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Q& A: Black Holes

Q:
Do black holes have the ability to devour stars whole, instead of pulling matter off of them bit by bit ? Analogous to taking small bites of an apple instead of eating the apple in one bite.

A:
Whether a black hole devours stars whole or bit by bit depends on the size of the star and the mass of the black hole. In general supermassive black holes with a mass greater than a few billion solar masses swallow stars in one bite, whereas smaller black holes will tear them apart.

For a star orbiting near a black hole, the black hole's gravitational pull on the side of the star nearest the black hole will be a stronger than its pull on the far side of the star. This is called the gravitational tide of the black hole. The same effect causes the Earth to be pulled slightly out of round by the moon's gravity, and produces the ocean tides, which is why the forces are called gravitational tidal forces. If the tidal force of the black hole is larger than the star's own gravity, it will pull the star apart. The radius of the event horizon of a black hole increases with mass, so for very massive black holes, the radius of the event horizon is large enough to make the black hole's gravitational tides smaller than the star's gravity, so the star remains intact until it is swallowed by the black hole. This occurs with gigantic black holes that have a mass of more than a few billion solar masses.

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