Two of the most prominent popularizers of science to have ever lived, Carl Sagan
..and Neil deGrasse Tyson
…have both made the same point:
Every atom in our bodies–hell every atom on earth (other than the hydrogen) was “cooked” in the center of some very large star (ten or more times the mass of our sun) or another, at least 4.6 billion years ago, then blown out into empty space when those monster stars exploded in a staggeringly violent way. Later the debris from these supernovas coalesced to form new stars and planets, like our familiar sun and the earth.
As I’ve seen people say it, “Jesus didn’t die for you, stars did.” And how.
The fancy word for this process of building up heavier atoms is nucleosynthesis. And I thought I’d talk a bit more about it.
I glossed over this a bit in this post in the section The Modern View of the Origin of the Earth. But maybe someone wants to know more, without getting so technical you need some serious math to understand it. So here’s my shot at an explanation. But one thing I suggest you do if you need to learn what nuclei and isotopes are, is to read Quick Review of Atomic Theory and Neutrons and Isotopes, which are two other sections of that same post.
The Starting Point
When the universe finally settled down from the massive subatomic particle soup chaos that followed the big bang, it turned out that something like 75% of the normal matter was hydrogen (symbol H, 1 proton) and the rest was almost entirely helium (He, 2 protons, 2 neutrons). There were traces of lithium (Li, 3 protons) and maybe even some really tiny amounts of beryllium (Be, 4 protons).
Well this mix isn’t very interesting, to say the least. Each hydrogen atom can combine with one other atom. And since all the other atoms out there for it to combine with are either hydrogen, or helium, and helium atoms don’t react, you get hydrogen molecules (H2), just two hydrogen molecules bonded to each other. Not a lot of exciting chemistry can happen in such a universe. Certainly no life.
The hydrogen/helium mix collapsed under its own gravity, to form denser areas, and those in turn got more and more concentrated, and would heat up as the potential energy in the gas got released. The combination of heat and pressure would eventually strip the electrons off the atoms, and now you have a plasma where nuclei are free to fuse. Once the pressures and temperatures (about 10 million degrees) get great enough at the centers of the clouds, this starts to happen and a star is born, with fusion taking place in the very core of the star.
These first stars consisted of nothing but hydrogen and helium, and most of the hydrogen was “protium” or just a bare proton.
The first stage was to get two protons to stick together; of course this is unstable. One proton turns into a neutron, releasing a positron and a neutrino, and now you have a deuterium nucleus.
What are positrons and neutrinos? Positrons are actually antimatter; they are anti-electrons. They have a positive charge, and carry the positive charge of the proton away from the new neutron. The positron eventually finds a free-floating electron, they annihilate each other, and a bunch of energy is generated. Note that instead of two protons and two electrons, there is now one proton and one electron. The electrical balance is always maintained. (That’s a fundamental law of nature.)
The neutrino is a very ghostly particle, and will probably never interact with anything. Ever. It flies out of the star at light speed (or close to it), never to be seen again.
When another proton is added, you have helium-3, which is stable.
Two helium 3s get fused together to form helium 4, with two free protons left over. This entire process is called the proton-proton chain.
The process of making helium from hydrogen releases a lot of energy, and this is what makes most stars shine. For example, our sun is “burning” hydrogen right now, 700 billion tons of it every second; it will probably burn a grand total of 15 percent of its hydrogen before it dies after 10 billion years. It’s about halfway through that time span now.
At this point though, all we’ve done is raise the ratio of helium to hydrogen in the universe; we haven’t created anything truly new.
Beyond Helium: Metals
Eventually the core of the star runs low on hydrogen, and the reaction starts to peter out. The core of the star no longer is generating energy and heat to counteract gravity’s pull and the core begins to shrink. But when this happens, the temperature and pressure will increase (just as it does when you compress a gas on earth). If the star is massive enough, the temperature might reach as high as 100,000,000 degrees. If this happens, the star can start fusing helium. At this point things start to get interesting. Metals begin to be made, and we begin to create stuff that did not get made as a direct result of the big bang.
Now to an astrophysicist, there are three kinds of matter in the universe: hydrogen, helium, and “metals.” “Metals” is their shorthand for everything else, including such distinctly non-metallic things as carbon, nitrogen, oxygen, phosphorus, sulfur, chlorine, argon, and iodine. But most of the chemical elements are metals of one kind or another, so it’s a bit of shorthand that makes sense.
Three helium-4 nuclei form a carbon-12 nucleus. Wait, though, what happens if you fuse only two of them? You get beryllium-8 which is very unstable; it doesn’t stay together long. Hopefully, another helium nucleus will fuse with it before it breaks apart. That implies a lot of nuclei in a very tiny space, hence the dramatically higher temperatures.
Carbon-12 is quite stable, you have plenty of it in you. It’s also liable to pick up another helium-4 nucleus, and become oxygen-16, or perhaps some protons that didn’t previously get “burned,” creating nitrogen. All of this releases some energy.
Meanwhile in a layer outside the core–slightly cooler and under less pressure–hydrogen that was formerly not hot and pressurized enough to fuse is getting heated up by all this energy being released, and it can start to fuse with the proton-proton reaction. So the star has a thick outer layer of hot hydrogen, the outside of which is what we actually see shining, a thinner layer just outside the core where hydrogen is fusing to form helium, and the core itself where helium is fusing to form metals.
Now this phase of a star’s life is very short compared to what came before. For the sun, which is just barely big enough to go through this, it will last a million years, versus the ten billion years for the proton-proton chain. Why is this? It’s because the creation of carbon from helium releases much less energy than creating helium from hydrogen. Yet the pressures at the star are crushing, and in order to generate enough energy to counteract the contraction, more energy has to be created every second, not less. So the sun (or any other star) has to burn its helium fuel that much more quickly. And once it’s done, the core will again shrink, temperatures will increase, and pressures will go up. But for the sun, they will not go up enough to go to the next part of the process. The sun will blow off some of its outer layers (hydrogen and helium, mostly) into space, and what’s left will be a white dwarf, slowly cooling from residual heat.
More massive stars, say ten times the mass of the sun, can proceed; their cores collapse more and the pressures and temperatures increase. Oxygen (O, 8) and carbon (C, 6) can combine to form silicon (Si, 14), oxygen and oxygen combine to form sulfur (S, 16). Silicon fuses to form nickel (Ni, 28), isotope 56, which is unstable and decays to iron (Fe, 26)-56. All along the way though, you will sometimes only add single helium nuclei, and more rarely single protons, so all of the elements in between can form as well. But from what I’ve said so far, it’s the even numbered elements (after beryllium) that will tend to be more common, with odd numbered ones somewhat less so, because most of the action consists of combining helium with itself, or other things that were made by combining helium. And when we look around us this turns out to be true, here on earth the even numbered elements tend to be more common than the odd numbered ones. Calcium (20) and titanium (22) are fairly common but scandium (21) and vanadium (23) are not.
“Ah ha,” some might say. “Nitrogen (N, 7), an odd numbered element, is much more common in the atmosphere than oxygen (O, 8) is! Your theory is wrong!”
Well it’s true that most of the atmosphere is nitrogen. But there is plenty of oxygen on earth. It’s just not all in the atmosphere; in fact you can ignore as less than a rounding error the oxygen in the atmosphere when talking about how much oxygen there is in the earth. Let’s even ignore the oxygen in the water of the oceans. Below your feet are the rocks that form the earth’s crust and mantle, 2900 kilometers deep. And those rocks, believe it or not, are 46% oxygen when analyzed by weight. More than 50% oxygen in a solid layer 2900 kilometers thick just completely blows away the thin gaseous layer of mostly nitrogen in the 100km or so thick atmosphere, if you are running any sort of contest to see what’s more common.
We have a better idea of what’s in the crust than we do for what is in the mantle, and it turns out the crust is 46% oxygen by weight and 0.002% nitrogen. There is more yttrium (Y, 39) in the earth’s crust than there is nitrogen. “What the hell is yttrium,” you might ask? Exactly the point.
OK, back to our massive star, burning through heavier and heavier fuel faster and faster, frantically staving off collapse.
Once it gets to iron and starts looking at all that iron in its core hungrily, wondering how much energy iron will release when it fuses, it has reached a critical point. Up til now, every reaction has released energy, and that energy has helped stave off the star’s tendency to collapse. But once a super massive star starts to “burn” carbon, it’s only going to live about 600 years. When it starts “burning” silicon to form iron, it’s going to go through all that silicon in a single day. So you can see a trend here. Iron won’t last very long at all, maybe minutes. But it’s not just that the star will have to go through it voraciously, but rather that if you fuse iron, you consume energy, rather than generating it. The core of any star that reaches this stage is no longer generating any energy at all, and collapses with nothing to stop it. The core will certainly grow hotter, for all the good that does. But the heat hits the outer layers of the star where all sorts of prior types of fusion are still going on, and at this point anything goes. With this much heat and varied material in the same neighborhood, just about any conceivable reaction will occur, even ones that cost energy. Any element can be created, clear up past copper (Cu, 29), zinc (Zn, 30), molybdenum (Mo, 42), silver (Ag, 47), tin (Sn, 50), barium (Ba, 56), tungsten (W, 74), gold (Au, 79), lead (Pb, 82), uranium (U, 92), plutonium (Pu, 94), possibly even up to californium (Cf, 98).
And the star explodes. That’s an understatement. It blows up so dramatically it will likely outshine the other hundred billion stars in its galaxy, put together.
And a good thing too. When the star explodes, it blasts all this wealth of chemicals out into the void. Now what is blasted out is still mostly hydrogen that was in the outer layers of the star and never burned, but a decent portion of it is not. Eventually the cloud of metal-enriched hydrogen and helium can contract again, and form new stars. And around those stars there will be planets with the sorts of elements needed to create minerals, and life.
The Earth Made of Star Stuff
Our planet ended up with at least some of almost everything, though most unstable things have long since disappeared. We have no technetium (Tc, 43) or promethium (Pm, 61) here, those were unstable and long since decayed away. And we don’t have any californium either, but we have a tiny trace of the original plutonium (94) and a lot of uranium (92) and thorium (Th, 90) because they are more stable. But everything else, from lithium to lead, we have and we can thank long-dead giant stars for it.
Jesus did not die for us, but stars did.
We have hydrogen, much of it tied up in water molecules (fun fact: there is more hydrogen in a gallon of liquid water than in a gallon of liquid hydrogen), but that is hydrogen that didn’t get burned in a star. Perhaps it was never in a star, or maybe it came from the outer layers of a supernova. We really can’t tell where particular hydrogen atoms came from.
Helium, though is more interesting. We have some of it on earth, but virtually none of it is “big bang” or primordial helium, nor is any of it star stuff, at least not directly. Helium captured by the early earth is long gone. At room temperature, helium atoms move faster than the earth’s escape velocity, and if a helium atom sticks around at all, it’s because it keeps ricocheting off other atoms and molecules in the atmosphere. But eventually it happens to reach the upper atmosphere and has a clear path out of this taco stand and is gone. So where do we get our helium (for party balloons and for cooling superconducting magnets) from? It comes from underground, from gas wells. All of the helium down there is in fact just old radioactive decay. When uranium or thorium decay they give off alpha particles, which are just helium nuclei. They get shot out of the nucleus, eventually lose their speed, and pick up a couple of electrons. The atom is trapped in solid rock rather than . So your party balloon is filled with chilled out radiation. (Which if you think about it is freaky, but cool.) But it means that all of our helium used to be part of uranium and thorium, so it’s all busted up star stuff, none of it from the big bang.
OK, so all the “metals” on earth are genuine, vintage star stuff. And with them all sorts of interesting chemistry can occur, especially with carbon. And that is a gigantic understatement.
Carbon can form long chains, and rings, and groups of rings, without end, perhaps literally an infinite number of different molecules, especially when you throw in the occasional phosphorus, nitrogen or oxygen atom. Of course hydrogen is involved too, taking up all the carbon bonds that don’t connect with anything else. (Chemists drawing diagrams of molecules often just leave the hydrogen out of the diagrams; it’s understood to be there.) These molecules with carbon in them are called organic molecules because almost every type of molecule in living things has carbon in it; the exceptions being water, salt and air.
And of course we could not live–there could be no life–without that sort of chemistry (now whether it must be carbon that is the basis of life is another question, but it’s commoner than any of the other candidates, so I suspect all natural life in the universe will turn out to be carbon based). And thus, life would not be possible without star stuff.
I suppose if you want to express thanks that you are alive, rather than praying to a mythical god, you can thank your lucky stars.
(Well, no, that’s really an astrological reference. But I couldn’t pass up the pun.)
Well this was another in a series of science posts here, hopefully you find them interesting.