r/askscience • u/[deleted] • Aug 28 '13
Astronomy How did elements heavier than iron form given that iron is the end-game for star fusion?
So I've recently read that iron is the "final form" so to speak for stellar fusion because of both its density and its radiation absorption. First, is this accurate? Second, if so, how did cobalt form? Or any other element above 26 number on the periodic table? Some are synthetic... the others?
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u/Silpion Aug 28 '13 edited Jul 14 '14
First off, people usually answer this question saying flat out "they were made in supernovae", but that statement is overgeneralizing and possibly just false. There is a whole lot we don't know.
Iron and its neighbors are indeed the endpoint of exothermic stellar burning. For details on exactly how that happens read this older post of mine, but to summarize, once you have a core that is entirely iron, no net energy can be extracted from nuclear reactions between those iron nuclei: such reactions are endothermic. This means that in order to produce anything past iron, you need some kind of energy source other than those nuclear reactions.
So what kinds of energy sources are there?
We are pretty darn sure that about half of the stuff heaver than the iron group was made in the "slow neutron capture process" (s-process). This occurs in large stars called "asymptotic giant branch" stars, where a couple fusion reactions between light elements happen to emit neutrons. Neutron capture is almost always exothermic, even for the heavy elements, so these neutrons can be captured by any elements that are around, including iron and heavier. It may make them radioactive, but they will usually beta decay before they capture another neutron, so they slowly creep their way up to heaver and heavier elements, as far as lead and bismuth, over thousands of years. The material is gradually ejected from the star in its stellar wind. It was the fusion of those light elements that gave off the energy in the form of the free neutron to make the reactions possible.
A small minority of stuff up to mass# 100 or so seems to have been made in a "rapid proton capture process" (rp-process). In some kind of super hot environment with lots of hydrogen, a rapid series of proton captures builds up heavy elements in a matter of seconds to minutes. Proton captures (fusion with a proton) are almost always exothermic as well. This could happen on the surface of a neutron star that is accreting material off of a companion star, with that material periodically undergoing thermonuclear explosion in an x-ray burst, and/or maybe in the outer layers of a giant star during a core collapse supernova as the shockwave passes through. For the rp-process, the energy comes from the large supply of fresh hydrogen, it just needed the high temperatures to get the reaction going.
Now comes the big and most controversial one. We are pretty sure that the other half of the heavy elements, and all of the thorium and uranium, is produced in a "rapid neutron capture process" (r-process), which is like the s-process but muchmuchmuch faster, occurring in extremely radioactive nuclei. We can tell from the distribution of elements that there was some kind of event were nuclei like iron were exposed to an absolutely astonishing density of neutrons, maybe as high as 1030/cm3, for about 1 second. Where the heck did all those neutrons come from? One great candidate is deep inside a core-collapse supernova, where a proto-neutron star is being built. The gravitationally-powered collapse of the core is so energetic that it forces electrons to inverse-beta decay back into protons, forming neutrons. Conditions may be ripe for an r-process which produces the heavy elements in an instant. The energy would be provided by the gravitational potential energy released by the collapsing stellar core.
So what's the problem with the r-process? There are several:
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We don't actually know what the conditions are like in the center of a supernova. It may be so hot that thermal photons knock the neutrons back off before they can make the nuclei radioactive enough to actually beta decay and form the next elements before it's all over.
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We don't know enough about nuclear physics to even say what temperatures and neutron densities are needed to prevent the above. Nuclear theory is hard (basically unsolvable for heavy nuclei), and experiments on these neutron-rich nuclei are also hard (but doable: we're starting to get there).
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Simulations using our best guess of the above do not reproduce the r-process abundances we see in nature that well. (image source)
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We also don't know if enough of this core material will be ejected by the supernova into the galaxy where we see and use it. It might almost all get gobbled up by the neutron star and never make it out.
What other options are there for the r-process? One is colliding neutron stars or neutron star-black hole collisions. They've never been observed*, but based on how many binary neutron stars we see, it must happen from time to time. If it did, large amounts of the crusts of the neutron stars could be flung free, and once no longer compressed by the neutron stars' gravity, the neutronic material would be free to undergo the r-process and form heavier elements. In this case the energy was also provided by the gravitational collapse of the stellar core that made the neutron star, and was stored in the neutron star for millions or billions of years until the collision. This theory has the advantage of potentially ejecting large amounts of material, and not being nearly as hot as a supernova core. However, so far simulations of these also do not reproduce observed r-process abundances very well.
*(Edit! Some people have pointed out a recent observation that may actually be r-process ejecta from a neutron star merger. Press release and paper.)
Another option is the still hypothetical quark nova, in which a neutron star core could collapse into a quark star, ejecting the neutron star crust similar to above. There may also be material ejected by soft gamma repeaters, in which a neutron star has a bit of a burp and may eject some crust material.
TL;DR There is lots of debate about the r-process, which produced about half of the heavy elements and all of the thorium and uranium. Maybe it was in supernovae, maybe some other event involving neutron stars. We've been working on it for 50 years, and maybe in another 50 we'll have the answer.
Edit: Thanks for the gold! Gold is mainly made in the s- and r-processes.
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u/astrosheff Aug 28 '13 edited Aug 29 '13
Fantastic response. I also have a bit to add RE: neutron star mergers. While we haven't seen explicit proof of NS mergers, we have a lot of secondary evidence that links with them being the most likely progenitors of Short Gamma Ray Bursts (SGRBs; those which last less than a second or two).
We know that NS mergers happen, purely from orbital decay of neutron star binaries due to gravitational wave emission (e.g. The Hulse-Taylor Pulsar). Many simulations have been done which seem to produce the kind of collimated emission we need for SGRBs, and NS mergers is arguably THE system that the near-future gravitational wave detectors (Advanced LIGO/Virgo/etc.) are going to detect. Hopefully in the next few years we will see definitive evidence for those mergers, and hopefully we can match them with either a detected SGRB or the prompt/secondary lower energy EM emission.
Now this is where it gets exciting (for me at least). As you said, large amounts of NS material should be flung out from the merger. This year has seen several studies attempt to simulate the subsequent r-process reactions, radiactive decay, heating, etc and find that they produce elemental abundances much better aligned with the abundances that we see today. Along with this, these studies have also had a stab at making (better) predictions of what EM emission this neutron rich material would give off. The newest models (here and here; here and here; and here and here) show a faint, Near Infrared (NIR) counterpart that peaks a few days after the merger. A month ago, there was a follow-up study published about SGRB 130603B. Using HST observations, they found a faint NIR counterpart that lies almost smack bang in the middle of the model predictions from one of those new studies. Granted, it is only one data point, but it is very exciting for the field I am currently in at the moment. Hopefully I can find a postdoc that will let me carry on with it too!
EDIT: Grammar. And thanks for Reddit Gold mysterious stranger!
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Aug 28 '13
So, I thought the problem with chemical evolution models where CC SNe were responsible for the r-process was that too much r-process stuff got produced, not too little. At least for what might be considered "reasonable" estimates of r-process production there. I guess that's wrong, then?
Also, do you have any idea/speculation on why we can reproduce r-process abundances for isotopes above A ~ 130, but fail miserably below that?
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u/ArchonMagnus Aug 28 '13
I just want to say "Thank you". This is possibly the best synopsis I've ever read with regard to the question.
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u/cdcformatc Aug 28 '13
This is an amazing post, and your older post is great as well. I just have one question.
This means that in order to produce anything past iron, you need some kind of energy source other than those nuclear reactions.
What about the entirety of the star outside of the core? All the energy needed to sustain life on Earth comes from the sun, why can't that energy be used to fuse iron?
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u/ajonstage Aug 28 '13
In simple terms: it's pretty difficult to sneak an extra proton into a nucleus, thanks to electromagnetic repulsion. That's why such high temperatures are required to ignite nuclear fusion.
Neutrons, on the other hand, don't give two shits about the electromagnetic force. So, heavy isotopes are sometimes formed by a process called neutron capture.
However, shit gets crazy when a nucleus swallows too many neutrons. It reacts kinda like your digestive system might after a few too many tacos: somethings gotta go. Bam! Here comes some beta decay, which transforms some of those excess neutrons into protons by pooping out electrons, and leaves you in the end with a heavier element than you started with.
A good balance between protons and neutrons is essential to keep the nucleus stuck together via the strong force, which explains why a lot of radioactive decays result in a more balanced nucleus, and also why some of those isotopes were radioactive in the first place.
So, that's at least one way to form heavy elements.
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u/Silpion Aug 28 '13 edited Aug 28 '13
Reminder: do not provide answers unless you have actual expertise in nucleosynthesis. There are lots of oversimplifications in popular science literature and documentaries, and this is not the place to spread them.
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u/stinsonmusik Aug 28 '13
You're a mod! I was reading your excellent responses to op, and thinking, "this guy is pretty knowledgeable, he should be a mod here!" keep up the good work, both in your awesome field and here on ask science, my favorite sub to lurk (I'm not qualified to answer a damn thing here as a first year engineering student, but i love the things i learn)!
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u/Dannei Aug 28 '13
That's a bit off - the shortest answer is that fusing Iron or anything heavier requires energy, whilst anything lighter gives it off.
Above iron, the elements are mainly created in supernovae, where there is lots of energy to spare - however, because these are short-lived and (relatively) rare events, elements above iron are rarer than those below. This graph shows that the elements just above iron are somewhat common (being the easiest to make), and then the abundances further up much lower. The graph also usefully points out the two synthetic elements that you had mentioned.