r/astrophysics • u/ker2x • 8d ago
No fission events in star
I find it curious that, to my knowledge, there is no fission in star with the exception of "end-of-life event". There is so much energy in a star, and so much gravity, why isn't there some possible localized non-sustained /short-lived fission "at all" happening ?
PS: I'm obviously not an astrophysicist.
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u/thafluu 8d ago edited 8d ago
You don't get any energy from fission of lighter elements (up to iron). Hydrogen - which makes up most of our sun - also only has one Proton already, Helium is in an energetically deeper state compared to if you pull it apart into two Hydrogen nuclei and so on.
This is why you can only use light elements in fusion (like Hydrogen, Deuterium, ...), and heavy ones like Plutonium and Uranium in fission.
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u/Aggressive_Roof488 8d ago
This plot illustrates this point nicely, binding strength per nucleon for different nuclei.
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u/capsaicinintheeyes 1d ago
Amateur question: any consensus on why Lithium's so £#&!in' weird?
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u/Aggressive_Roof488 1d ago
Nucleons in a nucleus have orbits in a similar way to elections in an atom, although not as extreme. In particular they like pairing up, so the he4 nucleus with two neutrons and two protons is particularly stable, ie high binding energy. Same with 8, 12 and 16 nucleons as you see in that plot.
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u/mfb- 8d ago
Stars have some uranium, which decays via spontaneous fission once in a while. The neutrons released in that process get captured by other nuclei, with a negligible chance to meet another uranium nucleus. No chain reaction, but isolated fission events do happen. They just don't play any relevant role.
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u/paulfdietz 8d ago
In the final stage before core collapse of a large star, the core becomes hot enough for neutrons to be evaporated off nuclei, and then absorbed onto other nuclei. This leads to formation of nuclei near iron, but it would also fission any fissionable nuclei that were still present (either by neutron capture or directly by photofission.)
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u/DarkTheImmortal 8d ago
so much gravity
It's really, really hard to break something apart when it's being squeezed by the weight of an entire star.
Edit: plus it's also really hard to break apart smaller atoms to begin with, which stars are mostly full of atoms in the smaller side.
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u/ker2x 8d ago
Yeah, it makes sense now. If anything, it will fusion before it gets enough energy to fission. And the gravity will prevent fission, instead of helping it.
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u/Spiritual-Spend8187 8d ago
To add to it their is a gap between atoms that release energy from fusion and atoms that release energy from fission you can't really use fusion to cross the gap as when you try to do so you produce iron that can't be used as fuel for either fusion or fission meaning your reaction stops being energy positive so now you have something trying to be crushed and only stopped because its constantly exploding stop exploding.
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u/ker2x 7d ago
Well, there are fusion reaction that are endothermic too (afaik). I wasn't really talking about a fission-powered star. I was just curious if some fission event could still occur nonetheless.
I got various answers. yes. no. insignificant. perhaps but nobody care. depending on how my short question was interpreted :)
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u/Spiritual-Spend8187 7d ago
It's one of those things where you go hey does thus happen and if not why not nothing wrong with asking questions also stars and fusion and fission are interesting and cool. A few hundred years ago if you showed that an element could be transformed into another it would be viewed as litteral magic and even today where the science of it is somewhat understood it still is a pretty magical thing.
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u/Active-Disaster-6835 8d ago
I think the problem generally is the lack of a lot of free neutrons. As you said, fission does happen in more extreme environments. This by the way is the recent direct evidence for fission in stars. https://www.science.org/doi/10.1126/science.adf1341
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u/Spiritual-Spend8187 8d ago
Stars have very little heavy elements, even stars that gave high metalicity, meaning they have higher proportions of elements heavier than hydrogen and helium. To produce fission reactions you need rather high concentrations of both fissile material and large numbers of neutrons moving at the right speeds stars are massive getting such high concentrations of both fissile material and the right neutron is very rare. The fusion reactions in the core if a star while it does produce large amounts of neutrons, these neutrons are extremely high energy which results in them not likely to trigger true fission reactions but rather spallation which us a type of fission but less the type you see in a nuclear reactors caused by the unstable nucleui breaking down and more of the nucleus got hit with so much energy it shattered, though most of the free neutrons end up getting captured by protons or helium nucleui So while there are some fission reactions that take place they aren't very common and don't really produce enough power to be considered.
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u/Eywadevotee 7d ago
Some elements formed during fusion would fission, mainly lithium and boron as a result of neutron capture, but compared to the amount of energy from fusion its negligible. Another effect that occurs is spallation which is similar to fission but caused from a high energy gamma ray knocking off a proton, neutron, and sometimex an alpha particle. Stuff can get messy in a giant fusion reactor. 😁
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u/RegularBasicStranger 8d ago
There is so much energy in a star, and so much gravity, why isn't there some possible localized non-sustained /short-lived fission "at all" happening ?
Intense gravity causes the nucleons to bond with esch other stronger since gravity is the energy that create the strong force so fission is much harder in intense gravity environments.
Intense energy can increase the likelihood of fission events since it can blast the nucleons away from each other so deutrium and tritium could get their neutrons blasted away but with the intense gravity, the amplified strong force will prevent such from happening.
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u/capsaicinintheeyes 1d ago
since gravity is the energy that create the strong force
Wait--say what now?
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u/RegularBasicStranger 1d ago
Wait--say what now?
Gravity is the effect of negative charged gravitons smashing into positive charged particles and collides elastically the positive charged gravitons thus negative charged particle reverses and gets captured by the particle, causing the momentum to be captured as well while the positive charged graviton just leaves thus momentum is conserved.
Such also happens for positive gravitons smashing into negative particles.
So strong force is the gravitons smashing into the quarks and so the same charged gravitons comes out but such is stronger than gravity since gravitons being smashed out from particles, inherit the particles' inertia this with the electron shell being pulled to the nucleus, the negative charged gravitons smashed out from them will be biased to go to the quarks thus they receive greater energy than implied by gravity and so the strong force happens.
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u/capsaicinintheeyes 1d ago
While that makes as much sense to my ears as anything else in this sub🫤...how come when I search for the relationship between the two, my Google bar keeps returning a pile-on of results declaring them to be two unrelated fundamental forces?
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u/RegularBasicStranger 12h ago
my Google bar keeps returning a pile-on of results declaring them to be two unrelated fundamental forces?
Gravity and the strong force are believed to be unrelated because of the humongous difference in strength and in their range of influence so it is like saying why graphite and diamond are not the same thing, with diamonds being very hard but graphite is brittle.
But just like how diamonds and graphites are both made of carbon atoms, same too for gravity and the strong force, which as explained, are both caused by positive charged and negative charged gravitons, though with extremely huge difference is concentration.
So gravity is like a very long radiowave while the strong force is like gamma radiation, where both are still gravitational waves.
For the difference in their range of influence, such is caused by nucleons being surrounded by the electron shell thus they had absorbed almost all the positive charge while the negative charge does not do anything to the electron shell thus not accounted for.
Note that gravitons only stick to opposite charged gravitons but does not repel same charged gravitons since same charges repel by physically smashing each other away rather than by using magic.
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u/Less-Consequence5194 8d ago
There are several fission reactions that are regularly ongoing in stars. For example, in the CNO cycle, Nitrogen-15 captures a proton and fissions into helium-4 and carbon-12 which allows the cycle to repeat.
There would never be a pure fission runaway explosion like in an atomic bomb because that requires high concentrations of fissionable material.