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Huge Black Hole At Hub Of Milky Way Eating Up Old Stars Astronomers Say This Is ‘Best Evidence Yet’

Thu., Jan. 8, 1998

Astronomers reported new evidence Wednesday that an immense black hole, weighing as much as 2,600,000 suns, squats at the hub of the Milky Way, gobbling up elderly stars while the rest of the galaxy wheels majestically around it.

Younger stars, like one nicknamed the Bullet, are zipping by the center of our galaxy at such high speeds that even the enormous gravity of the suspected black hole cannot capture them, they said.

The unusual motions of these stars and the tremendous mass of the stationary object in their midst provide the strongest confirmation yet that the center of the Milky Way is occupied by a black hole, astronomers told a news conference here sponsored by he American Astronomical Society.

“This object has to be a massive black hole,” said Andreas Eckart, of Germany’s Max Planck Institute for Extraterrestrial Physics. NASA astronomer Stephen Maran agreed: “This is the best evidence yet.”

According to the theory of relativity, black holes are created by collapsing stars or star clusters and continue feeding on their neighbors until the end of time. They are so dense that their gravity prevents anything, even light, from escaping once it has fallen in. Since the black holes cannot be seen, their existence has to be demonstrated indirectly.

Mark Reid, a radio astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., compared the situation to the O.J. Simpson murder case.

Proof of Simpson’s guilt, or of the black hole at the center of the Milky Way, has not been established “beyond a reasonable doubt” as required in a criminal trial, Reid said. But he said it has now been confirmed by a “preponderance of evidence,” as needed in a civil trial.

The center of the Milky Way - 26,000 light years from Earth - is obscured by thick clouds of gas and dust, screening it from earthly telescopes that use visible light. But radio waves pass through the screen, and reveal the objects behind.

One such radio source, known as Sagittarius A, was thought to be either a black hole or an unusual group of stars. Reid’s team observed it for two years, using an array of ground-based radio telescopes stretching from Hawaii to New England.

Along with other earlier measurements, the team found Sagittarius A was virtually motionless, while some nearby stars were speeding by at velocities up to 20 million miles an hour.

“The fact that it is almost stationary rules out that it could be a star or a star system,” said Reid. “It is totally consistent with a black hole.”

Other stars in the neighborhood are old and slow moving. Eckart predicted one of them will fall into the black hole about every 100,000 years and be swallowed up.

Thus the center of the Milky Way is “sort of a retirement village for old stars,” said Farhad Yusuf-Zadeh, another member of the team from Northwestern University in Evanston, Ill.

Meanwhile, other astronomers described the strange antics of a smaller black hole, on the far side of the Milky Way, which they nicknamed “Old Faithful” because it spurts out tremendous jets of hot gas every 30 minutes.

This object is located in the center of a spinning disk of fiery matter known as GRS-1915 in the constellation Aquila, 40,000 light years from Earth. The disk is formed by a steady stream of hot gas pulled off from a companion star by the gravity of the black hole.

“This is the ultimate Niagara Falls,” said Ronald Remillard, a researcher at the Massachusetts Institute of Technology in Cambridge, Mass.

Using an orbiting X-ray telescope, astronomers observed that the interior of the disk repeatedly empties, then refills with material. Each time it empties, a powerful gas flare is detected within minutes by ground-based radio and infrared telescopes.

The flares move at 92 percent of the speed of light, or about 600 million miles an hour, according to Remillard.

“This system behaves like a celestial version of Old Faithful,” said Craig Markwardt, a NASA scientist, referring to the famous geyser in Yellowstone Park.

Periodically, however, the disk quiets down and nothing happens for months or years.

“It has mood swings,” Remillard said. “When it’s in the mood, it gives out jets.” When it’s not in the mood, he said, it should perhaps be called “Old Fickle.”



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