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Solving Mysteries About Black Holes: Comments from Sterl Phinney
I'd like to start by commending NASA for its bravery: most NASA press releases say "scientists have discovered something", but this one says "NASA scientists have failed to discover something". This is a bit like Sherlock Holmes' famous "Silver Blaze" mystery about the theft of a race horse, where Sherlock Holmes describes "the curious incident of the dog in the nighttime." When Inspector Gregory says, "the dog did nothing in the nighttime", Holmes replies, "that was the curious incident". This is an important principle of science, as well as detective work: when expected things do NOT happen, it can be just as important a clue as discovering something that did happen. Star Formation
Giant Black Holes Why does all this matter? Although the black hole in the center of our Milky Way is now lurking, dim and incognito, there is good evidence that it was once active like a quasar. Quasars are brilliant beacons of light that astronomers can see to the edges of the universe. They consist of giant black holes swallowing huge amounts of hot gas. They are rare, incredibly bright, but short-lived. Forty years ago Donald Lynden-Bell pointed out that although quasars are rare, they are so short-lived that most galaxies must have been through a quasar phase, and so most galaxies must have a giant, but "quiet" (no longer swallowing gas) black hole in them. Lynden-Bell's prediction has since been spectacularly verified by the discovery of giant black holes in the center of our Milky Way and all the other galaxies near it. Puzzles About Quasars
A related puzzle: the gas around even the most distant quasars, only one tenth as old as the Milky Way, is as full of elements made in exploding stars as is the Milky Way. How did so many stars form quickly so near the black hole? Finding the Answers We had no way of seeing the answers to these puzzles until very recently.
Second, infrared observations with the Keck and VLT telescopes revealed the disk of massive young stars around the Milky Way's black hole. These Chandra observations seem to rule out the idea that the stars formed elsewhere and were dragged in, so they probably formed from the gas around the black hole. Third, in 1971 the final flight of the Stratoscope II balloon telescope (the NASA test predecessor of Hubble Space Telescope) discovered, and ground-based telescopes and HST confirmed that in the center of the Milky Way's nearby sister galaxy, Andromeda (M31), there is a disk of young blue stars only a light year across, inside a ring of older stars. So if stars are forming so close to the big black holes in the two galaxies nearest to us, stars are probably forming in the gas disks around all big black holes. Stars Falling Into Black Holes NASA and the European Space Agency are building a really exciting thing called the Laser Interferometer Space Antenna (LISA), to detect gravitational waves from these giant black holes and stars falling into them. The bad news is that if there had been enough very low mass stars, LISA might have seen some of them orbiting in our own Milky Way. This now looks unlikely. The good news is that lots of massive stars around all big black holes will create lots of small black holes to fall into the supermassive black holes. So, LISA will see hundreds of these, to great distances, on their way in, providing fantastically precise tests of black hole physics and relativity. Return to Sagittarius A* (13 Oct 05) |
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Revised: August 30, 2006
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