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Astronomers watch black hole destroying star


WASHINGTON - Astronomers using NASA's orbiting Chandra X-ray Observatory say they have detected the death cry of a sun-like star being ripped apart and partly devoured by a huge black hole.

The star's last gasp, described yesterday by scientists at NASA headquarters, was a powerful X-ray flare that erupted from the center of a previously quiet galaxy.

Albert Einstein theorized the existence of black holes in 1915 - objects so massive that even light could not escape their gravitational grasp. Astronomers suspected for decades that "supermassive" black holes, with a mass millions of times larger than our sun, might lurk at the center of many galaxies, growing by slurping up unlucky stars.

But until now they had never caught one snacking.

"This is one of the Holy Grails of astronomy," said astrophysicist Alex Filippenko, of the University of California, Berkeley.

He was not involved in the discovery, but said, "I believe it provides definitive evidence that stars are being ripped apart by supermassive black holes."

Billion light-years away

Until it met its fate, the doomed star orbited near the center of a galaxy known as RXJ1242-11, nearly a billion light-years from Earth. (A light-year is the distance light travels in one year - about 59 trillion miles.)

The galaxy had been quiet, but in 1992, NASA's ROSAT satellite spotted an enormous X-ray flare erupting from something inside it.

X-ray bursts are created when gas is heated to millions of degrees Celsius. And this one was one of the most powerful ever detected - thousands of times brighter than all the other stars of that galaxy combined.

"The observation strongly suggested a doomed star wandered too close to a black hole at the center of this galaxy," said Stefanie Komossa, a post-doctoral researcher at the Max Planck Institute in Germany.

In 2001, Komossa and astrophysicist Gunther Hasinger, also of the Max Planck Institute, decided to try to prove it.

Aiming the Chandra X-ray telescope at RXJ1242-11, they found the flare. It had faded to a 200th of its peak brightness, but even that fit nicely with theoretical models of how the wreckage of a star would dim as it was gradually consumed by a galactic black hole.

With Chandra's high resolution, they could also see that the X-rays were coming from the galaxy's core, exactly where the black hole should be.

Second look

"We then immediately pointed the XXM Newton Telescope at this galaxy to capture the bright flare before it faded away completely," Komossa said.

Black holes can't be seen directly because they don't emit light. But the Newton telescope's analysis of light from other stars near the galaxy's core produced data that could be explained only by the presence of a galactic black hole.

The team estimated its mass at 100 million times that of our sun. That clinched it.

Kimberly Weaver, an astronomer at the Goddard Space Flight Center in Greenbelt, said the doomed star likely was nudged from a safe orbit by the pull of another star.

As it drifted closer to the black hole, an intensifying gravitational field began to stretch the star like taffy.

It's called "tidal disruption," the same force that enables the moon's relatively feeble gravity to stretch Earth's rocky crust slightly and to create ocean tides.

Destruction of comet

In 1994, Jupiter's much stronger gravity created tidal disruption on Comet Shoemaker-Levy 9. It broke the comet's icy nucleus into 21 pieces as it flew by the planet.

Two years later, the fragments plummeted into Jupiter's swirling atmosphere.

A star falling straight into a black hole "can be eaten whole, before it's ripped apart, and you don't detect a flare in X-rays," Weaver said. Some on safer trajectories might be stretched but pass by safely.

But in this case, the forces of tidal disruption exceeded the gravity that had held the star together, and it flew apart in a matter of hours or days.

Its seething gases, heated to many millions of degrees, have since gushed a torrent of X-rays, "the last cry for help before it falls into the black hole," Hasinger said.

Astronomers believe only 10 percent of the star's gas actually fell into the black hole. The rest was likely hurled like road spray into surrounding regions of the galaxy.

Such events are rare in "mature" galaxies, where black holes have already devoured nearby dust, gas and stars. In a galaxy like our own Milky Way, black holes probably make meals of unlucky stars only about once every 10,000 years on average, scientists said.

Astronomers who study the Milky Way's core say they see no stars on a collision course with the black hole believed to lie at its center.

If one did stray in, it would produce an X-ray flare 50,000 times brighter than anything else in the sky except the sun. That would pose no threat to people, Weaver said, "but it would fry the instruments on Chandra."

With thousands of other galaxies to watch, Hasinger said, astronomers are now on alert for more X-ray flares.

By comparing them, they hope to learn more about black holes and about the hard-to-see inner regions of distant galaxies.

An article in yesterday's Sun about a star being destroyed by a black hole incorrectly stated the distance traveled by light in one year. The correct distance is about 5.9 trillion miles.
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