Runaway black hole. (Astronomy).

Observing a black hole and a companion star caroming through our galaxy, astronomers say they've found the best evidence to date that small black holes are born during supernova explosions.

The black hole and its partner, collectively known as GRO J1655-40, are streaking across the galaxy at 400,000 kilometers per hour, four times the average speed of neighboring stars. The duo's speed and elliptical orbit about the galaxy's core suggest the bodies were kicked out of their presumed birthplace within the Milky Way's inner disk, where most stars are formed.

The gargantuan power required to set GRO J1655-40 into motion most likely came from a supernova that created the pair, asserts I. Felix Mirabel of the French Atomic Energy Commission in Gif-sur-Yvette, France, and the Institute for Astronomy and Space Physics in Conicet, Argentina.

In such an explosion, the core of a massive star implodes, sending out shock waves that eject the star's outer layers. If the core has a mass greater than three times that of the sun, gravity crunches it down into a black hole. If the explosion isn't perfectly spherical, it can impart a kick to the black hole it created.

"GRO J1655-40 is the first black hole [system] for which there is evidence for a runaway motion imparted by a natal kick in a supernova explosion," Mirabel and his colleagues report in the Nov. 19, 2002 Astronomy and Astrophysics.--R.C.