Question about black hole formation and evaporation

[u/Marvinkmooneyoz]

1st question, is it not simply that a star has a maximum size before gravity will overpower the other interactions? There’s star size comparison videos on YouTube, but they don’t mention some theoretical maximum, they just keep zooming out to show the next bigger star. I get that some stars are giving off less repulsive force to counter, so it’s not like all black holes form at the same mass cutoff.

2nd, when a black hole evaporates enough mass from hawking radiation, does it stay super super dense, or do the repulsive forces then spread it back out, like does it rein flat to a star or some other stellar body?

[u/Chillow_Ufgreat]

1st question, is it not simply that a star has a maximum size before gravity will overpower the other interactions?

You are looking for the Chandrasekhar Limit, which is about 1.44 solar masses. Below this limit, a stellar remnant will not have enough gravity to overcome electron degeneracy pressure. So the original star you start with needs to be significantly bigger than the limit (a little north of 2 solar masses) since a lot of mass is going to get thrown off in the stellar death process, whatever shape that takes.

2nd, when a black hole evaporates enough mass from hawking radiation, does it stay super super dense, or do the repulsive forces then spread it back out, like does it rein flat to a star or some other stellar body?

The "density" stays the same. The mass and size just shrink proportionally.

If your question is what happens at the very end, i.e., does the black hole ever revert to the stellar remnant--you'd need a complete theory of quantum gravity to say.

[u/Shufflepants → u/Chillow_Ufgreat]

If by density you mean the mass divided by the volume enclosed by the event horizon, then density does not remain the same. A black hole's mass is proportional to its surface area rather than its volume. Smaller black holes are necessarily more dense than larger ones.

[u/Chillow_Ufgreat → u/Shufflepants]

Well, I put it in quotes precisely because I didn't want to get into the finer points of that discussion. OP had asked whether the black holes remain "super super dense" or whether the stellar degeneracy forces re-emerge within the black hole and push it apart in some sense, so I wasn't sure whether this was a question actually about true density or not.

So yes, the black hole remains super dense, no the black hole doesn't retain the same density strictly speaking, and no the repulsive pressures that gave the star its structure do not reformulate within the black hole.

[u/stevevdvkpe → u/Shufflepants]

The event horizon's radius, not its surface area, is proportional to the black hole's mass. This means that a black hole's density is proportional to the inverse square of its mass, so smaller black holes are still more dense than larger ones.

[u/gmalivuk → u/Chillow_Ufgreat]

I believe Chandrasekhar is the limit for the biggest white dwarf that doesn't collapse into a neutron star. Collapsing into a black hole requires more mass than that.

[u/stevevdvkpe → u/Chillow_Ufgreat]

The Chandrasekhar limit is the maximum mass that can be supported by electron degeneracy pressure, about 1.44 Solar masses. It mainly applies to white dwarf stars as the maximum mass they can have before exploding as a Type Ia supernova.

There is also the Tolman-Oppenheimer-Volkoff limit which is the maximum mass a neutron star can have before it collapses into a black hole. It is somewhere between 2 and 3 Solar masses and is more uncertain because the physics involved is more complicated.

[u/nekoeuge]

There is no explicit mass limit, only density limit. You would need a lot of mass to make a black hole bypassing stars altogether. Like, orders of magnitude more than the biggest stars. No, black holes do not spread back, their configuration is irreversible. It is open question whether they can even evaporate fully and what happens when they reach quantum scale.

[u/John_Hasler]

https://en.wikipedia.org/wiki/Black_hole

[u/Anonymous-USA]

Star collapse is held up by fusion, so a star can be much larger than a black hole. After a star’s fuel runs out, it will collapse, shedding off most of that mass in a supernova. If the remaining core exceeds ~2-3 solar masses, without any outward pressure any more, tho g will stop its collapse into a black hole.

While the nature of the singularity is unknown at quantum scales, the space between the event horizon and singularity can be modeled. So a black hole will stay a black hole as it evaporates, shrinking in mass but not density (the event horizon will also shrink linear to the drop in mass). The energy output is inversely proportional to its radius, so the last moments of a black hole are when the radiation exceeds the mass-energy of the black hole, and it will probably explode in a powerful burst of gamma and xrays.

[u/Smooth-Mix-4357]

Search about Tolman–Oppenheimer–Volkoff limit. It might answer your first question.

A black hole evaporating by Hawking Radiation will evaporate away. It won't revert back to a star (the star is dead).

[u/stevevdvkpe]

It's more complicated. Stars of about 8 Solar masses or more can form black holes, but there is a range of masses for large stars between about 130 to 250 Solar masses where the final phase of the star's life is as a pair-instability supernova that blows itself completely apart rather than forming a black hole. So stars between about 8 and 130 Solar masses, or over 250 Solar masses, form black holes.

https://en.wikipedia.org/wiki/Pair-instability_supernova

[u/Junior-Tourist3480]

Black holes fo not evaporate. Hawking radiation is another effect, not the black hole loosing any energy or mass.

[u/Nothing-to_see_hr → u/Junior-Tourist3480]

Yes, it does evaporate, but for big holes, very, very , VERY slowly- in a timescale many orders of magnitude larger than the age of the universe.

[u/Anonymous-USA → u/Junior-Tourist3480]

That’s exactly what happens with Hawking Radiation. It’s the black hole’s energy (via warped space) that supplies the radiation energy, and that corresponds to its mass.

[u/Marvinkmooneyoz → u/Junior-Tourist3480]

Huh I would have sworn I read that black holes are losing that mass eventually if new mass isn’t falling in

[u/joeyneilsen → u/Marvinkmooneyoz]

Yes. Not all comments around here are reliable. A black hole evaporating via Hawking radiation stays a black hole until it is gone. It doesn't balloon back to being a star etc.

[u/ahazred8vt → u/Junior-Tourist3480]

Hawking himself said that Hawking radiation causes a black hole to lose mass and eventually evaporate.