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This artist’s concept shows a view across a mysterious disk of young, blue stars encircling
a supermassive black hole at the core of the neighboring Andromeda Galaxy.
Pictures: NASA, ESA and A. Schaller

Supermassive Black Hole Confirmed
By Larry O’Hanlon, Discovery News

Sept. 21, 2005— A supermassive black hole with the tonnage of 140,000,000 suns has been found simmering at the center of the Andromeda galaxy, accompanied by a hornet’s nest of furious blue stars.

All of those blue stars, detected as just a single blue dot a few pixels wide by the Hubble Space Telescope (HST), are the key to confirming, once and for all, the existence and mass of the black hole, astronomers said.

Careful analysis of the Hubble data revealed that the blue light is from hundreds of the fast-burning stars that are orbiting a relatively small and dark region at speeds that make sense only by the presence of a monster black hole.

“At this point the least extraordinary explanation of what we’re seeing is a supermassive black hole,” said astronomer John Kormendy of the University of Texas.

Kormendy and his colleagues Ralf Bender and Richard Green have reported their discovery in the Sept. 20 issue of Astrophysical Journal.

“It turns out to be bigger than we thought,” said Kormendy of the black hole’s extraordinary size.

The existence of the black hole at the center of Andromeda has been suspected for decades, he said. But there was also the possibility that the dark region seen by telescopes was just a stellar bone yard crowded with dim, burned-out star corpses.

That view has been changed because of data gathered by a now-defunct spectrograph on the HST, explained Kormendy. His team found that the blue light, when split into its composite colors (a spectrum), has the “bomb-proof” hallmark of stars, he said. And not just any stars: they are big, hot blue stars like Sirius, the brightest star in Earth’s night sky, he said.

The spectrum of what is thought to be a disk of some 400 blue stars also shows shifts that indicate the stars are orbiting the black hole. It’s the rate of the rotation of those stars about the black hole that helps the researchers to calculate the approximate mass of the black hole.

“The really special thing about what they’ve done is to use spectroscopy to measure the velocity of stars” located close to the center of Andromeda, said astrophysicist Scott Tremaine of Princeton University.

In all, there are about 35 other galaxies with suspected supermassive black holes at their center, said Kormendy. Only in two other galaxies have the black holes been considered proven: NGC 4258 (also known as M106) and our own galaxy, the Milky Way.

In fact, says Kormendy, it’s unlikely that the others will be confirmed, because they are so far away.

In contrast, Andromeda is the closest full-fledged galaxy to our own and its core is actually easier to see than the Milky Way’s, Tremaine said.