Physics 7
Introduction to Astronomy

H. E. Smith                                                                                                       Winter 1999

FAST-MOVING STARS HINT AT BLACK HOLE ( 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.'' 


 By MICHAEL SMITH




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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