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February 19, 2004

Black Holes' Vast Power Is Documented


New X-ray observations by orbiting satellites have given astronomers their first telling evidence that appears to confirm what had been only theory: that a star is doomed if it ventures too close to a supermassive black hole.

The National Aeronautics and Space Administration and the European Space Agency announced yesterday the detection of a brilliant flare of X-rays from the heart of a distant galaxy, followed by a fading afterglow.

After analysis, an international team of scientists concluded that the telescopes had witnessed the overpowering gravity of a black hole as it tore apart a star and gobbled up a hearty share of its gaseous mass.

It was an act of cosmic mayhem known as a stellar tidal disruption. It removed any lingering doubt, astronomers said, that the reputation of black holes as star-destroyers is fully deserved.

"Stars can survive being stretched a small amount, but this star was stretched beyond its breaking point," said Stefanie Komossa of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, who led the discovery team. "This unlucky star just wandered into the wrong neighborhood."

Astronomers said they suspected that the ill-fated star was thrown off course by a close encounter with another star. Then it fell under the gravitational influence of a black hole and its enormous tidal forces, nothing so benign as the Moon's tug on Earth's oceans. The black hole, in effect, reached out and squeezed and stretched the star until it disintegrated.

In a televised news briefing at NASA in Washington, Günther Hasinger, also an astrophysicist at the Max Planck Institute, said, "For the first time, we really are convinced that we are seeing a star being ripped apart by a black hole."

Alex Filippenko, an astronomy professor at the University of California, Berkeley, who was not involved in the research, agreed.

"This is really fantastic," Dr. Filippenko said. "This is very strong evidence that stars are being ripped apart by supermassive black holes."

The astronomers estimated that about 1 percent of the victimized star's mass was consumed by the black hole. This small amount, they said, was consistent with theoretical predictions that the momentum and energy of star-destruction process would cause most of the star's gas to be flung away from the black hole.

The initial outburst of high-energy radiation was detected in 1992 by the German Rosat X-ray spacecraft, but the observations were fuzzy. At the time, puzzled scientists could only speculate about the nature of the flare or its origin.

The more definitive observations were made three years ago by two orbiting X-ray telescopes, NASA's Chandra and the European XMM-Newton satellites. They determined that the X-rays were coming from the center of a galaxy, RX J1242-11. The most awesome black holes, with densely packed masses equivalent to millions or billions of Suns, are found at galactic cores. This one is estimated to have a mass of about 100 million Suns.

More detailed examinations of the energy spectrum of the X-rays by the European satellite, astronomers said, revealed physical conditions similar to the expected surroundings of black holes, ruling out other possible astronomical explanations. The energy liberated by the tidal disruption was reported to be equivalent to that of a supernova, an exploding star.

Dr. Filippenko said the findings should advance understanding of black holes and provide a critical framework for theoretical models of how they grow and evolve over time.

Black holes cannot be observed directly; their gravity is so strong that nothing, not even light, can escape their clutches. What is known about black holes is mostly deduced from observations of the whirlpool motions of gas and dust and stars under their gravitational influence.

The X-ray discovery may give scientists another means of identifying the presence of black holes and learning more about their behavior.

Kim Weaver, an astrophysicist at NASA's Goddard Space Flight Center in Greenbelt, Md., said the high-energy flares of a star's disintegration by black-hole gravity could serve as flashlights to illuminate the otherwise obscuring gas and dust and to provide glimpses into the inner regions of galaxies.

The astronomers gave assurances that the Sun is far enough away from a suspected black hole at the center of the Milky Way galaxy to be well out of danger.

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