How to Weigh a Black Hole

Astronomers have managed to weigh the massive black hole at the center of M87, one of the galaxies in our local cluster, and have found that it has swallowed approximately 6.6 billion suns.  A team headed by Karl Gebhardt at the University of Texas at Austin used the Gemini North telescope on top of Hawaii’s Mauna Kea volcano to observe the speeds at which different groups of stars within the M87 galaxy orbited the galaxy’s center, and from that calculated the mass of the black hole.

Professor Gebhardt presented a paper describing the experiment on January 12 meeting of the American Astronomical Association.  The web site makes papers available to members only.  Ron Cohen at posted this rather good article on the discovery.  Paul Sutherland at has also posted a nice description for non-scientists.

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