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1993-04-08
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THE WEEK, Page 27HEALTH & SCIENCEMilky Way Monster
Bursts of gamma rays point to a black hole at the center of
our galaxy
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.