For the first time, two orbiting telescopes have seen the violent death throes of a star being partially swallowed by a black hole. The observation supports a long-held theory of how black holes capture unsuspecting stars that wander into their neighborhood.
One of the long-standing questions in astrophysics has been what happens when something falls into a massive black hole. These are immensely dense objects believed to be at the center of most galaxies -- so dense that their phenomenally strong gravity pulls in everything within reach and will not even let light escape.
But the U.S. Chandra and European XMM-Newton space telescopes have witnessed the light of a star in a distant galaxy as part of it was falling in.
Astrophysicist Guenther Hasinger of the Max Planck Institute in Germany says the observatories caught a powerful x-ray death blast from the star -- light that was able to penetrate the obscuring dust of the distant galaxy in a way visible wavelengths cannot. He describes the event near the black hole.
"The poor star is moving around it and one of its turns is coming so close to the black hole that it is stretched, then ripped apart," says Mr. Planck. "Then part of the debris of the star is falling onto the black hole, is heating up, then basically the last cry of help before the matter falls into the black hole is radiated in x-rays, which we have observed."
The star's final days were first witnessed in the early 1990s by another European x-ray satellite named Rosat, but its images were too fuzzy to let scientists know where the light was coming from. It took the newer, higher resolution Chandra and Newton telescopes to show them that the x-ray flares were coming from the center of a galaxy 700 million light years away, with a black hole estimated at 100 million times more massive than our sun.
"This is really fantastic stuff," says University of California astronomer Alex Filippenko. "This is one of the holy grails of astronomy."
Mr. Filippenko, who did not take part in the research, says the findings are the first strong evidence that black holes actually do feed on stars as hypothesized. "If such events are not seen, then they call into question the whole hypothesis that supermassive black holes exist in these galaxies," he says. "But now this combination of data from Rosat, Chandra, and XMM-Newton provides the definitive evidence. I think this is very strong evidence that stars are being ripped apart occasionally by supermassive black holes."
The researchers say that if a massive black hole disrupted a star at the center of our Milky Way 25,000 light years away, the resulting x-ray outburst would be 50,000 times brighter than the brightest x-ray source in the galaxy. It would not endanger Earth, but it would destroy the light gathering instruments aboard the orbiting telescopes. The energy output of such an event is enormous -- enough to provide power for 100-billion Earths for as long as the universe has existed.
The chances of this happening, however, are considered very rare -- about once every 10,000 years in a typical galaxy. But Guenther Hasinger told reporters at U.S. space agency headquarters in Washington that with billions of galaxies in existence, there should be many chances to see a black hole destroy a star.
"We very much hope that future x-ray survey missions, which scan a large part of the sky, will actually pick up these sources regularly," says Mr. Hasinger. "At that time we will be alert -- we know the event now -- and we will [alert] all the astronomers around the globe to follow this and we hope we can really learn from this experiment that nature is providing for us."
In the distant galaxy the German astronomers witnessed, the dying star's x-ray blast lingers, but is much fainter as it gradually fades from view.
