Preposterous Universe

Wednesday, November 10, 2004
Middleweight black hole

We've known for a long time that the center of our galaxy harbors a supermassive black hole, perhaps two million times the mass of the Sun. (Still only a very tiny part of the galaxy, which is perhaps one hundred billion times the mass of the Sun.) There are different pieces of evidence in favor of this, all of which come down to mapping out a gravitational field that implies a huge mass in a small area, without seeing the radiation you would expect if the object weren't a black hole. You can even see time-lapse movies of stars orbiting the black hole. These days we suspect that most large spiral galaxies contain such black holes at their center, and in earlier days these black holes were powering quasars or active galactic nuclei.

Now scientists have found another black hole near the galactic center, much smaller than the first but still big -- about a thousand solar masses. Careful observations resolved a single blob into a cluster of a few individual stars zipping around this black hole. The size is especially intriguing, as it is "intermediate-mass" -- not just a few solar masses, as you might get from the explosion of a massive star, nor a million solar masses, as you find at the center of large galaxies. Not surprising that such objects could form by having smaller holes gradually accumulate mass in very dense environments, but it's good to see it directly.

Small and intermediate black holes will occasionally spiral into supermassive black holes, giving off gravitational radiation. This will be a primary target for next-generation gravitational-wave observatories, like the LISA satellite. This particular black hole is in no danger of inspiralling any time soon, but probably its cousins are common in other galaxies.

Ideas on culture, science, politics.
Sean Carroll

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