Is it possible for a giant star to have a black hole inside it?

[Thadriel]

TL;DR Summary

For an extended period of time?

I’m sure that when a star is in the process of becoming a black hole, there must therefore be one inside it at some point during the process (correct me if I’m wrong on that). But if so, how long does that take? Could there exist a supergiant star that has a black hole inside it for a long period of time, say, thousands of years, before fully collapsing?

Is it possible for a black hole to be in a star with long term stability, with the star just not collapsing entirely? Like maybe it spins so fast that the outside can stay away from the event horizon?

Thanks for insight.

 

[Orodruin]

No.

 

[Thadriel]

No.

So what I gather from this is that if a star begins to collapse into a black hole (or neutron star), there is simply no going back?

 

[russ_watters]

I’m sure that when a star is in the process of becoming a black hole, there must therefore be one inside it at some point during the process (correct me if I’m wrong on that). But if so, how long does that take? Could there exist a supergiant star that has a black hole inside it for a long period of time, say, thousands of years, before fully collapsing?

I would have guessed minutes or hours, but google tells me less than a second.

Is it possible for a black hole to be in a star with long term stability, with the star just not collapsing entirely? Like maybe it spins so fast that the outside can stay away from the event horizon?

No. It's tough to picture what you are imagining and isn't clear how much thought you've put into this; if it's spinning that doesn't help for matter at the poles (it still falls toward the center). And "spinning" so fast it can't collapse is just another word for "orbiting". Such a thing could not even be a star because if it isn't trying to collapse under gravity it can't be undergoing gravity-induced fusion. That's more like rings.

So what I gather from this is that if a star begins to collapse into a black hole (or neutron star), there is simply no going back?

Right.

 

[Ibix]

So what I gather from this is that if a star begins to collapse into a black hole (or neutron star), there is simply no going back?

Yes. A star's material is not in orbit nor held in a solid structure by internal forces, so if a black hole formed it would quickly swallow the bulk of the rest of the star because there's not much stopping it doing so.

 

[Baluncore]

Is it possible for a black hole to be in a star with long term stability, with the star just not collapsing entirely?

No. The event horizon would immediately expand outwards at the speed of light, until the BH included all the available material of the star.

 

[Ibix]

No. The event horizon would immediately expand outwards at the speed of light, until the BH included all the available material of the star.

I'm not sure 'speed of light' is a useful measure here, is it? Event horizons are null surfaces whether they are growing or not, so in some senses are always moving at the speed of light. I agree growth will be very rapid - I'm just not clear on how one might quantify the speed and for whom.

 

[Orodruin]

I'm not sure 'speed of light' is a useful measure here, is it?

It is not. As you mentioned, the event horizon is a null surface by definition. The stellar material would be falling into the black hole and as a result the black hole would grow, but there are several problems in ascribing a speed of growth to the event horizon.

 

[Lord Crc]

According to Wikipedia there's some speculation this might have happened in the very early universe, but I can't judge how reasonable this is.

https://en.wikipedia.org/wiki/Quasi-star

 

[ohwilleke]

I would have guessed minutes or hours, but google tells me less than a second.

As a concrete analogy, which isn't precisely on point, but should be similar in order of magnitude of its duration, the merger of two intermediate sized black holes detected by gravitational wave detectors on May 21, 2019 at a distance of 5 parsecs away form Earth (about 16.3 light years and hence from an event that actually took place about 16.3 years before it was detected) took about 0.1 seconds. This is consistent with the post from @russ_waters quoted above.

NASA asserts that "A stellar-mass black hole, with a mass of tens of times the mass of the Sun, can likely form in seconds, after the collapse of a massive star."

And, it is inevitable that a black hole will once the conditions for the formation of a black hole are met:

According to Einstein's theory, for even larger stars, above the Landau–Oppenheimer–Volkoff limit, also known as the Tolman–Oppenheimer–Volkoff limit (roughly double the mass of our Sun) no known form of cold matter can provide the force needed to oppose gravity in a new dynamical equilibrium. Hence, the collapse continues with nothing to stop it. Once a body collapses to within its Schwarzschild radius it forms what is called a black hole, meaning a spacetime region from which not even light can escape. It follows from general relativity and the theorem of Roger Penrose[6] that the subsequent formation of some kind of singularity is inevitable.

Citation #6 in the quotation above is to Penrose, Roger (1965-01-18). "Gravitational Collapse and Space–Time Singularities". Physical Review Letters. American Physical Society (APS). 14 (3): 57–59. Bibcode:1965PhRvL..14...57P. doi:10.1103/physrevlett.14.57. ISSN 0031-9007.

Describing the length of time for this requires a rigorous definition to provide a rigorous answer, however.

Of course, the times being discussed in this thread are the times from the star reaches the "tipping point" until the process of black hole formation being completed, i.e. from the "straw that broke the camel's back moment". It can, of course, take millions or billions of years for a star to reach that tipping point.

Also, when one is talking about the duration of this process, this sloppy question and correspondingly sloppy answer, is implicitly talking about the duration of the event from the perspective of an observer so distant that gravitational time dilation from the forming black hole itself is negligible (e.g. an observer on distant Earth watching in a telescope as the star disappears).

Gravitational time dilation in the immediately proximity of the event greatly slows down the passage of time in a highly observer location specific way due to General Relativity, so it isn't rigorously correct to state that this process happens in any specific duration of time, without specifying the location of the observer whose rate of time passage you care about.

Being punctiliously careful about precisely which observer's passage of time is relevant to the physics calculations you are doing is a critically important conceptual task in any situation that involves very strong gravitational fields. Every now and then you see debates between scientists over papers one of them has written, in published criticisms of the other person's papers, asserting that the wrong observer's time has been used in calculations, causing the original calculations to be fatally flawed.

 

[Thadriel]

According to Wikipedia there's some speculation this might have happened in the very early universe, but I can't judge how reasonable this is.

https://en.wikipedia.org/wiki/Quasi-star

But would that be stable, or a quasi-star so ridiculously massive that it just takes a long time to collapse? (according to a distant observer)

The Wikipedia article says when they'd run out of fuel they'd "dissipate, leaving behind the intermediate mass black hole." I do not understand what this means. Is that like evaporating? Should that stuff not fall into the black hole?

As a concrete analogy, which isn't precisely on point, but should be similar in order of magnitude of its duration, the merger of two intermediate sized black holes detected by gravitational wave detectors on May 21, 2019 at a distance of 5 parsecs away form Earth (about 16.3 light years and hence from an event that actually took place about 16.3 years before it was detected) took about 0.1 seconds. This is consistent with the post from @russ_waters quoted above.

NASA asserts that "A stellar-mass black hole, with a mass of tens of times the mass of the Sun, can likely form in seconds, after the collapse of a massive star."

And, it is inevitable that a black hole will once the conditions for the formation of a black hole are met:
Citation #6 in the quotation above is to Penrose, Roger (1965-01-18). "Gravitational Collapse and Space–Time Singularities". Physical Review Letters. American Physical Society (APS). 14 (3): 57–59. Bibcode:1965PhRvL..14...57P. doi:10.1103/physrevlett.14.57. ISSN 0031-9007.

Describing the length of time for this requires a rigorous definition to provide a rigorous answer, however.

Of course, the times being discussed in this thread are the times from the star reaches the "tipping point" until the process of black hole formation being completed, i.e. from the "straw that broke the camel's back moment". It can, of course, take millions or billions of years for a star to reach that tipping point.

Also, when one is talking about the duration of this process, this sloppy question and correspondingly sloppy answer, is implicitly talking about the duration of the event from the perspective of an observer so distant that gravitational time dilation from the forming black hole itself is negligible (e.g. an observer on distant Earth watching in a telescope as the star disappears).

Gravitational time dilation in the immediately proximity of the event greatly slows down the passage of time in a highly observer location specific way due to General Relativity, so it isn't rigorously correct to state that this process happens in any specific duration of time, without specifying the location of the observer whose rate of time passage you care about.

Being punctiliously careful about precisely which observer's passage of time is relevant to the physics calculations you are doing is a critically important conceptual task in any situation that involves very strong gravitational fields. Every now and then you see debates between scientists over papers one of them has written, in published criticisms of the other person's papers, asserting that the wrong observer's time has been used in calculations, causing the original calculations to be fatally flawed.

Sadly I missed that.

In this case, I had meant from the perspective of an observer within an orbital distance to it. That's obviously very ambiguous, so I'm failing again, but I imagine there comes a point when other large bodies would have more influence on the observer. Is that right? Or does everything orbit anything from the right observer's perspective? But just to skip all that drama, let's say the observer is far enough away that it's kind of like Earth watching the nearest star outside of our solar system.

 

[ohwilleke]

But just to skip all that drama, let's say the observer is far enough away that it's kind of like Earth watching the nearest star outside of our solar system.

From that observer's perspective it takes on the order of a fraction of a second to a few seconds from reaching the "tipping point" that create the conditions necessary for its formation.

For an observer in a circular orbit around a black hole the formula for gravitational time dilation is as follows:

The minimum photon-sphere radius of the smallest possible stellar black hole is about 13.5 km so that would be "r_s" in the formula. The nearest star to Earth (Alpha Centuri) is about 40,208,000,000,000 km away which is "r" in the formula. So the time dilation experienced by an observer on Earth from this massive system is about 252 parts per quadrillion, which is probably too small to detect experimentally, particularly after considering other possible sources of gravitational time dilation "noise" from other stuff in the universe in the general vicinity of Earth.