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The Question

(Submitted March 17, 1997)

Admittedly, I am very ignorant of the practical study of astrophysics based on relativity and quantum mechanics. I do enjoy studying astrophysics though, and would like to ask a few questions concerning a black hole and a possible neighboring star. I read the article on X-ray emission from a star into a black hole and wanted to know if there is any speculation that goes a step further; what would happen if a star were to fall or be launched into a black hole? Question two: would a star continue to fusion just outside and or inside the event horizon of a black hole? If a star could continue fusion, would the change in surface area of the star (due to the gravitational increase as it is closer to the singularity) allow for a greater consumption of fuel leading to an increase in temperature? I would truly appreciate any and all insight you can provide for me.

The Answer

One thing you need to consider is the relative sizes of a star and that of (the event horizon of) a black hole. For example, the Sun has a radius of ~700,000 km; a black hole with the same mass would have a radius of ~3km.

(See, http://imagine.gsfc.nasa.gov/docs/science/know_l2/black_holes.html for the formula that describes the Schwarzschild radius.)

So if the black hole in question is a few times more massive than our Sun (astrophysicists believe massive stars go supernova and leave such black holes behind), then it's much smaller than the star. In X-ray binaries, a black hole and an ordinary star are in orbit around each other. The star is distorted by the tidal force of the black hole, but otherwise normal, and only slowly (over many millions of years) loses gas onto the black hole. A direct, head-on collision is rare but if this happens, it would be very violent and the star would be torn apart very quickly. So quickly, in fact, that the core of the star (where the fusion is happening) probably won't have time to respond to the changing conditions before it gets torn apart. A completely torn-apart remnant of the star may form a disk around the black hole.

What tears the star apart is the tidal force, or the gradient of the gravitational field. The gradient is less for more massive black holes, so if the black hole is about a billion times more massive than the Sun, then normal stars may be able to fall inside the event horizon of a black hole without being disrupted by the tidal force. Many galaxies are believed to contain Such supermassive black holes at their respective centers, although the typical inferred mass is rather less than a billion times solar (10 to 100 million may be more typical).

Best wishes,

Koji Mukai and Tim Kallman

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