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Thursday, September 21, 2000

Image courtesy of UCLA Division of Astronomy and Astrophysics
Scientists have determined precisely the position of a giant
black hole at the center of the Milky Way by observing
orbits of stars around it.

Big Isle’s Keck
scope sheds light
on black hole

Keck scope spots space object
more than 2 million suns across

By Helen Altonn

RESEARCHERS using the Keck telescope on Mauna Kea have accurately pinpointed a gigantic black hole with more than 2 million suns at the center of the 100 billion stars swirling in the Milky Way galaxy.

The discovery has given astronomers a rare chance to see a little patch of stars in the galaxy spin around in the next few decades, said a former University of Hawaii astronomer.

"Those of us who work on the structure and evolution of galaxies are always faced with the same problem," said John Kormendy, who left the Hawaii Institute for Astronomy in January for an endowed chair at the University of Texas in Austin.

"We're working on a snapshot of things evolving over a time scale of millions and millions of years."

Measurements of a University of California, Los Angeles, team are so close to the center of the galaxy, he said, that astronomers will be able to see it rotate about its center and watch how it distorts and changes the orbits of nearby stars.

"We're actually going to see something happen in the next few decades. To see something galactic in a human lifetime is marvelous."

Findings of the UCLA team, led by physics and astronomy professor Andrea Ghez, are reported in the journal Nature published today. The issue also contains a commentary on their work by Kormendy.

Researchers included UCLA physics and astronomy professor Eric Becklin, also a former UH astronomer, who identified the center of the Milky Way in 1968.

In a telephone interview, Kormendy said astronomers have known quite a long time about the black hole at the center of the earth's galaxy, but the UCLA team's measurements "made the story much cleaner and nicer."

The researchers spent four years watching stars spin closer and faster around the black hole in the galaxy.

They clocked star velocities at orbital speeds of more than 3 million miles per hour near the central black hole. Observations more than four years ago showed the stars traveling at less than 2 million miles per hour.

"Our own Earth, by contrast, orbits the sun at a relatively stately 67,000 miles per hour," the team noted.

"Black holes are really bizarre critters," said Keck spokesman Andrew Perala.

He said Einstein's theory of general relativity nearly a century ago predicted that space was not separate from objects within it.

"Like some sort of cosmic trampoline carrying the mother of all bowling balls, the 'fabric' of space at our galaxy's center has wrapped around the enormous mass of these compacted stars.

"The mass has curved the space so sharply that nothing within a black hole, including light itself, can escape," Perala said.

A black hole is invisible to telescopes and seems to have vanished from the universe, yet its gravitational effects on nearby stars are "powerful and observable," he said.

Kormendy said astronomers had measured how fast stars near the center of the galaxy were moving, but they did not know exactly where the solar mass is.

The UCLA team used the infrared images of three stars to measure their speed. "The new measurements are not just of stars moving, but how fast the motion is changing with time -- acceleration measurements," he pointed out.

Ghez uses the 10-meter Keck I Telescope, the world's largest optical and infrared telescope, to study movement of 200 stars close to the galactic center.

The resolution is so high that galactic center observations are similar to detecting two flies in Japan buzzing less than 10 feet away from each other, she said.

"We are actually seeing stars begin to curve in their orbits," she said.

One may complete its orbit around the supermassive black hole in as few as 15 years, she said, pointing out that the light from the stars takes 24,000 light-years to get to Earth.

Kormendy's research team has documented at least 37 similar galaxies that appear to have black holes at the center.

He uses data collected from Mauna Kea and does some observing there, but also does a lot of observing with the space telescopes.

He said he and other researchers find "every time we point the Hubble space telescope at a galaxy that's got a bulge -- one of these more or less spherical clusters of stars in the middle -- we find a black hole. ... We haven't failed yet."

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