AUSTIN, Texas -- Astronomers have used the Hubble Space Telescope to see closer to the center in our neighboring Andromeda galaxy than in any other galaxy except our own. In doing so, they dramatically improved the evidence that a supermassive black hole lurks in Andromeda's core. They also found a new mystery -- the black hole lives inside a tiny cluster of blue stars whose origin is not understood. This work was done by a team of astronomers led by Ralf Bender of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany and John Kormendy of The University of Texas at Austin. It is published in the September 20 issue of the Astrophysical Journal.

They key to both advances is a tiny source of blue light that is only one light-year across and that coincides with the center of the galaxy. Andromeda is more than 200,000 light-years across and about two million light years away from us.

"Only Hubble has the resolution in blue light to observe the blue source. It is so small and so distinct from the surrounding red stars that we can use it to probe into the dynamical heart of Andromeda," says Richard Green of the Large Binocular Telescope Observatory in Tucson. Green adds, "These observations were taken by the members of our team that built the Space Telescope Imaging Spectrograph (STIS). We designed its visible channel to seize just such an opportunity -- to measure starlight closer to a black hole than in any other galaxy outside our Milky Way."

The astronomers used this superb resolution to discover that the blue light, first spotted by Hubble in 1995, comes from a disk of hot, young stars. They whiz around an invisible, dark central object like planets in our solar system revolve around the Sun. By measuring the motions of the stars, the astronomers found that the dark object weighs as much as 140 million Suns. It is about three times bigger than previously thought.

The new Hubble observations greatly strengthen the evidence that the dark object is a black hole. In 1988, in independent ground-based studies, John Kormendy and the team of Alan Dressler (Carnegie Observatories) and Douglas Richstone (University of Michigan) discovered the central dark object in Andromeda. This was one of the first of what are now about 40 such detections, most of them made with Hubble.

"Right from the beginning, there were compelling reasons to believe that these are supermassive black holes," says Kormendy, but he cautions that "extreme claims require extremely strong evidence. We have to be sure that these are black holes and not dark clusters of dead stars."

So far, dark clusters have been ruled out in only two galaxies, NGC 4258 and our Milky Way. Kormendy adds, "These two galaxies give us important proof that black holes exist. But both are special cases -- NGC 4258 contains a disk of water masers that we observe with radio telescopes, and our Galactic center is so close that we can follow individual stellar orbits. Andromeda is the first galaxy in which we can exclude exotic alternatives to a black hole using Hubble and using the same techniques by which we find most other supermassive black holes."

Kormendy emphasizes, "Looking for black holes always was a primary mission of Hubble. Nailing the black hole in Andromeda is an important part of its legacy. It makes us much more confident that the other central dark objects detected in galaxies are black holes, too."

Ralf Bender adds, "And now we have proved that the black hole is at the center of a tiny disk of blue stars. But this uncovers a new mystery." The new observations by the Space Telescope Imaging Spectro- graph show that the blue light consists of more than 400 stars. Their properties suggest that they formed only 200 million years ago. Bender asks, "How could stars form so close to the powerful gravity of a black hole? Gas that might form stars must spin around the black hole so quickly -- and so much more quickly near the black hole than farther out -- that star formation looks almost impossible. But the stars are there."

Bender and his colleagues believe that the present disk of blue stars may not be the first to form at Andromeda's center. "The blue stars are so short-lived that it is unlikely, in the 12-billion-year history of the galaxy, that a disk of them would appear just when we are ready to look for it," says team member Tod Lauer of the National Optical Astronomy Observatory. "That's why we think that the mechanism that formed this disk probably formed other stellar disks in the past and will form them again in the future."

The mystery of the blue disk shows why it was important to check that the central dark object is a giant black hole. Kormendy notes, "If we don't understand how the blue stars formed now in the hostile environment around a supermassive black hole, then how can we be sure that a lot more stars didn't form in the benign absence of a black hole?"

Bender explains, "The stars in the blue disk will die in a few hundred million years. They will leave behind a cluster of dark neutron stars and small black holes. If stars formed and died many times in the past, then there is a danger that Andromeda contains 10 or 20 million dead stars at its center and not a supermassive black hole. Now we have shown that this is impossible. Andromeda becomes the third galaxy in which plausible alternatives to a supermassive black hole are excluded."

Notes: This release is put out concurrently with a release from the Space Telescope Science Institute, which can be found online at: http://hubblesite.org/news/2005/26