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What would happen to a star...


Flybuild
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If a star somehow survived the iron fusion stage, than what would happen next? What stage would kill the star? Would anti-matter form in its core? What would a cutaway model look like? How much radiation would it emit? (you get the point, just whats the answer to ALL the questions that come with overcoming iron in a star be?)

My theory about what the death of such a star be, would be when it fused so many atoms together, that the density of the atoms is that of a black hole, obveusly then eating itself.

Edited by Flybuild
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A star couldn't overcome iron fusion: because iron has the most tightly bound nucleus of all atoms. Therefore fusing iron actually takes more energy than it gives out, this is why stars explode when they reach that point: outwards radiation pressure falls, the star implodes and when the outer regions collide with the extremely dense collapsed core, they bounce back and boom: supernova.

Anti-matter wouldn't form (some random anti-matter always form, but is instantaneously anihilated by normal matter) as it is not linked to fusion.

Answering the other questions is harder because it simply can't happen. The star can do nothing but collapse once fusion stops.

Edited by Gaarst
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1 minute ago, Gaarst said:

A star couldn't overcome iron fusion: because iron has the most tightly bound nucleus of all atoms. Therefore fusing iron actually takes more energy than it gives out, this is why stars explode when they reach that point: radiation outwards pressure falls, the star implodes and when the outer regions collide with the extremely dense collapsed core, they bounce back and boom: supernova.

Anti-matter wouldn't form (some random anti-matter always form, but is instantaneously anihilated by normal matter) as it is not linked to fusion.

Answering the other questions is harder because it simply can't happen. The star can do nothing but collapse once fusion stops.

Hypothetically speaking, if some miracle let the star pass.

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Well let's say some magic force supports the iron core against collapse, then the shells surrounding it would continue fusing lighter elements and it would carry on as a giant star. Eventually what you'd be left with is a giant ball of iron plasma, possibly with an outer of lighter elements, that should collapse into a neutron star or black hole but Magic stops it doing so.

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After the star reaches the iron stage and fusion comes to a halt (and after the resultant supernova), the star's final fate depends on its mass. If the mass remaining is less than about 1.4 solar masses, the star will shrink until repulsion between atoms' electron shells produces sufficient outward pressure to stabilize it (the fancy term for this is "electron degeneracy"). Now you've got a white dwarf. But if the remnant is more than 1.4 solar masses, things get weird--electron degeneracy isn't enough, and the star continues to collapse until it's about the size of a city. In the process, it becomes energetically favorable for protons and electrons to combine to form neutrons--and voila! You've got a neutron star.

By the way, science fiction has it wrong--"neutronium" (i.e. matter composed of super-dense neutrons) isn't a solid. It's a fluid with really strange properties. Also, neutrons outside of an atomic nucleus are extremely unstable at room temperature and pressure! Try to take neutronium out of a star and build a spaceship with it, it will explode and the neutrons will disintegrate back into protons and electrons.

If the stellar remnant is massive enough, degeneracy pressure between neutrons still isn't enough to halt the collapse of the star......and you get a black hole. In theory, neutron stars that get too massive could decay further into quark stars, electroweak stars, and others that rely on ever deeper and more fundamental forces, but those are entirely theoretical.

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Slight fix to above : White dwarf don't emerge from stars that have iron core. They emerge from lower mass stars that doesn't even have enough pressure to create them. The only "iron white dwarf" is the iron core itself before re-ignition.

For OP : Then you can have a star core made of Uranium or something. The pressure can be enough for that kind of thing, because the main reason for SN II is actually because of not enough energy/outward pressure (acting in reverse to gravitational compression/inward pressure) left immediately after initial iron flash burning. Fusion is mainly due to gravitational compression, equilibrium is mainly from produced energy (in form of photon pressure).

EDIT : Actually, that's for pair-production SN, where the photon are so energetic they immediately create particle-antiparticle pair, reducing photon pressure. Standard SN II happens when the iron core suddenly shrinks, due to change from being held by electron degeneracy to neutron degeneracy (white dwarfs are about the size of Earth while neutron stars are the size of a city - plenty of distance to fall). Then the envelope fall off, much like you pour water onto solid surface - they splatter around, and that's the blast you saw. If enough matter coming in during this phase is assimilated to the neutron star, it can collapse again into a black hole.

 

 

 

Edited by YNM
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4 hours ago, WedgeAntilles said:

After the star reaches the iron stage and fusion comes to a halt (and after the resultant supernova), the star's final fate depends on its mass. If the mass remaining is less than about 1.4 solar masses, the star will shrink until repulsion between atoms' electron shells produces sufficient outward pressure to stabilize it (the fancy term for this is "electron degeneracy"). Now you've got a white dwarf. But if the remnant is more than 1.4 solar masses, things get weird--electron degeneracy isn't enough, and the star continues to collapse until it's about the size of a city. In the process, it becomes energetically favorable for protons and electrons to combine to form neutrons--and voila! You've got a neutron star.

By the way, science fiction has it wrong--"neutronium" (i.e. matter composed of super-dense neutrons) isn't a solid. It's a fluid with really strange properties. Also, neutrons outside of an atomic nucleus are extremely unstable at room temperature and pressure! Try to take neutronium out of a star and build a spaceship with it, it will explode and the neutrons will disintegrate back into protons and electrons.

If the stellar remnant is massive enough, degeneracy pressure between neutrons still isn't enough to halt the collapse of the star......and you get a black hole. In theory, neutron stars that get too massive could decay further into quark stars, electroweak stars, and others that rely on ever deeper and more fundamental forces, but those are entirely theoretical.

What happens to red dwarfs (when they die, which they will, but the universe is too young for that to happen)

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Just now, RainDreamer said:

Something I wonder is, how are elements heavier than Iron get fused?

Because of the enormous amount of energy released during the supernova: the nuclei collide so violently that they fuse together making elements heavier than iron. Note that this process absorbs more energy than it releases.

Though this process doesn't explain the formation and quantites found of some elements, especially the heaviest naturally found metals (gold, lead, uranium, etc...)

Some theories mention that some of the heaviest elements are formed when neutron stars collide or with hypernovae.

Edited by Gaarst
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13 hours ago, fredinno said:

What happens to red dwarfs (when they die, which they will, but the universe is too young for that to happen)

Nothing. They just fizzle like a burned-out campfire, leaving behind a dead cooling ball of helium.

 

17 hours ago, YNM said:

Slight fix to above : White dwarf don't emerge from stars that have iron core.

It's entirely possible an iron-core star could leave a white dwarf behind. It depends on how much mass is left over. Even if the supernova does crush the core remnant into a neutron star, the neutron star won't be stable if it's not massive enough; insufficient mass means insufficient inward pressure, which means the neutrons become unstable and decay into protons and neutrons.

The problem is, supernovas can be very unpredictable. There are observed cases where nearly-spherical stars suffered extremely lopsided supernovas. In some cases most of the mass of the exploding star went to one side. Science isn't sure how that happened.

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As mentioned, iron fusion isn't impossible by any stretch, but it is indeed endothermic. Thus a star fusing iron would fail to keep itself alive by that process. The iron at the core would continue to be compressed, eventually all globbing together into one massive pile of nucleons, and since the protons and electrons would come into contact and merge, we get left with a neutron star.

So let's imagine a scenario in which we've cast a magic spell on the star that prevents atoms from being thusly crushed. I predict that the region of primary energy production will have to be the series of shells around the core fusing different elements. The star would have to continuously shrink in order to fuel these, and thus the iron core would continue growing. Eventually I foresee, when all the usable fuel has run out, the star will have turned into iron all the way up to the surface.
Inside, some iron nuclei would probably end up fusing due to sheer pressure and effects along the lines of quantum tunneling. This wouldn't produce heat or in any other way induce further fusion, so it would basically only happen randomly like radioactive decay. The rate would have to gradually increase over time as more of the star is converted into heavy elements. Eventually the heavy elements being formed would themselves be radioactive and try to decay back into lighter ones, and in time the rates of fusion and decay would reach an equilibrium. From this point, elements in the star would continually reconfigure themselves until the whole star is composed of the heaviest stable element: lead.
So my hypothesis is that the end result is a massive, cold, compressed, very smooth sphere of lead. Probably electron-degenerate lead, like a black dwarf made of lead instead of carbon and hydrogen.

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8 hours ago, WedgeAntilles said:

Nothing. They just fizzle like a burned-out campfire, leaving behind a dead cooling ball of helium.

 

It's entirely possible an iron-core star could leave a white dwarf behind. It depends on how much mass is left over. Even if the supernova does crush the core remnant into a neutron star, the neutron star won't be stable if it's not massive enough; insufficient mass means insufficient inward pressure, which means the neutrons become unstable and decay into protons and neutrons.

The problem is, supernovas can be very unpredictable. There are observed cases where nearly-spherical stars suffered extremely lopsided supernovas. In some cases most of the mass of the exploding star went to one side. Science isn't sure how that happened.

But stars that small can't fuze to iron anyways. Our own star can only get to the Carbon-Oxygen-Nitrogen cycle before collapsing.

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Uhhhh.....think there might be crossed wires here; my last post had replies to two different people.

True, red dwarf stars don't reach the iron fusion stage. But with other stars that are big enough to reach iron fusion, the end result depends on the mass of what's left over after the Big Kaboom. It takes around a hundred billion gravities to crush atoms in on themselves and force protons and electrons to merge into neutrons; anything less and you get a white dwarf instead of a neutron star.

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Just now, fredinno said:

Stars fuze in this order: Hydyrogen>Helium>Carbon/Nitrogen/Oxygen>Silicon>Iron. Then Kaboom.

Thanks. 

And here's sone general stuff people know about stars.

But when a star hits iron, it can't fuse it. Simply can't. You're gonna need magic. The reason it dies is because the energy balance is upset. The gravity starts to push in, because there's less energy from the core pushing out. Then the star gets much denser, and it does fuse, but mostly, if not entirely, the extra hydrogen and other elements in the outer layers. Then it gets a huge burst of energy. But our sun will die differently. It'll expand. Then the outer layers will just float off, leaving a dead core.

Probably got a bit wrong. I don't care much, iron is plainly a star killer.

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On 1/15/2016 at 0:59 AM, YNM said:

Dead red dwarfs will leave a helium white dwarf. Which haven't been observed...

I guess in the heat death of the universe, we can merge these things together to form helium-burning stars...

2 hours ago, parameciumkid said:

Isn't there also a stage with sodium and magnesium before the iron stage?

I simplified it enormously. https://en.wikipedia.org/wiki/Stellar_nucleosynthesis#Key_reactions There are actually two main reactions before the silicon stage. It doesn't really matter much for the star itself, because these fusion stage 'eras' get smaller and smaller in length as time goes on.

massive_star_struct.jpg

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On 1/15/2016 at 3:57 AM, WedgeAntilles said:

It's entirely possible an iron-core star could leave a white dwarf behind. It depends on how much mass is left over. Even if the supernova does crush the core remnant into a neutron star, the neutron star won't be stable if it's not massive enough; insufficient mass means insufficient inward pressure, which means the neutrons become unstable and decay into protons and neutrons.

This (theoretical ?) paper says otherwise though. The ball of degenerate iron (because it's not fusing) can keep itself stable - held only by electron degeneracy - until it reaches the Chandrasekhar limit, then it collapses into a neutron star, with the shell falling in and splat against the neutron star surface. If the iron can keep itself stable enough, maybe an actual iron fusion will take place - ultimately you'll be left with an "onion" star with some heavy-atom (say, uranium ? Should count it I suppose) core.

9 hours ago, fredinno said:

I guess in the heat death of the universe, we can merge these things together to form helium-burning stars...

Well, dark energy will work away from your favor...

Edited by YNM
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On 1/17/2016 at 0:45 AM, YNM said:

This (theoretical ?) paper says otherwise though. The ball of degenerate iron (because it's not fusing) can keep itself stable - held only by electron degeneracy - until it reaches the Chandrasekhar limit, then it collapses into a neutron star, with the shell falling in and splat against the neutron star surface.

And, as I understand it, that "splat" is what triggers a type II or IIb supernova. If that leaves behind a remnant of less than 1.4 solar masses......then what?? Presumably a neutron-star remnant of less than 1.4 won't be stable.

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16 hours ago, WedgeAntilles said:

And, as I understand it, that "splat" is what triggers a type II or IIb supernova. If that leaves behind a remnant of less than 1.4 solar masses......then what?? Presumably a neutron-star remnant of less than 1.4 won't be stable.

Probably the entire thing just breaks apart in a supernova, leaving behind a white dwarf.

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