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The Question
(Submitted November 20, 2001)
How many black holes are known to man that are in the universe?
And out of those known black holes what are the names?
The Answer
The answer to your first question depends a lot on how strong an
evidence you would want to accept something as a black hole.
Astrophysicists generally agree that when the compact object in
an X-ray binary system is shown to be more massive than about
3 times the mass of the Sun, then this compact object is a black
hole beyond reasonable doubt. These are called "dynamically
confirmed black holes." Dr. Orosz maintains a list of such
systems (currently 14) on his home page:
http://mintaka.sdsu.edu/faculty/orosz/web/
If you accept a less strict standard of evidence, then there
are many more black holes that (we think) we know of. These
include additional X-ray binaries such as Cygnus X-1, the mysterious
object at the center of our Galaxy, and the central objects in
many (perhaps even most) luminous galaxies.
For example, the Sloan Digital Sky Survey aims to measure the
distances to more than a million galaxies and quasars:
http://www.sdss.org/
A large fraction of these galaxies, and all the quasars, are
thought to contain a supermassive black hole. Given such a huge
number, there is no plan to individually name these black holes;
astrophysicists use designations based on their positions on the
sky (you see examples on Dr. Orosz's page).
Best wishes,
Koji & Georgia
for "Ask an Astrophysicist"
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