麻豆传媒

FIRST it ripped the body apart; then it ate the shreds. A giant black hole in a galaxy a billion light years away has been caught in the act of butchering a star 鈥� the first time this has been seen, astronomers announced this week.

It supports the idea that black holes all over the universe must be eating stars, a habit which may be their chief method of growth.

A powerful flare of X-rays was the star鈥檚 final scream. The flare, from the centre 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 satellite observatory picked up emission as strong as that from many active galaxies. Unlike our own Milky Way, which has a relatively quiet black hole at its centre, active galaxies contain giant black holes feeding off a constant gas supply, which appear as bright blue points. 鈥淵et in visible light, RXJ 1242-1119 is just a normal galaxy,鈥� says Stefanie Komossa of the Max Planck institute for extraterrestrial physics in Garching, Germany.

Komossa suspected these X-rays might be a brief flare from a dying star, rather than a constant emission, but she needed follow-up observations to be sure. In 2001 Komossa, G眉nther Hasinger and their team looked at RXJ 1242-1119 again with two more space-based telescopes. The Chandra observatory showed that the flare had almost subsided, and pinpointed its location to the core of the galaxy. Komossa鈥檚 group also used the orbiting XMM-Newton telescope to show that the X-ray energies have exactly the broad spread that astrophysicists expect to see from gas being consumed by a black hole.

Komossa and her group can now reconstruct the crime. First, a star about the size of our sun ventures too close to a black hole. 鈥淚t then feels enormous tidal forces exerted by the black hole which finally rip apart the whole star,鈥� Komossa 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 where no light can escape. But the black hole is a very messy eater. Only a small percentage of the star actually goes in; the rest is flung outwards by the force of the flare.

This discovery tells us a lot about how black holes grow, according to Kimberly Weaver of NASA鈥檚 Goddard Space Flight Center in Greenbelt, Maryland, an expert in active galaxies. 鈥淗ere for the first time we see that a whole star can be ripped apart 鈥� you can have stuff falling in for the life of the galaxy, and the black hole can just grow and grow and grow.鈥�

Supposedly less active galaxies, including the Milky Way, may grow in a similar way. 鈥淩oughly once in 10,000 years, our galaxy鈥檚 giant black hole will eat a star,鈥� Komossa says. 鈥淭hen the centre of our Milky Way would flare up to become about 100 billion times brighter than it is now.鈥�