Milky Way Monster

It is known as the Great Annihilator: a mysterious region close to the center of the Milky Way galaxy that spews out bursts of high-energy gamma rays. A popular theory held that the Great Annihilator was actually a gigantic black hole, a million stars collapsed into a single object so dense that its gravity wouldn't let even light escape. New information gathered by the Very Large Array radio telescope in New Mexico and published in Nature has found that this theory is slightly off the mark. The Great Annihilator does indeed seem to be a black hole, but it's only as massive as a single star, and it's merely close to, but not right at, the center of the Milky Way.

An important part of the theory, however, still seems valid. It holds that as additional gas falls into the black hole, it is compressed and heated. This process creates positrons, one form of the strange stuff known as antimatter; as the positrons are flung out into space, they eventually collide with interstellar clouds. Result: enormous explosions.

Every subatomic particle has a corresponding antiparticle, identical in mass but differing in one crucial characteristic, like electric charge. Positrons, which carry a positive charge, are the antiparticles of electrons, which are negative. Matter and antimatter destroy each other whenever they meet, and because interstellar clouds are full of electrons, these particular collisions have been fingered as the power behind the Great Annihilator.

Black holes are thought to be rather common in the galaxy, so why has only one Great Annihilator been found? The answer may be that the galactic core is ! unusually full of gas clouds; these provide lots of matter for the antimatter to run into. Other parts of the galaxy are just too empty for black holes to create much of a bang.