Feb 18, 2004

Black Hole Mayhem


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Black Hole Mayhem

Two x-ray telescopes have caught a black hole in the act of ripping a star apart.





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February 18, 2004: Thanks to two orbiting X-ray observatories, astronomers have the first strong evidence of a supermassive black hole ripping apart a star and consuming a portion of it. The event, captured by NASA's Chandra and ESA's XMM-Newton X-ray Observatories, had long been predicted by theory, but never confirmed ... until now.

Astronomers believe a doomed star came too close to a giant black hole after being thrown off course by a close encounter with another star. As it neared the enormous gravity of the black hole, the star was stretched by tidal forces until it was torn apart. This discovery provides crucial information about how these black holes grow and affect surrounding stars and gas.


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Above: An artist's visualization of a star that wandered too close to a supermassive black hole in galaxy RX J1242-11. [More]

Observations with Chandra and XMM-Newton, combined with earlier images from the German Roentgen satellite, detected a powerful X-ray outburst from the center of a galaxy named "RX J1242-11." This outburst, one of the most extreme ever detected in a galaxy, was caused by gas from a star that was heated to millions of degrees Celsius before being swallowed by the black hole. The energy liberated in the process was equivalent to a supernova.




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"Stars can survive being stretched a small amount, as they are in binary star systems, but this star was stretched beyond its breaking point," said Stefanie Komossa of the Max Planck Institute for Extraterrestrial Physics (MPE) in Germany, leader of the international team of researchers. "This unlucky star just wandered into the wrong neighborhood."

The black hole in the center of RX J1242-11 has a mass of about 100 million times Earth's Sun. By contrast, the destroyed star probably had a mass about equal to the Sun. Astronomers estimate that only one percent of the star's mass was ultimately consumed, or accreted, by the black hole. The rest was flung away from the black hole.

The force that disrupted the star in RX J1242-11 is an extreme example of the tidal force caused by differences in gravity acting on the front and back of an object. The tidal force from the Moon causes tides in Earth's oceans. A tidal force from Jupiter pulled Comet Shoemaker-Levy apart, before it plunged into the giant planet.


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Left: Black holes aren't the only things that cause strong tides. Jupiter can do it, too. This illustration shows Comet Shoemaker/Levy crashing into Jupiter in 1994, after the comet was torn apart by the giant planet's tides. Although on a very different scale, the physical mechanism for the breakup of Shoemaker/Levy also caused the disruption of the star in RX J1242-11.


The odds of a stellar tidal disruption in a typical galaxy are low, about one in 10,000 annually. If it happened at the center of the Milky Way Galaxy, 25,000 light-years from Earth, the resulting X-ray outburst would be about 50,000 times brighter than the brightest X-ray source in our galaxy, but it would not pose a threat to Earth.

Other dramatic flares have been seen from galaxies, but this is the first one studied with the high-spatial resolution of Chandra and the high-spectral resolution of XMM-Newton. Both instruments made a critical advance. Chandra showed the RX J1242-11 event occurred in the center of a galaxy, where the black hole lurks. The XMM-Newton spectrum revealed the fingerprints expected for the surroundings of a black hole, ruling out other possible astronomical explanations.

Supermassive black holes in the centers of galaxies are familiar to astronomers. There are many of them, including one at the heart of our own Milky Way. Now astronomers have a way to find more: look for x-ray outbursts when stars are ripped apart by black-hole tides. Observations like these are needed, say researchers, to determine how quickly black holes can grow by swallowing neighboring stars.