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NASA's Chandra Reveals New Star Generation

October 14, 2005
Chandra X-ray Center
NASA's Chandra X-ray Observatory revealed a new generation of stars spawned by a super-massive black hole at the center of the Milky Way galaxy. This novel mode of star formation may solve several mysteries about these super-massive black holes that reside at the centers of nearly all galaxies.

Chandra's image of the Galactic Center (left) has provided evidence for a new and unexpected way for stars to form. A combination of infrared and X-ray observations indicates that a surplus of massive stars has formed from a large disk of gas around Sagittarius A*, the Milky Way's central black hole (illustration on right). Credit: X-ray: NASA/CXC/MIT/F.K.Baganoff et al.; Illustration: NASA/CXC/M.Weiss

NASA's Chandra X-ray Observatory revealed a new generation ofstars spawned by a super-massive black hole at the center of the MilkyWay galaxy. This novel mode of star formation may solve severalmysteries about these super-massive black holes that reside at thecenters of nearly all galaxies.

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

Black holeshave earned their fearsome reputation because any material, includingstars, that falls within their "event horizon" is never seen again.These new results indicate immense disks of gas, orbiting many blackholes at a safe distance from the event horizon, can help nurture theformation of new stars. This conclusion comes from new clues that couldonly be revealed in X-rays. Until the latest Chandra results,researchers have disagreed about the origin of a mysterious group ofmassive stars discovered by infrared astronomers.

The stars orbitless than a light year from the Milky Way's central black hole, whichis known as Sagittarius A* (Sgr A*). At such close distances to Sgr A*,the standard model for star forming gas clouds predicts they shouldhave been ripped apart by tidal forces from the black hole. Two models,based on previous research, to explain this puzzle have been proposed.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 themigration model, the stars formed in a cluster far away from the blackhole and then migrated in to form the ring of massive stars. Themigration scenario predicts about a million low mass, sun-like stars inand around the ring. In the disk model, the number of low mass starscould be much less.

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

This research, coauthored by Nayakshin andRashid Sunyaev of the Max Plank Institute for Physics in Garching,Germany, will appear in an upcoming issue of the Monthly Notices of theRoyal Astronomical Society.

"In one of the most inhospitableplaces in our galaxy, stars have prevailed," Nayakshin said. "Itappears star formation is much more tenacious than we previouslybelieved." "We can say the stars around Sgr A* were not deposited thereby some passing star cluster, rather they were born there," Sunyaevsaid. "There have been theories that this was possible, but this is thefirst real evidence. Many scientists are going to be very surprised bythese results."

The research suggests the rules of star formationchange when stars form in the disk surrounding a giant black hole.Because this environment is very different from typical star formationregions, there is a change in the proportion of stars that form. Forexample, there is a much higher percentage of massive stars in thedisks around black holes.

NASA's Marshall Space Flight Center,Huntsville, Ala., manages the Chandra program for the Science MissionDirectorate. The Smithsonian Astrophysical Observatory controls scienceand flight operations from the Chandra X-ray Center in Cambridge, Mass.For more information about this research on the Web, visit:

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