10 September 1998

There's Nothing At The Centre

The presence of an enormous black hole at the center of our galaxy has been detected by a researcher funded, in part, by the National Science Foundation (NSF). The evidence will be reported today at the Central Parsecs Galactic Center Workshop '98 in Tucson, Arizona, by Andrea Ghez, of the University of California-Los Angeles.

"What lies in the center of the Milky Way has been one of this century's 'big' science questions," said Terry Oswalt, NSF program manager for Stellar Astronomy and Astrophysics. "Ghez's work has massive implications on our understanding of how galaxies evolve."

Black holes are formed from the remnants of collapsed stars. A black hole consists of a large mass compacted so densely that not even light can escape its force of gravity. Since Ghez could not directly see a black hole, she inferred its presence by searching for the gravitational influence it imposes on nearby objects she could see, namely stars.

In 1995, using the Keck I Telescope atop Mauna Kea in Hawaii, Ghez began tracking the movement of 200 stars near the galactic center. She found at least 20 stars that exhibited the telling signs of influence by extreme gravitational forces.

These stars are spiraling around the black hole at speeds of up to three million miles per hour-about 10 times the speed at which stars typically move. In order to account for the rapid speeds of these stars, Ghez determined that an object 2.6 million times more massive than our Sun must be concentrated into a single black hole.

Just getting a clear view of the center of our galaxy is an impressive feat in itself. To overcome the distortion created by the Earth's atmosphere, Ghez made her observations using a technique called "infrared speckle interferometry." The procedure, which she helped develop, uses computers to analyze thousands of high-speed, high-resolution snapshots.

The result: an image that has at least 20 times better resolution than those made by traditional earthbound imaging techniques. "It's like putting on glasses," said Ghez.

Using this technique in 1995, Ghez witnessed the disappearance of a star that was, at the time, the closest object to the black hole. Whether the star was sucked into the black hole, or simply went behind it, scientists may never know.

But we have little to fear about a similar fate for Earth, since the center of the Milky Way galaxy is approximately 24,000 light years away. Because of the Earth's position on an outer arm of the spiraling Milky Way, much of our knowledge about galaxies does not come from our own. Ghez's research, however, gives us a definitive view about a part of Galaxy that we have never seen before.

"There is an incredible amount of matter between us and the center of the Milky Way to obscure our view," said Oswalt. "Ghez has pulled the living room shades open a bit and finally given us a good look at what's going on in our own backyard."

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