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As far as i understand, black holes radiate away energy in form of Hawking Radiation. Thus, they lose mass, i suppose. Is there a point where the mass becomes too small for the object to still be a black hole? What happens then, will it turn into a neutron star supported by degeneracy pressure? Or is it all about density, and once a black hole is formed, it will stay a black hole (i guess until it completely radiates away)?

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    $\begingroup$ No, it eventually will evaporate into nothing, but for the sun's mass black-hole you need about 10^67 years for that. So clearly nobody in the universe will out-live massive black holes. However, micro black-holes with the mass of electron will evaporate into nothing almost instantly in the blink of eye. $\endgroup$ Commented Aug 19, 2019 at 12:42
  • $\begingroup$ Related: physics.stackexchange.com/q/734825/226902 $\endgroup$
    – Quillo
    Commented Nov 7, 2022 at 12:49

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I think that once a black hole forms then that is it, because although its mass is finite, its density (in GR) becomes infinite at the central singularity. The loss of energy(mass) will then result in a shrinkage of the event horizon but no change in the black hole nature - the BH nature of an object is not determined solely by its mass, the density of the object is crucial.

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  • $\begingroup$ Apart from the very nice and simple "density" argument here, there is also an "entropy" argument: physics.stackexchange.com/a/652267/226902 $\endgroup$
    – Quillo
    Commented Nov 7, 2022 at 12:45
  • $\begingroup$ Sorry for deleting my comment; I felt frustrated because it seemed I was not understood. I think "nobody who understands" might phrasing some strangers could misinterpret as disparaging. I have asked a friends who work in physics this, and the answer has always been "we still don't know". I'll try another phrasing. Assuming black holes evaporate, has material that started falling into the black hole actually "reached" the central singularity before this occurs? Of course, my friends didn't know much; They'd be very interested in further reading, if indeed this has a settled answer. $\endgroup$
    – MRule
    Commented Mar 5, 2023 at 17:30
  • $\begingroup$ @MRule see physics.stackexchange.com/questions/3706/… among others. $\endgroup$
    – ProfRob
    Commented Mar 5, 2023 at 17:37
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The neutrons will never return once a black hole has formed and outgrown the neutron star. A black hole would exist within a neutron star until added mass increases the radius of the Event Horizon to engulf the neutron star matter. Once that happens things progress very slowly.

Hawking Radiation has still not been proved conclusively. Even if it's true there is still a delay ahead. Black holes with mass about two thirds of our Moon's upwards would be colder than the Cosmic Microwave Background and would therefore absorb energy from it faster than they would emit Hawking Radiation.

Black holes get colder as their mass increases so the CMB would be almost used up before evaporation would start and it's a hard guess as to how long that would take. Smaller black holes would evaporate more quickly for two reasons.

  1. is that the space curvature is more extreme at the Event Horizon so Hawking Radiation would occur more quickly due to a higher number of particle-pairs created.

  2. is that smaller Black Holes are predicted to have a higher temperature implying higher radiative power.

If a neutron star has a high enough angular momentum it can delay a Black Hole from forming because the centrifugal force would combat gravitational force but this would be a rare occurrence because neutron stars typically only lose mass via thermal radiation. Therefore they don't slow down by much, maybe by a second every few millenia if we're overestimating.

By gaining enough mass from its surroundings via accretion (consuming nearby planets, dust and debris etc) the object may collapse further. A neutron star's rotation can also be slowed by several mechanisms.

One would be in the emission of pulses of electromagnetic radiation if it's a pulsar type.

Another would be an "anti-glitch", which is an observed case of a reversed "glitch" - when a portion of the crust cracks and matter is tossed off which speeds up rotation. Anti-glitch causes are still under investigation.

A third would be if accrued matter is moving in the opposite direction to the star's rotation. This is called retrograde accretion (see this paper https://arxiv.org/abs/1704.06364v1) and is thought to be conclusive because slowing-down and speeding up rates are very similar in observation.

It seems for now that whatever ends up inside a Black Hole is destroyed. Evidence points to the fact that matter would choose any form rather than allow Black Hole physics to happen. It just seems to be the inevitable fate of large stars.

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  • $\begingroup$ "the neutron star will eventually run out of momentum" By what mechanism would it shed the momentum?? $\endgroup$
    – kutschkem
    Commented Feb 6, 2020 at 7:22
  • $\begingroup$ @kutschkem - Good point - bad explanation. I've edited the answer. $\endgroup$
    – Clock
    Commented Feb 6, 2020 at 10:42
  • $\begingroup$ That's better, thank you. $\endgroup$
    – kutschkem
    Commented Feb 6, 2020 at 12:45
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The BH stays a BH and radiates till it becomes a micro BH. At this point we need a quantum theory of gravity, that we still do not have... But when this becomes relevant we already have a microscopic BH, while neutron stars have (more or less) one solar mass. Note that neutron stars have an associated conserved baryonic number, BH only have mass and spin (astrophysical BH are likely to be neutral)... So, to have a NS you have to somehow recreate the baryonic number out of nothing. Photons or neutrino-antineutrino pairs in the Hawking radiation do not carry fundamental conserved charges, so it is difficult to say that the leftover are degenerate baryons.

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