Volume: 04, Issue: 04 10/19/2005 
Chandra's image of the Galactic Center has provided evidence for a new and unexpected way for stars to form. Image courtesy NASA/CXC/MIT/F.K.Baganoff et al.
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A combination of infrared and X-ray observations indicates a surplus of massive stars has formed from a large disk of gas around Sgr A*, the Milky Way's central black hole. Illustration courtesy NASA/CXC/M.Weiss.
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Other Articles in This Issue:
Hubble Searches for Possible Moon Resources
Max Q: The All-NASA Rock Band
Saturn Moon Dione Displays Icy Tranquility
Enhance Your Teaching Skills
 

Black Holes Spawn New Generations of Stars

New research has proven black holes may not be as villainous as their reputation indicates. NASA's Chandra X-ray Observatory has revealed a new generation of stars spawned by a super-massive black hole at the center of the Milky Way galaxy.

"Massive black holes are usually known for violence and destruction," said Sergei Nayakshin of the University of Leicester, United Kingdom. "So it's remarkable this black hole helped create new stars, not just destroy them."

Black holes have earned their fearsome reputation because any material (including stars) that falls within their "event horizon" is never seen again. The new Chandra results indicate immense disks of gas, orbiting many black holes at a safe distance from the event horizon, can help nurture the formation of new stars. This conclusion comes from new clues that could only be revealed in X-rays.

Until the latest Chandra results, researchers disagreed about the origin of a mysterious group of massive stars known as Sagittarius A* (Sgr A*). Discovered by infrared astronomers, Sgr A* orbit less than a light year from the Milky Way's central black hole. At such close distances, the standard model for star forming gas clouds predicts they should have been ripped apart by tidal forces from the black hole.

Based on previous research, two models have been proposed to explain this puzzle. In the disk model, the gravity of a dense disk of gas around Sgr A* offsets the tidal forces and allows stars to form. In the migration model, the stars formed in a cluster far away from the black hole and then migrated in to form the ring of massive stars. The migration scenario predicts about a million low mass, sun-like stars in and around the ring. In the disk model, the number of low mass stars could be much less.

Researchers used Chandra observations to compare the X-ray glow from the region around Sgr A* to the X-ray emission from thousands of young stars in the Orion Nebula star cluster. They found the Sgr A* star cluster contains only about 10,000 low mass stars, thereby ruling out the migration model. Because the galactic center is shrouded in dust and gas, it has not been possible to look for the low-mass stars in optical observations. X-ray data have allowed astronomers to penetrate the veil of gas and dust and look for these low mass stars.

"In one of the most inhospitable places in our galaxy, stars have prevailed," Nayakshin said. "It appears star formation is much more tenacious than we previously believed."

The research suggests the rules of star formation change when stars form in the disk surrounding a giant black hole. Because this environment is very different from typical star formation regions, the proportion of forming stars changes. For example, a much higher percentage of massive stars exist in the disks around black holes.

For more information about this research on the Web, visit these websites:
http://chandra.nasa.gov
http://chandra.harvard.edu

    
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