Physics 7
Introduction to Astronomy

H. E. Smith Winter 1999

( United Press International )

 Fast-moving stars hint at black hole

UPI Science News

TUCSON, Ariz., Sept. 7 (UPI) _ A California astronomer says some stars near the center of the galaxy are moving at speeds of up to 3 million miles per hour _ about 10 times the speed at which stars usually move.

Andrea Ghez (as in ``says'') of the University of California at Los Angeles said today the speeding stars are the best evidence yet that there's a giant black hole at the center of the galaxy. She said their unusual speed occurs because they are being pulled by the powerful gravity of the black hole.

Ghez's announcement _ the results of three years of study using Hawaii's Keck I telescope _ came at an international workshop in Tucson for astronomers who study the center of the Milky Way galaxy.

Ghez said: ``This is the best evidence yet. There's been a growing case for a central black hole over the past few years...but there's no longer any ambiguity.''

The first evidence for a central black hole came in 1932, when American radio engineer Karl Jansky discovered powerful radio waves coming from the center of the galaxy. Later scientists concluded the radio waves were probably caused by matter falling into the black hole at high speeds.

More recently, astronomers have pinned down the source and given it a name _ Sgr A* (read ``Sagittarius A Star). Ghez's new study shows it is 2.6 million times more massive than our sun and is ''confined to an extremely small volume.``

Black holes are formed from the remnants of collapsed stars. They have a gravitational pull so great that nothing can escape, not even light. Ghez couldn't see the black hole directly, but she could observe its gravitational effect on nearby stars.

She said: ``They're going like a bat out of hell. They're really moving.''

The central black hole itself is difficult to detect because it does not emit radiation. Another barrier is the huge clouds of interstellar dust lying between Earth and the galactic center, blocking visible light and forcing astronomers to fall back on infrared and radio telescopes.

But until recently, according to Angela Cotera of the University of Arizona, the workshop's co-chair, those instruments weren't as powerful as the best optical telescopes. Now, she said, ``We're starting to get detail that we couldn't get before.''

Ghez said the powerful Keck telescope, which uses infrared radiation, allowed her to see stars very close to the black hole _ about four times the width of the sun's solar system away.

She said, ``Given that it's 24,000 light-years away, it's impressive that we can distinguish something the size of our solar system that far way.'' A light-year is the distance that light, traveling at 186,000 miles per second, can cover in a year.

Terry Oswalt of the National Science Foundation, which supported Ghez's research, said: ``What lies in the center of the Milky Way has been one of this century's big science questions. Ghez's work has massive implications on our understanding of how galaxies evolve.''


Copyright 1998 United Press International (via Comtex). All rights reserved

FAST-MOVING STARS HINT AT BLACK HOLE., United Press International, 09-07-1998.

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