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Whenever I read about black holes, it is normally involving how they devour anything that comes too close... regardless of how big anything is.

But what if it were a really small black hole vs something really large? How about something like a large star or neutron star... could a black hole be small enough to get devoured / consumed by something like that or would it just not be possible?

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Black holes don't "devour". They can't "eat" things. But things can "fall into" a black hole. It really is just gravity, but really really intense gravity.

A really small black hole would have a mass of about 3 times the mass of the sun. All neutron stars are smaller than that (or they would turn into black holes). A really large star could be more massive and would be much much larger in volume. A black hole could orbit with such a star, but if it got close enough it would pull gas off the star which would then fall into the black hole, causing it to get larger.

There is no way that a large star can in any way "pull apart" or otherwise "consume" a black hole.

There is a theoretical notion of the quasi-star. These are similar to a Thorne–Żytkow objects. In normal stars, the outer layers are prevented from collaping by energy from nuclear fusion in the core. In a quasi-star, the core collapses into a black hole and the release of energy from matter falling into the black hole prevents the collapse of the star. A steady state can be achieved, as if more matter starts to fall in, more energy is released causing the star to grow, and reducesing the amount of matter falling in. Nevertheless, ultimately all the matter from the star would fall in.

Smaller black holes can't form from stars. We don't know if they exist at all, but if they do, very very small black holes could be too small to interact much with regular matter. A very small black hole could be smaller than a proton, and even if one fell into the Earth, it could pass through the gaps between and inside atoms. Such a black hole would be very hot, due to Hawking radiation.

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  • $\begingroup$ What if it isn’t orbiting? Wouldn’t a collision between a black hole (above some threshold size) and a large star result in a larger black hole? Which one consumed the other would be moot. $\endgroup$
    – WGroleau
    Aug 14, 2019 at 0:24
  • $\begingroup$ FWIW, a 3 solar mass BH has a Schwarzschild radius of ~8.862 km. $\endgroup$
    – PM 2Ring
    Jan 12 at 8:20
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That would be a bigger black hole. Black holes are indestructible but a big one can consume a smaller one.

The collision between anything and a black hole results in a black hole plus some optional debris from whatever the other thing is, so the result of a collision between two black holes is simply a bigger black hole.

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It's "in principle" possible. For example if we had a black hole the size of a small mountain on Earth (this kind of black hole are possible in theory but cannot form from stellar collapse) then it'd be impossible to confine in a laboratory, and it would fall through the Earth to eventually settle at the Earth's core. We could then argue that the Earth has "consumed" the black hole.

It would be temporary, however, because eventually the particles at the Earth's center would fall into the black hole, it would gain mass, eat more particles, etc, until everything falls in and the Earth is consumed by the black hole.

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  • $\begingroup$ A 1 kg black hole has a Schwarzschild radius of 1.48517E-27 m, far smaller than a proton. And assuming that Hawking radiation is real (and that some quantum gravity effect doesn't stabilise micro black holes) its lifetime is only 4.65118E-17 seconds. Please see vttoth.com/CMS/physics-notes/311-hawking-radiation-calculator $\endgroup$
    – PM 2Ring
    Jan 12 at 8:27

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