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Ask an Astrophysicist

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:

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:

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
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