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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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