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Re: Center Of Galaxy Contains A Large Black Hole

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Posted by nitesh gupta/">nitesh gupta on September 24, 1998 16:48:31 UTC

: Subject: : Unambiguous Evidence for Monstrous Black Hole at Center of Our Galaxy (Forwarded) : Date: : Wed, 9 Sep 1998 20:03:26 GMT : From: : Andrew Yee : Organization: : UTCC Campus Access : Newsgroups: : sci.astro : : University of California-Los Angeles : Contact: Harlan Lebo, harlanl@college.ucla.edu : Date: September 7, 1998 : Unambiguous Evidence for Monstrous Black Hole at Center of Our : Galaxy Presented by UCLA Astronomer Andrea Ghez : Answering one of astronomy's most important questions, UCLA : astronomer Andrea Ghez reported today at a conference in Tucson, : Ariz., that a monstrous black hole resides at the center of our : Milky Way galaxy, with a mass more than two million times that of : our sun. : The question of what lies at the center of our galaxy, 24,000 light : years away, has been the subject of a raging debate among astronomers : for more than a quarter-century. Scientists have suspected that the : galactic center contains either a single "supermassive" black hole : or a cluster of millions of smaller stellar remnants. Black holes : are collapsed stars so dense that nothing can escape their : gravitational pull, not even light. : "Our galaxy is rather mild-mannered and quiet, and was one of the : least likely galaxies to have a black hole at its center," said Ghez, : an associate professor of physics and astronomy at UCLA, who spoke : at The Central Parsecs: Galactic Center Workshop '98. "Yet a : supermassive black hole at the center of our galaxy is precisely : what we have found. The evidence for the black hole is very strong. : One implication is that massive black holes may be found at the : center of almost all galaxies." : The Milky Way is one of approximately 100 billion galaxies : containing at least 100 billion stars each. : In her research, Ghez used the 10-meter Keck I Telescope -- the : world's largest optical an infrared telescope -- atop Mauna Kea in : Hawaii to study the movement of 200 stars that are close to the : galactic center. Ghez studied these stars each year since 1995, : using a technique she refined called "infrared speckle : interferometry." : "Black holes cannot be seen directly, but their influence on nearby : stars is very visible and provides a signature," said Ghez, 33. "We : have found that signature in the rapid movement of the 20 or so : stars that are most affected by its gravitational influence." : These 20 stars are orbiting ever closer to the black hole at a : blinding speed of up to three million miles per hour -- about 10 : times the speed at which stars typically move. The rapid speed at : which the stars closest to the galactic center are moving reveals : that the mass of the black hole -- 2.6 million solar masses -- : must be concentrated in a single object, she said. : The star that was closest to the black hole in 1995 has since : disappeared. Ghez has a number of possible theories to explain : its disappearance, ranging from the mundane to the exotic. One : explanation for observing a bright source in only one image, Ghez : said, is that it was a "gravitational lensing event," which occurs : when the light path from a star passing behind the black hole : is bent by the strong gravitational field of the black hole. : Alternatively, it could have been a flare due to a star falling : into the black hole. Ghez, however, acknowledges that scientists : may never learn which theory is correct. : One reason why astronomers previously had been unable to determine : whether a black hole is at the galactic center is that our : atmosphere distorts the images of stars. Ghez's speckle : interferometry involves taking thousands of very quick, high- : resolution snapshots that correct for the distortions produced by : the Earth's atmosphere. She has developed algorithms - specific : computer commands based on sophisticated mathematics -- and software : for analyzing the data. : Using traditional imaging techniques at the center of the galaxy : would cause the stars closest to the galactic center to look fuzzy : and indecipherable. Ghez's technique, however, improves the : resolution by a factor of at least 20. : "The atmosphere blurs your vision," Ghez said, "but speckle : interferometry clears the picture up; it's like putting on glasses. : Think of seeing a coin that looks distorted at the bottom of a pond. : We take thousands of freeze frames, and then can determine what is : distorted and what is really at the bottom of the pond." : The center of the Milky Way was identified in 1968 by Eric Becklin, : a UCLA professor of physics and astronomy. Its general location in : the galaxy is known, but not its precise location. The center of : the Milky Way is located due south in the summer sky. : The black hole at the center of our galaxy came into existence : billions of years ago, perhaps as very massive stars collapsed at : the end of their life cycles and coalesced into a single, : supermassive object. : Ghez studied the stars closest to the galactic center using the : W.M. Keck Observatory's 10-meter Keck Telescope, and has returned : to the Keck Observatory four times this year to observe the : movement of these stars. She has been able to accurately predict : the locations of the stars closest to the galactic center. (She : identifies the stars based on their location and brightness.) Ghez : has the highest resolution images of the galactic center ever : obtained, which allow precise measurement of a group of stars close : to the galactic center. Keck's large diameter allows Ghez to see : fine details and to position the stars more accurately than a : smaller telescope would permit -- details which were crucial in : establishing the existence of the supermassive black hole. : "The Keck Observatory is the best facility in the world for this : research," Ghez said. "The Keck Telescope enables us to track stars : very precisely." The telescope's resolution is so high, Ghez added, : that it could detect two flies in Japan that are less than 10 feet : away from each other. "That's the resolution we are reaching," she : said, "if you scale it out to 24,000 light years." : Ghez's research is supported by the National Science Foundation : through an NSF Young Investigator Award, the Packard Foundation : and the Alfred P. Sloan Foundation. : "Ghez's research is a Areal tour-de-force," said Ferdinand Coroniti, : chair of UCLA's physics and astronomy department. "She continues to : dazzle and amaze the astronomical community with her technical : virtuosity and scientific accomplishments." : She is now searching for additional black holes or other dark : matter near the massive black hole. Her research has been accepted : for publication in the December issue of Astrophysical Journal. : Ghez's co-authors on the paper are former UCLA graduate student : Beth Klein and UCLA astronomy professors Mark Morris and Eric : Becklin. : : --- : Andrew Yee : ayee@nova.astro.utoronto.ca

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