of atoms of that element. So if you look at those periodic tables you might have picked up on the way in, you'll notice that hydrogen has an atomic mass of one.

So those two hydrogen atoms in water have a total atomic mass of two. Oxygen, on the other hand, has an atomic mass of sixteen. And therefore, by mass, you are composed primarily of oxygen. What's up with this room? What is this room mainly made of? What do you think is the most common element in this room? It's a little more difficult. But, if you think about the walls for a second, those are probably some of the biggest things here. I assume the walls are mostly made of concrete. Concrete is largely made of sand. Sand is silicon dioxide.

And silicon dioxide is one silicon atom and two oxygen atoms. The silicon atom has a mass of 28. The two oxygen atoms have a mass of 32. So it turns out that the walls are also made mostly of oxygen. It turns out that oxygen is probably the most common element in this room en masse. Without a doubt it is the most common element in you. And many people find this surprising. We often think, when we study astronomy, that oxygen is one of the rarest things that exist. But in fact, oxygen makes up about 65% of the body. After oxygen, carbon is the next most important element.

And then hydrogen, just under 10%. Nitrogen, about 3%. And then all those other things represent just a couple of percent down here. Calcium, phosphorus, potassium. And they make up only a small percentage of the human body. So, you are made primarily of oxygen. Something we often consider rare, but in fact, as we will discover, is quite common in the universe. Astronomers not only know what you're made of, but we can also tell what things in space are made of. Even things we have never visited and things we probably will never visit. Take this star cluster as an example.

This is a newborn star cluster. Young, hot, blue stars, right here, just born. This is the cloud of dust and gas from which they were born. Astronomers can analyze the light from these stars and the gas around them, and we can tell what they are made of. Although with our current technology it would take us millions of years to reach these stars. And the thing is, even if we do, even if our children develop spaceships that can go ten times faster than our current spacecraft, it will still take them millions of years to reach this star cluster.

If your grandchildren do 100 times better than that, it will still take them hundreds of thousands of years to reach this star cluster. We will never, not even in the next thousand years or more, sample these stars. However, astronomers can tell exactly what these stars are made of. And in fact, they are 74% hydrogen, 25% helium and everything else is 1%. So, all the oxygen, the nitrogen, the carbon, the iron, the nickel, the gold. All. All of those elements are 1% and stars are 74% hydrogen, 25% helium. Turns out that's the same composition as our sun. 74% hydrogen, 25% helium and 1% everything else. And astronomers determine what distant objects are made of by breaking down the light from those objects into their component colors.

We know that if you take white light and pass it through a prism, you get a whole spectrum of colors. What scientists discovered back in the 19th century is that if you take special elements, particular elements, and excite them to glow, either by heating electricity or passing electricity through them, when they glow, they emit only colors of certain light. So if you make hydrogen glow, for example, the hydrogen emits a particular color of red light, a particular color of blue-green light, and two blue lines. Helium. If helium is excited to glow, it gives off particular colors of light.

And you will notice that the colors that helium emits are different from the colors that hydrogen emits. You can think of them as a fingerprint or a barcode. Each element of the universe shines with its own particular set of colors. And all hydrogen atoms, regardless of whether they are on Earth or in deep space, glow these colors. But only hydrogen shines with those particular colors. So when we are faced with a distant object and want to know what it is made of, we can take the light from that object, break it down into its component colors, analyze the specific colors we see, and use them to determine what the object is made of.

We can do the same with our sun, for example. This is a spectrum of our sun. And when we break down the sunlight, we get the full rainbow here. But now we see that particular colors are missing from the spectrum. And those particular colors that are missing correspond to hy- In this case, this red line here, corresponds to hydrogen atoms in the atmosphere of our sun that are absorbing that particular color of red light. That red color that is being absorbed by the atmosphere of our sun is the same red color that is there. This blue-green color here is the same as the one there.

So these lines come from hydrogen atoms in our sun. And astronomers can determine the composition of objects using a variety of different telescopes. Telescopes like the Large Binocular Telescope located on Mount Graham, a couple of hours northeast of Tucson, Arizona. I mention this image just to let people know that the University of Virginia is a partner in this telescope. It is the largest telescope in the world mounted on a single mount. And the people of Virginia own a part of this telescope. And the University of Virginia, in particular, has built a spectrograph, one of those devices to break down light into its component colors, which is used in that telescope.

But we can also use other telescopes such as the Hubble Space Telescope. And in particular, one that I've worked on is the Far Ultraviolet Spectroscopic Explorer. The name tells you everything you need to know. Look at ultraviolet light, not the visible light we can see with our eyes. Spectroscopic means that it divides light into its component colors. And Explorer means it was a small satellite mission, not one of the big flagship missions like Hubble, but a smaller telescope mission. Not only can we use visible light, infrared light, and ultraviolet light, we can even use radio waves to determine what things are made of.

Just a few hours west of Charlottesville, so probably about 5 hours west of us here in Green Bank, West Virginia, is the largest fully steerable radio telescope in the world. And this radio telescope picks up radio waves from space and different molecules in space emit different radio waves, including water. And we can use radio telescopes like this one to track the distribution of different atoms in our galaxy and in the universe. So let's go back to the periodic table of elements for a few minutes. I should mention that I took this periodic table of elements from the JLab website.

There's a really nice It's Elemental page where you can click on each of the elements and get information about each of the elements. So you should visit that website after tonight's talk. But there are a couple of things I'd like to tell you about the periodic table of elements and... Perhaps the most surprising thing I can tell you about the periodic table of elements, and it's a really extraordinary thing to be able to say. The periodic table of elements is not just the elements we know. They are all the elements from which things can be made in the universe.

There is nothing that can be made that is not on our periodic table of elements. And it's amazing to be able to say that if there is something in the universe and it's made of matter, it's on our periodic table of elements. So it's worth explaining why this is the case for a few seconds, to try to justify that statement. This often comes up, for example, when people talk about UFOs. They say that if a UFO landed, we would know it was not from this world because it would be made of material that we do not have in our periodic table of elements.

And the answer is, if it is made of matter, it will be made of material that is on our periodic table. Has to. Because it turns out that nothing is missing from our periodic table. The first thing we should know about the periodic table is that each element has a number above it. So hydrogen is element number one. Helium is two. Lithium is three. Beryllium, four. Boro, five. Again and again going up the periodic table of elements. That number tells us the number of protons inside the nucleus. There are three basic components of matter. There are protons, with a positive charge.

Neutrons, unchanged. And they are contained in the nucleus of the atom. And then there are the negatively charged electrons that orbit the atom. And they orbit quite far from the nucleus. The core is remarkably small. The best way to think of the nucleus is that it is about the size of a pea in the middle of a football field. So if you imagine the size of a football field with the electrons here representing the edge of the football field, then the nucleus is about the size of a pea in the center of the football field. So the nucleus is very small compared to the distribution of atoms.

Different elements in the periodic table have different numbers of protons. So, we mentioned before that hydrogen was element one and it is element one because, down in its nucleus, deep in the center, it has a positively charged proton. And what that means is that it also has a negatively charged electron orbiting around it. Helium is element number two on the periodic table. Its atomic number is two because it has two positively charged protons.Normal helium also has two neutrons. And so it has an atomic mass of four, but its atomic number is two. Carbon has atomic number six because there are six positively charged protons down there.

So if we look back at the periodic table of elements, the element number tells us the number of protons. So, we have a proton. Two protons. Three protons. Four. Five six seven eight. As you go through the periodic table of elements, you will see that there are no missing numbers on the periodic table. So, for example, let me choose this. Forty-one, forty-two, forty-three, forty-four, forty-five. Each number is represented on the periodic table. And the thing about protons is that they don't come in halves. Forty-five and a half protons do not exist. You either have forty-four protons or forty-five protons.

You don't have something in between. And if you lookLook carefully at the periodic table and you will see that there is no missing number in the table. We have discovered all the elements in the periodic table. I'll get to these big ones in a second. Now, it's important to say that that hasn't always been true. When the periodic table was invented, there were holes in it. This is the father of our modern periodic table, Mendeleev. And Mendeleev, when he compiled the first version of the periodic table, organized it in rows instead of columns. But the important thing is, as you'll notice, there were some question marks here.

He understood that the periodic table told us something about the way matter is composed. And it wasn't just a convenient way for humans to organize all the different items. He was actually telling us something about the structure of the atom. And what he discovered were holes in the periodic table. And these holes corresponded to elements that had not yet been discovered. And when I told you about the elements forty-two, forty-three and forty-four here on the periodic table, I didn't choose them at random. Element number forty-three is a beautiful example. It's called technetium. Until the 1920s, there used to be a hole in our periodic table.

There was no technetium in the periodic table. And that's because there is no stable version of technetium. Each version of technetium is radioactive and decays into something else. If you build, we can do it today. We can produce technetium today. If you do it today... The most common version disappears after a few days. It is highly radioactive. And it disappears after a few days. If you look at the rocks on the surface of the Earth, which are hundreds of millions or billions of years old, if there was technetium there, it decayed and disappeared a long time ago.

So there was a hole in the periodic table. But today we can take element forty-two, molybdenum, and bombard it with neutrons, and we can produce element forty-three, technetium. So much so that it is possible that some of you have ingested or been injected with technetium. It is commonly used in medical imaging today. If you have ever had a radioactive tracer test, approximately 80% of all radioactive tracer tests today are performed with technetium. That's why we use it today in medical tests. But, for a long time, there was a hole here in the periodic table. So today there are no more gaps.

Except, one could argue, well, what about these big elements? Scientists always seem to be creating new elements, all the way up to ununoctium here on the periodic table. The thing about these big, giant elements down here is that we create them by breaking up smaller elements. And, when we crush them, for a small fraction of a second, they come together and form one of these larger, more massive elements. But all of these elements are so unstable that the vast majority of them disintegrate in less than a second. So even if you create one of these atoms in one of these atom smashers, it will disappear in less than a second.

It disappears because it is so unstable that it breaks down again into smaller atoms. Even the most stable of these elements is not stable for much more than a second in time. So you can't really build a spaceship with ununoctium or any of these heavy elements because they decay in a second. Therefore, I would say that nothing in the universe can be created from any of this. Surprisingly, our periodic table contains everything in the universe. If it is made up of protons, neutrons and electrons, it is on our periodic table. And, if a UFO ever lands and explains all those things in astronomy that I'd love to know and that we haven't discovered yet, I can guarantee that, if it's made of protons, neutrons, and electrons, it's in our path. periodic table.

So that begs the question: where did those protons, neutrons and electrons come from in the first place? The stuff that we atoms are made of is made of those fundamental building blocks, where did they come from? And the answer is that they came directly from the Big Bang. The next time you get up in the morning. Young students won't understand this, but those of us my age will. When you get out of bed and you're sitting there and you're feeling a little old and creaky that day, no matter how old you are, remember that the protons, neutrons, and electrons that make up your atoms are many. older than you.

In fact, they date back to the Big Bang. And, in fact, they are 13.7 billion years old. That number, the age of the universe, was determined about 10 years ago by a NASA satellite called the Wilkinson Microwave Anisotropy Probe. We'll see a photo here in a minute. And it is one of the most contested numbers in astronomy. We fought for 100 years to discover the exact age of the universe and now we know it. It is 13.7 billion years old. And the protons, neutrons and electrons in your body were formed in the Big Bang. Now, the Big Bang is not just a hypothetical idea that astronomers have come up with.

Today the Big Bang can be tested directly in laboratories. In fact... This is a simulation of a collision at the gigantic particle accelerator at CERN, in Switzerland, on the border between France and Switzerland. They take protons and smash them at high speed because when those two protons meet head-on, the pressures, temperatures and densities in that collision are the same as they were a small fraction of a second after the Big Bang. Here on Earth we can recreate the conditions of the Big Bang when we collide atoms like this in our atom colliders. In fact, that's one of the main reasons we do it: to create these kinds of conditions that we don't normally experience around us.

So we understand the Big Bang quite well. We know that at the Big Bang, during the Big Bang, the universe was really hot. And it was so hot that the universe was filled with light and that light was made up of high-energy photons, a type of light called gamma rays. And from time to time, these two gamma rays collided with each other. And, when they collided with each other, they would form a piece of matter and a piece of antimatter. And the only difference between matter and antimatter is the charge. So this, for example, is a positively charged proton and this is a negatively charged antiproton.

And then, when these photons collided, they would form two pieces of matter. And the only thing that was needed for these two photons to matter was that they had enough energy. Einstein's famous equation, E=mc^2, tells us how much energy is needed to produce a given amount of matter. If you know the mass of the proton and the mass of the antiproton, you can calculate how much energy it took to produce them. And it turns out that the only form of light that has enough energy to produce these things is high-energy gamma rays. Now, fortunately for you and me, gamma rays are pretty rare these days, unless you work in one of the nuclear industries or work here at JLab, you probably don't encounter gamma rays very often.

But in the early universe it was full of gamma rays, this happened all the time. In the first seconds of the universe, matter and antimatter emerged as these photons collided. The problem is that matter and antimatter have opposite charges. Well, remember that opposite charges attract each other. The positive proton and antiproton attract and annihilate each other in a burst of energy, like in Star Trek. When matter and antimatter come together, they annihilate and you get those two photons of light back. If this were the only way to produce matter in the universe, then every time we made a piece of matter, we would produce a piece of the corresponding antimatter.

But, for reasons that physicists don't understand now, from time to time in the early universe, the universe formed a chunk of matter, but not a corresponding chunk of antimatter. Or he created antimatter, and the antimatter was so unstable that it decayed and all we have left is matter. But the thing is this, the protons that you and I are made of. They were created by light in the early universe and those protons are the ones left because their companions were not created or disintegrated a long time ago. Physicists or scientists who answer the question why antimatter sometimes did not form or decay will undoubtedly win the Nobel Prize in Physics.

So. So, my generation has been working very hard to solve this problem. We haven't gotten there yet, so maybe one of the students here will finally be the person to figure out why that antimatter wasn't created and only matter was left. Because today, in our universe, there is essentially no antimatter. Almost none, in the universe as a whole. Well, for the first... few minutes of the big bang, the first three minutes, it was so hot that all the protons and neutrons that were created started colliding with each other and started undergoing nuclear reactions. And neutrons and protons can collide to produce a heavy form of hydrogen.

And that heavy form of hydrogen can collide with neutrons to produce an even heavier form of hydrogen. Or that it can collide with a proton to produce a form of helium. This hydrogen and that proton can combine to form another form of helium. In the first three minutes of the Big Bang, the universe was hot enough that these protons and neutrons could collide with each other and begin to form the elements of the periodic table of elements. And we get the formation of hydrogen atoms, which is easy, because hydrogen atoms are just a proton. But we have the formation of helium and lithium in the periodic table.

But there is a problem in the first three minutes. The universe is expanding rapidly. It is expanding so fast and cooling so quickly that just three minutes after the Big Bang the nuclear reactions stopped. It's too cold for nuclear reactions. So the universe doesn't have much time to produce elements because it only has three minutes of nuclear reactions to do so. On top of that... it turns out that right after lithium on the periodic table of elements there is a bottleneck. The next element from lithium, the form of beryllium that would be produced, is unstable. So unstable that the universe cannot do it.

This is what the periodic table of elements looks like today. This is what the periodic table of elements looked like three minutes after the Big Bang. There were only three elements in the entire universe. Hydrogen, helium and lithium. And here I only include lithium, there was a very, very small amount of lithium in the early universe, so I include it here because some lithium was produced. But, three minutes after the Big Bang, the universe was made up of 75% hydrogen and 25% helium. There was no gold. There was no carbon, no nitrogen, no oxygen. Nothing. I like to point out to students that chemistry class was a lot easier back then because there was only hydrogen and helium, and helium is a noble gas, so it doesn't form molecules.

The problem was that there could be no chemistry classes because there was no carbon or oxygen to train the chemist. Steve's website that has the periodic table of elements was much easier to build back then. But there couldn't be any Steve to build it because there was no carbon, nitrogen or oxygen to create Steve. So that was the Big Bang in the first three minutes. So where did the gold come from? Because gold didn't come from the Big Bang. The carbon and oxygen in your body did not come from the Big Bang. So where did they come from?

Well, we have to track this hydrogen and helium gas over time and see what happens to them. So the next step after the Big Bang... is for the universe to continue expanding. And at first, this gas is distributed very evenly, very uniformly throughout the universe. But gravity begins to group them together. And as gravity begins to clump this gas together, we can see clumps forming in the early universe. This is an image of the afterglow left over from the Big Bang. This is a complete image of the sky. It's like one of those weird maps of the Earth where they show you the entire map of the Earth, so they take it apart and spread it out completely.

This is a map showing the entire Big Bang globe across the entire sky. The light you're seeing was taken by the Wilkinson Microwave Anisotropy Probe, that NASA satellite I mentioned a minute ago, and this is the glow that was released 380,000 years after the BigBang. But you'll notice that just 380,000 years after the Big Bang, matter is already clumping together. Gravity is already starting to pull on him. And we can run simulations starting with a very smooth universe and just let gravity work in our simulations, and we can see how the universe starts to cluster into clumps. And we went from the very fluid universe that we had at the beginning to the very lumpy universe that we have today.

And any very, very deep image of the universe, like the extremely deep field that was recently released by the Hubble Space Telescope, the deepest image that humanity has ever taken of the universe, shows us that the current universe is very, very lumpy. The gas is not evenly distributed. It is grouped. And the basic component of the universe is the galaxy. This is an image of what our galaxy, the Milky Way, would look like if we could get out of it. But, as I said before, we have never been outside our galaxy. Our children will not leave the galaxy and their great-great-grandchildren will not leave the galaxy.

Because it would take hundreds of thousands of years to travel this distance and look back, even traveling at the speed of light. And we are very far from achieving it at this point. So this is another galaxy, NGC 4414, that we think looks like our Milky Way. If we could leave the Milky Way and look back, this is what our galaxy would be like. Our galaxy is made up of a few hundred billion stars andgiant clouds of gas and dust. And those gigantic clouds of gas and dust are where new stars are born. Now, we don't see the Milky Way like that, we see it like that because we live inside it.

It's like a giant pizza with a grapefruit in the middle. The grapefruit is a big lump down here, the pizza is the disk of material. But, because we live inside the pizza, in the middle of the pizza, about halfway down the center, when we look up at the constellation Sagittarius, we see the grapefruit in the center and then here's the rest of the pizza. around us. Everything you can see in the night sky, with one exception, is part of our Milky Way galaxy. These stars are in the Milky Way. It turns out that they are the stars above us.

These stars are in the Milky Way. Those are the stars that are below us. These are the stars that are in the Milky Way and that are around us. And of course, if you think about a thin pizza, if you live in the middle of a pizza, there's not a lot of pizza above you, there's not a lot of pizza under you, but there's a lot of pizza around you, which is why our galaxy It looks like a line that crosses the sky. In fact, the only thing you can see with the naked eye that isn't part of our galaxy is a faint, fuzzy object in the Andromeda constellation, right here.

And that faint, fuzzy object is another galaxy called the Andromeda Galaxy, about 2.4 million light years away from us. And I'm going to assume that here in the Hampton area they won't see that because of all the light pollution, they'll need to get out into a dark country sky to see the Andromeda galaxy. Within galaxies, the basic components of the universe are stars. And the stars are the next part of our story, like what happens to the elements. Stars are giant balls of hydrogen and helium gas. Our sun is a giant ball of hydrogen and helium gas.

It has about 300,000 times the mass of the Earth. It's 109 times the diameter of the Earth, so the Earth would be a small dot up here compared to the size of the Sun. But the Sun doesn't have a solid surface. It is a gas to the core. And it is, as I said before, 74% hydrogen, 25% helium and 1% everything else. In the center of the sun, because of the weight of all these layers of gas pushing down on the center, as they push down on the center, they compress that gas. That gas is very hot down there. It has a temperature of about 15 million degrees.

And nuclear reactions are taking place down there and that is what powers our sun. And they are two basic nuclear reactions in the universe. There are fission reactions, where you take large elements, like uranium, and break them down into krypton and barium and release a bunch of neutrons. Those are fission reactions. And there are fusion reactions, where you take light elements, like hydrogen, and collide them to fuse or build larger elements. This one is important to us. Fission is important because this is how much of our electricity is generated. A good portion of the electricity that runs the lights in this room and this projector comes from nuclear power plants in Virginia.

In central Virginia, we have the Lake Anna nuclear power plant, and at Lake Anna, they take uranium atoms, they split them, and in the process, they release energy, and that energy is converted into electricity. We also make nuclear weapons with this. Uranium bombs and plutonium bombs are fission weapons. Fusion, on the other hand, where you take light elements and combine them to build larger elements, is something we haven't mastered yet, at least for generating electricity. However, we have mastered this, if you can call it mastery, in the area of ​​nuclear weapons. Hydrogen bombs. We can carry out this reaction in an uncontrolled way, but we have not yet discovered how to control it, harness it, and convert it into electricity.

But we understand nuclear fission and fusion reactions quite well. And you can guess which one powers the sun because I already said that the sun is made up of 74% hydrogen. And so the sun is made primarily of hydrogen and it is its fusion reactions that power the sun. Deep inside the sun, in the center of the sun, but only in the core of the sun, not in the outer parts of the sun, just deep in the core, it is quite hot, about 15 million degrees, where it can be collide protons and form heavier elements. This is the set of nuclear reactions, the chain of reactions, that takes place deep in the center of the sun.

What happens in the sun is that a hydrogen atom, which is a single proton, which is positively charged, and another hydrogen atom, which is a single proton with a positive charge, collide with each other. Now, they don't want to come together because, remember, charges repel each other, so a positively charged proton and a positively charged proton. Normally when you try to put them together, they are positive. Both positive charges will come together. they repel and separate. But, if this happens under high enough temperatures, like the 15 million degrees we have at the center of the sun, you can bring them together so tightly that they get so close together that the two can react and form something heavier. .

A form of heavy hydrogen. And that heavy hydrogen can combine with another proton to form a form of helium. Two of them combine to produce another form of helium. The end result in our Sun is that four hydrogen atoms enter and one helium atom leaves. Hydrogen is then converted to helium in our sun. Every second inside our sun, 600 million tons of hydrogen are converted to helium in our sun, every second. Now you may remember from chemistry class in school that, in chemical reactions, mass is conserved. You start with a certain number: to begin with, an amount of mass, that mass must be conserved.

That is no longer true with nuclear reactions. In the case of nuclear reactions, what is conserved is the combination of mass and energy. This helium atom that leaves has less mass than the 4 hydrogen atoms that entered. It is a tiny fraction. It has 0.7% less mass than the 4 hydrogen atoms that entered, but that 0.7% of matter was converted into energy. And, according to Einstein's E = mc^2, that matter that disappeared comes out in the form of energy, and that is what powers the sun. So our sun is converting hydrogen into helium. This is a problem for our sun. Or at least the long-term future of our sun.

Because our sun is like your car. It runs on fuel and someday our sun will run out of fuel. This shows what our sun was like when it was born. This is the distance from the center of the sun to the surface of the sun. And this is the composition of the sun. So this is the core, this is the surface. You can see that throughout the sun, when it was born, at birth, the sun was made up mostly of hydrogen and a little bit of helium. Today, the sun is four and a half billion years old.

So, it's about halfway through its life. The sun has used about half of the hydrogen in its core and converted it to helium. Our sun is approximately halfway through its life. He is a middle-aged star. When the Sun is about 10 billion years old it will have converted almost all of its hydrogen into helium. And at this point, our sun will start to die, because there will no longer be nuclear reactions in the center, and our sun needs those nuclear reactions because gravity is trying to gather all that gas together. And nuclear reactions provide the force that balances the force of gravity.

Without those nuclear reactions, gravity will win and the center of our sun will begin to contract. And as the center of our sun contracts, it releases so much energy that it inflates the sun's outer layers into a red giant star. So the final destiny of our sun is to become a red giant star. This is the size of our sun today. This is the size of our sun when it is a red giant star. It will become huge compared to its current size. But the contradictory thing is that the center of our sun is actually shrinking.

The center shrinks and becomes smaller and the outer layers swell. And in fact, that center will eventually become unstable. And, when it becomes unstable, it will blow away those outer layers. Throughout its life, our sun converts hydrogen into helium. When it's a red giant star, at the end of its life, that core will get hot enough to be able to convert helium atoms to carbon and some carbon to oxygen on the periodic table. You will only be able to do this later because, at this time, it is not hot enough at the center of the sun to convert helium into carbon.

The problem with helium is that it has two protons. Another helium atom has two protons. That's plus two and plus two. It is much more difficult to drive them together. While a temperature of about 10 million degrees is needed to fuse hydrogen into helium, the temperature has to be above 100 million degrees to fuse helium into carbon, and our sun is still not hot enough. But it will be in the red giant phase. And it will fuse helium into carbon and some carbon into oxygen. But our sun is not big enough and will not do more than that.

And that's as far as our elements can go on the periodic table. Now, keep in mind that the carbon, nitrogen, and oxygen in your body do not come from our sun because our sun won't until it is dying. So, it's not like it happened that long ago. This is what the sun will do at the end of its life. And this is what will end: this is the final destiny of our sun. This little white dot in the middle is a white dwarf star. It is the burned core of a star like our sun. And this beautiful planetary nebula that you see around you, this bright nebula, is the outer atmosphere of the star.

The dead core becomes unstable and breaks off those outer layers and creates this beautiful planetary nebula right there. The other problem with stars like our sun is that all the carbon, nitrogen and oxygen remain locked in that white dwarf star. It does not return those elements to space for future generations of stars to use. So the carbon, nitrogen, and oxygen in your body don't come from a star like our sun. The carbon, nitrogen, and oxygen in your body actually come from a much larger star. Giant stars undergo different nuclear reactions. So let's talk for a minute about the nuclear reactions that occur in big, giant, massive stars.

And this is a really cool plot that shows how much of each element there is in the universe. This tells you the quantity of the item and these are each of the items listed here. And what you need to know about this table is that it is a logarithmic table, which means that each of these marks is a factor of ten. So just by looking at this, it appears that there is almost as much nitrogen as oxygen in the universe. But in fact, they are about one mark apart, which means there is ten times more oxygen than nitrogen in the universe.

So these are the abundances of the elements. And we have already said that hydrogen and helium are the most abundant elements, by far, in the universe. And then there is hardly any lithium, beryllium and boron, but here are the other elements. And, if you look at that periodic table of elements, you'll notice something very interesting about the abundant elements. So can someone tell me what atomic number carbon is? Six. So what number is oxygen? Eight. What is neon? Ten. Magnesium? Twelve. Silicon? Fourteen. Do you see a pattern developing here? They are the even elements. Six. Eight.

Ten. Twelve. Fourteen. Sixteen. Eighteen. Twenty. Twenty two. Twenty four. Twenty six. Even elements in the universe are ten times more abundant than odd elements. Why is that? Why does the universe prefer even elements to odd ones? By the way, one more thing before continuing, tonote from this table is that I mentioned that hydrogen and helium are the most abundant, look what is the third most abundant element in the universe. It's oxygen. You are made of oxygen. You are made of the third most abundant element in the universe. After oxygen, you are made primarily of carbon. Carbon is the fourth most abundant element in the universe.

You are made of the common matter of the universe. Carbon and oxygen are not rare. After hydrogen and helium, they are the two most abundant things in the entire universe. So let's get back to this question of even elements. What happens to the even elements? It has to do with the way massive stars live their lives. Massive stars can fuse hydrogen into helium. But, when they have finished fusing hydrogen with helium, they will be able to fuse helium with carbon. And then they can fuse, undergo other nuclear reactions in which helium and carbon can produce oxygen.

Helium and oxygen can produce neon. Carbon and carbon can produce magnesium. Oxygen and oxygen can produce silicon. And if you look at these reactions, you will notice that the basic element of all of them is the helium atom. Three helium atoms formed carbon. Helium and carbon produce oxygen. Since the basic component of all elements is the number two helium atom. If you start with a building block that consists of two and put two of them together, you get four. Six. Eight. Ten. Twelve. Fourteen. Sixteen. The way massive stars live their lives is by fusing heavier and heavier elements and they always use helium.

So, silicon and helium produce sulfur. Additionally, helium produces argon. Additionally, helium produces calcium. And so on until we reach iron in the periodic table. Massive stars, during their lifetimes, will fuse hydrogen to iron on the periodic table. This is the current periodic table. This is what massive stars can do. During their lifetime, they will fuse hydrogen into helium. And helium in carbon. Carbon to oxygen, neon, magnesium, silicon, sulfur, argon. All the way to the iron. But iron is really special. It turns out that iron is the most stable element in the universe. There is no element more stable than iron.

That is, iron is so bonded that if these atoms fuse to form iron, energy is released. If you fission these atoms, breaking them down to become iron, you release energy. But when you get to the iron, there's nowhere else to go. If you wanted to take in iron and accumulate heavier elements, you would have to add energy. And stars don't want to do that. Stars use these nuclear reactions to generate energy. Having to do nuclear reactions that you have to put energy into will simply absorb energy from the center of the star. So the stars end up here, in the iron.

And, in fact, a massive star, a really big star. Say, a star that has about twenty-five times the mass of our sun. It will fuse hydrogen into helium for about 7 million years. Our sun will do that for 10 billion years. Truly massive stars, although they have much more fuel available, do not live very long lives because they burn that fuel very quickly. It will burn helium into carbon for 500,000 years. Carbon to neon for 600 years. Neon to oxygen for about a year. Oxygen to silicon for about half a year: 6 months. And then silicon to iron in a day.

And on the last day of this star's life, it begins to fuse silicon into iron. And, in the last minutes of that star's life, the inside of the star looks like a giant onion. There is a big iron ball here in the middle. And, around that, there is a layer of silicon that is fusing with the iron. And around that is a layer of oxygen that fuses with the silicon and goes on and on and out. As I said, this iron is so stable that it cannot undergo any nuclear reaction. So without nuclear reactions, gravity is holding that iron ball together.

Gravity is trying to contract it. Iron atoms push against the force of gravity. And in particular, it's the electrons that are there that push back. But gravity squeezes it tighter and tighter and they push back harder and harder. However, over time, this layer of silicon continues to shed more and more iron onto that core. And that core becomes more and more massive. And increasingly massive. And eventually it becomes so massive and the gravity is so strong that the atoms can no longer resist the force of gravity and they give up. And the core collapses. And, in about two seconds, the core of this star, which is about the same size as our planet, but probably weighs two or three times the mass of our sun.

So it's many, many times the mass of the Earth, but about the size of the Earth, that core collapses. And when it collapses, it releases so much energy that the star explodes in a titanic explosion called a supernova. Only the most massive stars become supernovas. If you look here, this is in a nearby galaxy called the Large Magellanic Cloud. This is a giant star forming region right here. There is a star in this image that is about to go supernova. It went supernova in 1987. This was a photograph taken a couple of days before the star exploded.

When looking at that image, can you find the star that is ready to explode? If you could, you'd be doing a lot better than astronomers because we had no idea it was that star that was ready to explode. So if I come back, you can see the star. See, the thing is, with astronomy, we can see the surface of a star, but we can't see what's going on inside stars, so we had no idea that star was ready to explode. But, for a few days, that star eclipsed our entire Milky Way galaxy of hundreds of billions of stars.

That is the amount of energy they give off. And, when these stars explode, most of the star's mass is ejected into space. And all the elements that formed during the life of that star also extend back into space. So, we had said that this is the periodic table of elements. This is what it looked like three minutes after the Big Bang. This is what it looks like thanks to stars like the sun. Massive stars can accumulate into iron. And, because they are exploded at the end of their lives, they return all of these elements to space.

But remember that core collapse, where that big iron ball collapsed in on itself in two seconds? When that big iron ball collapses, the densities, pressures and temperatures are so high that, in about two seconds, all the other elements of the periodic table are formed. The reason gold is precious to us is because it is rare. And the reason gold is rare is that the universe had about two seconds to produce it when the core of that star collapsed. Because it is the only time when the pressures, temperatures and densities are high enough to produce some of these heavy elements like gold, mercury and silver.

They form when these massive stars implode at the end of their lives. So let us trace the history of your atoms, and in particular, your gold atoms, from the beginning of the universe to the present day. The atoms in your gold jewelry started out as hydrogen and helium atoms in one of these giant star-forming regions, like this one, this giant star cluster. The first time its atoms were found inside a star, they were not deep in the star's core. They were probably in the outer parts of the star. And remember, there are no nuclear reactions in the outer parts of a star.

Nuclear reactions only occur in the core of the star. So the first time its atoms were found inside a star, that star exploded in a titanic explosion and spewed those atoms back into space, but they were still hydrogen and helium atoms. The second time its atoms were found in one of these giant star-forming regions, like the Orion Cloud. Here is Orion's belt and his sword. And that is the Orion Nebula, a region of giant star formation. The second time they were found in a star, they were probably not found deep in the core of the star, but in the outer layers of the star.

And, when that star exploded, it sent all those elements back into space. The third time its atoms were found in one of these giant star-forming regions, they were likely found deep in the star's core. And this time, the hydrogen atoms became helium. Helium to carbon. From carbon to oxygen. Until we reach silicon. Even ironing. And then when it exploded, it formed the gold that's in your jewelry. And that gold was scattered back into space in that titanic explosion at the end of the star's life. After that gas spread into space, its atoms found themselves in a final star-forming region, like the Eagle Nebula in Sagittarius.

The famous one with these beautiful pillars of dust sticking out. And this is really interesting to astronomers because we actually see little clumps of gas up here. And those little gas clumps are forming stars. And, when we look at those forming stars, this particular group of stars comes out of the Orion Nebula, we look at the stars and we see, surrounding them, these black disks of dust. Those are planets in the process of formation. The last time you were in one of these star-forming regions, your atoms were not in the star. They were in that disk of dust that surrounded the star.

And as gravity brought those pieces of dust together, they formed small rocks. These rocks grouped together to form asteroids. These asteroids grouped together to form planets. And the fourth time their atoms were in a star-forming region they ended up, not in the star, but on the third planet from the sun. And they were not in the depths of the earth. They were actually close to the surface. Its atoms have spent most of their time near the Earth's surface here. And this is where your atoms come from. You are, as Carl Sagan said, literally made of star stuff.

The atoms in your body were forged in the furnaces of stars billions of years ago. But its atoms have not been in a single star. You've probably been to at least two or three different stars in the history of the universe. And, like I said, this time your atoms ended up, not on the star, but on the planet. The last thought I want to leave you with is "What is the final fate of your atoms?" So, you are made primarily of oxygen. What will happen to your atoms in the future? Because, I hate to tell you, you don't own your atoms, you are simply borrowing them for as long as you are here on Earth.

Well, when we all pass away, whether we are cremated or buried, our atoms become part of the surface of the earth. And, over millions of years, those atoms will be recycled through plants, animals and other living things on the surface. However, eventually our sun will die. When our sun dies, it will become a red giant. And our star will get so big... the diameter here is two astronomical units, which means the radius is one astronomical unit. An astronomical unit is the distance from the earth to the sun. Our sun will become so large that the outer layers of the sun will reach into the Earth's orbit around the sun.

When it does, it may not be enough to completely vaporize the Earth, but it will vaporize the outer layers of the Earth. Then its atoms, which are part of the outer layers of the earth, will be vaporized from the surface. They will become part of the outer layers of this red giant star. And, as you may remember, when our sun dies, the core shrinks to form a white dwarf. The outer layers are expelled to form a beautiful planetary cloud. That is the destiny of your atoms. Someday you will be a planetary nebula. And I didn't mention it before, but I will mention it now.

See that green glow coming from the center of the nebula? Those are oxygen atoms. So those are the oxygen atoms you are made of and that is where you will be headed one day. Because you will return to space. And then, billions of years into the future, these atoms will be in the next generation of stars. So the Milky Way is like a giant recycler. Its atoms have been in stars and will be again in future stars. But you have them only for this short period of time. So thank you very much. I'll be happy to answer questions for a few minutes.

I think we have about five minutes for questions. So thanks. Yes. Does the city of Charlottesville have a black sky right now? No, Charlottesville does not have a dark sky ordinance. Anyway, it's not a very good dark sky ordinance. I don't know if there's much hope of getting it in Virginia. I think we're really going to have to work in rural areas and work hard to preserve the dark skies we have in rural areas today. But I'm not very hopeful that there's any hope of taking cities like Charlottesville or Hampton and trying to reverse what we've already done.And we have worked, in fact.

I will say that we have worked very hard in Charlottesville to try to get a dark sky ordinance, but there is too much opposition from others. Yes. What is that grapefruit in the middle of the Milky Way? Yes, so half of the Milky Way, the grapefruit in the middle, is a big ball of stars called the galactic bulge. And those were some of the first stars that formed when our galaxy was forming. And so if you look at it in that image, it's just a big ball of stars sitting there in the center of the galaxy.

Really old stars. Yes. [Mostly inaudible, but something about black holes and whether the data is skewed if one is in the line of sight of an observation.] Okay, so the question is "How do we know, when we look at things that are very far away?" , like Millions of light years away, how do we know that there is no black hole between us? Or other objects. Well, the answer is that in many cases, there are other objects between us and we see them. to this. So when we look at star forming regions like this, most of the stars that you can see here are actually foreground stars that are in the foreground.

The thing to keep in mind about black holes is that. They are incredibly small. If we take our sun and shrink it down to a black hole, its radius is only a couple of kilometers, which is equivalent to a couple of miles across. So that's so infinitesimally small when we're talking about distances. so big that it has no effect at all. Well, see, that's the thing. Black holes have a very bad reputation for absorbing everything. And it turns out that only if you get close to them do they absorb everything. But something interesting to think about about our sun is that the Earth is kept in orbit by the mass of our sun.

If we take our sun and shrink it so small that it became a black hole, it turns out that we have not changed the mass of the sun and we have not changed the distance from the Earth to the Sun. So even if our sun became a black hole , the Earth would continue to orbit the sun, just as it does today. Therefore, it does not absorb things. The only way black holes suck things in is if you get very, very close to them. Near what is called its event horizon, where gravity is very strong. But that event horizon is what's only a couple of miles wide.

So, you have to get very close to black holes. Yes. So, there are two things there. One is: do black holes remove matter from stars? Yes. Stars are usually born in binary systems. Two stars orbiting each other. And, in those binary systems, if the most massive dies and becomes a black hole, when the least massive dies and becomes a red giant, that matter can be absorbed by the black hole. There is a beautiful example of this in the constellation Cygnus. It is the best candidate we have for a black hole. But isolated stars like our sun, which do not belong to binary systems, black holes are not at all common in the galaxy.

It's not common; I would say that it is, in fact, remarkably unusual for an isolated star, like our sun, to interact with a black hole. And the only truly massive black hole in our galaxy lies deep in the galaxy's center. And at the center of our Milky Way galaxy, there is a black hole 4 million times the mass of our sun. But that is 26,000 light years away from us and we orbit the galaxy. We never meet down there, in the center. The only stars that have to worry about that black hole are the ones that are close to it.

So. Can we thank Dr. Murphy again, please, for his talk tonight?