X-ray data from NASA's Chandra, the European Space Agency's XMM-Newton, and the German Roengtensatellite (ROSAT) X-ray observatories provide direct evidence for the catastrophic destruction of a star that wandered too close to a super-massive black hole.
A giant black hole in a galaxy a billion light years away has been caught in the act of butchering a star - the first time this has been seen, according to astronomers. It means that black holes all over the Universe must be eating stars, and that may be the main way they grow.
A powerful flare of X-rays was the star's final scream. The flare, from the center of a galaxy called RXJ 1242-1119, was thousands of times as bright as all the stars in the galaxy put together.
Its beginnings were seen back in 1992, when the ROSAT observatory picked up emission as strong as that from many active galaxies. Active galaxies contain a giant black hole feeding off a constant gas supply, and usually have a bright blue pinpoint in the center.
"Yet in visible light, RXJ 1242-1119 is just a normal, inconspicuous galaxy," says Stefanie Komossa of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany.
This animation shows a yellow star that travels too close to a giant black hole in the center of the galaxy RX J1242-11. As it nears, the star is stretched by tidal forces from the black hole and is quickly torn apart. Most of the yellow gaseous debris from the star escapes the black hole in parabolic orbits. However, a small amount of material is captured by the black hole and then forms a rotating disk of gas. X-rays are emitted as the gas in the disk is heated (as shown by the blue color) and is gradually swallowed by the black hole, eventually emptying the disk.
Komossa suspected that these X-rays might be a brief flare from a dying star, rather than constant emission, but she needed follow-up observations to be sure. In 2001, Komossa, Gunter Hasinger and their team looked at RXJ 1242-1119 again with two more space-based telescopes.
The Chandra observatory showed that the flare has almost subsided. Komossa's group also used XMM-Newton to show that the X-ray energies have just the broad spread expected when gas is being consumed by a black hole.
Komossa and her group are now able to reconstruct the murder scene. A star about the size of our Sun ventures too close to the black hole. "It then feels enormous tidal forces exerted by the black hole, which finally rip apart the whole star," she says.
Some of the debris circles the black hole for a while, heating up so much that it shines brilliantly in X-rays, before falling below the event horizon from beyond which no light can escape. But the black hole is a very messy eater - only a few per cent of the star actually goes in. The rest gets flung outwards again by the force of the flare.
This discovery tells us a lot about how black holes grow, according to Kimberly Weaver of NASA's Goddard Space Flight Center in Greenbelt, Maryland. "Here, for the first time, we see that a whole star can be ripped apart."
This means, she says, that it is not only active galaxies in which black holes are consuming matter. While those black holes are eating continuously, the new discovery shows that those in other galaxies can snack on stars in order to grow.
It must happen in our own galaxy too. "Here we see much milder flares. They could be something the size of a comet being swallowed," says Weaver.
But roughly once in 10,000 years, our own galaxy's giant black hole will eat a star, says Komossa: "Then the centre of our Milky Way would flare up to become about 100 billion times brighter than it is now."
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