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I'm speculating here, but it seems obvious that material and energy from the star would rush through the black hole's event horizon very quickly. Could this be enough to reverse the process that created the black hole in the first place, or will the entire large star get sucked into the black hole as mass rushes over the event horizon and causes the event horizon to expand?

Granted, the very large star would eventually turn into a black hole itself anyway, so it should be a zero-sum game from a mass perspective. But if a large star can incorporate a small black hole into its own core, then maybe when the large star goes supernova later, some very exotic materials or radiation from the reincorporation of the small black hole would be expelled?

For a size comparison, I'm thinking a star that has one or two thousand times the mass of our sun, versus a black hole that has 4 or 5 times the mass of our sun. Speed and trajectory obviously make a difference. But I'm looking for a situation where they could possibly merge into something stable.

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    $\begingroup$ Define "small" ? It's pretty important in this context. A black hole is a small size, but huge mass. So how small a mass ? $\endgroup$
    – StephenG - Help Ukraine
    Commented May 28, 2017 at 14:14
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    $\begingroup$ It... won't end well for the star. The size (i.e. mass) of the black hole will determine how long the process will take, but the end-game is most likely a sizeable black hole with a very active accretion disk. $\endgroup$
    – Emilio Pisanty
    Commented May 28, 2017 at 14:28
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    $\begingroup$ We should also consider what relative speed and trajectory we're discussing. Could a small black hole moving in a hyperbolic trajectory make it out of the star again with enough momentum to let it continue on into interstellar space ? $\endgroup$
    – StephenG - Help Ukraine
    Commented May 28, 2017 at 14:35
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    $\begingroup$ For a size comparison, I'm thinking a star that has one or two thousand times the mass of our sun, versus a black hole that has 4 or 5 times the mass of our sun. Speed and trajectory obviously make a difference. But I'm looking for a situation where they could possibly merge into something stable. $\endgroup$
    – MarkW
    Commented May 28, 2017 at 14:40
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    $\begingroup$ There has been research done about the effect of primordial/microscopic black holes passing through (stable) white dwarves. The basic idea being that they can induce temperature variations that cause the dwarf to go supernova, in a way which would be discernible from a normal type Ia supernova. But no significant evidence was found to support such events, constraining the contribution such black holes can give to the dark matter content. $\endgroup$
    – zibadawa timmy
    Commented May 28, 2017 at 20:29

2 Answers 2

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Let's imagine we've got a giant star, late in its life. From being a massive star, it will have many different layers undergoing different types of nuclear fusion, culminating in an inner core of (degenerate) iron. Towards the end of its life, this iron core will approach a maximum of $1.4M_\odot$, the Chandrashekar limit, and so is a a sizeable thing. For the purpose of your question, let's assume I'm an intergalactic wizard, and when I click my fingers, I can turn this iron core into a black hole of similar mass. (I think this should be representative of your question?)

Previously, the electron degeneracy pressure from the iron core was able to support the upper layers of the star. In my finger click, that pressure is gone. That means the star will start to collapse. However, it can't simply collapse radially inwards. Remember that most stars are rotating, and when things are rotating, we need to conserve angular momentum, $$L=mvr.$$

If we want to allow our material to fall in towards the centre, then $r\rightarrow R_S$, where $R_S$, is the Schwarzschild radius of the black hole. This is a massive change in angular momentum, and angular momentum is quite hard to get rid of. This means what will initially happen, is the collapsing star material will collapse into the plane of rotation of the star. That is it will form into a disc around the black hole. (This the same physics that explains why Saturn has rings). Note that as this happens, a tremendous amount of energy will be released. A similar collapse event is thought to be one of the causes of long gamma-ray bursts, which are some of the most luminous events in the universe.

After the initial burst of energy, the star will cease to undergo fusion, and will exist as an incredibly hot and active accretion disc around the black hole, in a larger collapsing cloud of ejected gas. As the matter in the accretion disc sheds angular momentum, it will be able to trickle into the black hole.

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    $\begingroup$ Why wouldn't fusion occur in the increadibly hot disc? Not enough pressure? $\endgroup$
    – Yakk
    Commented May 29, 2017 at 13:35
  • $\begingroup$ Fusion in stellar objects generally stops at iron, because starting with iron the fusion reaction requires more energy than it releases. $\endgroup$
    – Drunken Code Monkey
    Commented May 29, 2017 at 14:33
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I remember reading about evidence of a black hole collision with a star. I gave an attempt to find something on this, but I realized the search would take longer than I want. As I remember this was described as "stellar murder." The star exploded.

The black hole would be akin to a high velocity bullet. Just as a rifle bullet causes a melon to burst apart, much the same may happen with the star. The star would be terribly disrupted. As the black hole reaches the core it will create a runaway fusion region around it. This could then lead to a sort of induced supernova event. Even if that does not happen the black hole would likely burst through the star dragging with it a lot of material that forms an accretion disk and a nebula flying away from the star.

I think it is not likely that the black hole will becomes stuck in the star. The stellar material will be of course gravitating material, but the material under compression will be heated and tend to rebound away. The black hole would probably not acquire another matter to be stopped. On the other hand if it does the star would probably be doomed to explode with some of the material entering the black hole.

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    $\begingroup$ I feel like this is missing a source / proof / falsifability -- if you don't want to bother finding a source, don't bother making the claim (?) $\endgroup$
    – cat
    Commented May 28, 2017 at 19:03
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    $\begingroup$ I just spent another half hour trying to find something on this. This is a lot more time than I spent writing the answer. I read this on PhysOrg or BAUT or something like that maybe 5 years ago. $\endgroup$
    – Lawrence B. Crowell
    Commented May 28, 2017 at 20:35
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    $\begingroup$ @cat The facts support your feeling. Note that there is no downvote on comments so the data is skewed however. Also not having a reference to hand doesn't mean that the information is incorrect. Kudos to Lawrence for the half hour search. $\endgroup$
    – Tom Hale
    Commented May 29, 2017 at 1:42
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    $\begingroup$ I wouldn't expect runaway fusion unless the black hole is passing through degenerate matter. Which basically doesn't happen in very massive stars. Otherwise the standard nuclear thermostat should control things. Given that stars can undergo a helium flash (when a degenerate helium core undergoes fusion) without exploding, you're going to need a degenerate core that's pretty far along in the evolutionary stage to generate sufficient energy to unbind the star. $\endgroup$
    – zibadawa timmy
    Commented May 29, 2017 at 3:16
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    $\begingroup$ Are you sure this isn't it? Black Hole Caught Red-Handed in a Stellar Homicide It's the first result on Google for "stellar murder", and it does involve a black hole/star interaction. Although a brief skim suggests it wasn't a collision. $\endgroup$
    – jpmc26
    Commented May 29, 2017 at 7:40

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