I know stars fuse stuff all the way up to iron. But then fusion stops releasing additional energy at iron, which I remember from chemistry class. So I would assume stars don’t make much of anything heavier than iron. So where does everything heavier than iron come from?
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When a star makes iron it explodes and makes everything else. Very over simplified but that’s why we’re here.
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Fusion stops producing energy, but it doesn’t necessarily stop happening. That energy just has to come from elsewhere; it’s no longer self-sustaining.
It’s agreed upon that heavier elements mostly formed during cataclysmic explosions – originally it was believed to be supernovae but I think neutron star collisions are the prevailing theory now.
Gold, silver, platinum, etc, are all made during a supernova explosion. The fact that these elements are present on Earth, tells us we are a second generation star system, and that the star here before our yellow dwarf, mush have been much bigger.
There’s two main ways that elements past iron are produced in the universe.
The first happens in low-mass stars (around the mass of the sun). Here elements aren’t fused above iron, but they can form heavier elements by capturing neutrons. There’s quite a lot of free neutrons in these stars formed by nuclear reactions, every now and again one of these neutrons will be absorbed by a nucleus, turning it into a heavier nucleus. This combined with beta decay of neutrons within the nuclei to protons lets nuclei slowly climb up the periodic table. This process takes thousands of years, for an individual nucleus it will take decades for each neutron to be captured. As you get up the periodic table this gets less and less efficient, until past bismuth it effectively can’t happen because the nuclei are unstable and decay faster than new neutrons are absorbed. We call this the s-process (s for slow).
To get heavier masses we need neutrons to be absorbed faster, much faster, hundreds of times per second. For this to happen you need insane amounts of free neutrons, many more than present in the star. When this occurs you get rapid successive neutron captures which shoot the nucleus up the periodic table faster than it can decay back down. These neutron densities can be achieved in supernovae, but that only gets up to ~rubidium in the periodic table. For more massive elements we need even higher neutron densities, the kind which are only found in the mergers between neutron stars. We call this rapid neutron capture the r-process. Determining where exactly this mostly happens (supernovae or neutron stars) is a major open question in current astrophysics, though we now have very good evidence that neutron stars are the main contributor for the heavy elements.
During a supernova where a massive star collapses, iron can fuse with alpha particles to get up to nickel. And it’s the radioactive decay of this nickel which mostly powers the light we see as a supernova, minute amounts of heavier elements are also produced by this alpha fusion. Finally supernovae of white dwarfs can also act in a similar way to get up to zinc.
This Mendeleev Table of Nucleosynthesis identifies origins for elements: dying low mass stars, exploding massive stars, exploding white dwarfs, merging neutron stars, and laboratory.
Text contains links to more information.
Super novae
When the star starts to fuse iron, the energy output of the star suddenly drops, so all that energy pushing outward from the core suddenly dissappears, and the outer layers of the star slam into the core with incredible force. This has so much extra energy that it can form all the other elements up to uranium and possibly trace amounts of plutonium, neptunium, promethium, and technicium.
The explosion isn’t rapid fusion and it is more the outer layers of the star bouncing off the core.
Stars create elements up to and around Iron as a result of nuclear fusion during their lifestyle. If the star explodes as a supernova it will cast these elements into the cosmos.
Elements heavier than iron are created through neutron-capture inside of the star. Rathet than multiple small nuclei fusing together, a single large nuclei gradually grows in size and decays, grows and decays, etc… If the star explodes, they get cast out as well.
The collapsing star before it fully explodes into a supernova will crush itself enough to have one more quick fusion cycle to make the super heavies, and then they fly out in all directions, minus the white dwarves or neutron stars that remain.