* Astronomy

The Andromeda galaxy harbours a supermassive black hole at its core that is surrounded by an unexpected and unexplained disc of young stars.
As far back as 1995, Hubble revealed a strange blue light in Andromeda's core that astronomers thought could have come from a single, bright blue star or perhaps from a more exotic energetic process.

Click this for very large picture of the Andromeda galaxy.

However, new observations by the Hubble Space Telescope has answered that mystery; but has immediately created another in its place.
The blue light is coming from a disk of hot, young stars. These stars are whipping around the black hole in much the same way as planets in our solar system are revolving around the sun. Astronomers are perplexed about how the pancake-shaped disk of stars could form so close to a giant black hole.
In such a hostile environment, the black hole's tidal forces should tear matter apart, making it difficult for gas and dust to collapse and form stars.
The stellar orbital speed is controlled by the galaxy's central black hole, so such energetic activity was thought to prevent star formation. Stars form when a knot of gas and dust collapses under its own gravity.

"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" - Ralf Bender, Max Planck Institute for Extraterrestrial Physics in Garching, Germany.

The newly discovered disc is composed of over 400 very hot, young blue stars, that formed in a burst of activity roughly 200 million years ago, orbiting like a planetary system very close to the black hole. That puzzles astronomers because the black hole's intense gravitational field should have torn apart any clouds of matter long before they could coalesce to form new stars.
The stars are packed into a very flat disc that is just 1 light-year across.
An elliptical disc of older red stars surrounds it, spanning about five light years.
Since the two discs appear to be in the same plane, they are probably related, but no one yet understands how either disc came into being.

"The blue stars in the disk are so short-lived that it is unlikely in the long 12-billion-year history of Andromeda that such a short-lived disk would appear now. We think that the mechanism that formed this disk of stars probably formed other stellar disks in the past and will trigger them again in the future. We still don't know, however, how such a disk could form in the first place. It still remains an enigma" - Tod Lauer of the National Optical Astronomy Observatory in Tucson, Arizona.


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Spectroscopic observations made with Hubble's Space Telescope Imaging Spectrograph (STIS) show that the disc of blue stars is only about 200 million years old, while the galaxy itself has been around for about 12 billion years. Intriguingly, there are signs of young stars very close to the core of our own galaxy as well.
So this puzzling phenomenon may not be unusual, says Hubble team member Tod Lauer of the National Optical Astronomy Observatory in Tucson, Arizona, US.
Since it would be surprising if astronomers happened to catch the only example of such a young disc, it may be that such rings are constantly being formed.

"This behaviour may not be the exception but the rule. We still don't know, however, how such a disc could form in the first place. It’s an enigma" - Tod Lauer.

Measurements by the STIS instrument also allowed astronomers to determine the movement of the blue stars and therefore pinpoint the black hole's mass.
This proved conclusively that it really is a supermassive black hole, with a mass 140 million times that of our Sun - three times more massive than previous estimates.
Until these observations, it had been impossible to rule out some highly unlikely alternatives for the object in the centre of the galaxy - including an extremely dense star cluster, says team member John Kormendy at the University of Texas, US.

"Nailing the black hole in Andromeda," will be remembered as one of Hubble's most important findings "It makes us much more confident that the other central dark objects detected in galaxies are black holes too" - John Kormendy.

While astronomers are surprised to find a disk of stars swirling around a supermassive black hole, they believe the puzzling architecture may not be that unusual.
"The dynamics within the core of this neighbouring galaxy may be more common than we think. Our own Milky Way apparently has even younger stars close to its own black hole. It seems unlikely that only the closest two big galaxies should have this odd activity" - Tod Lauer.

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