LONDON (Oct 2, 1996 6:54 p.m. EDT) - German astronomers said on
Wednesday they had all but proved there was a massive black hole at
the centre of our galaxy, the Milky Way.
Reinhard Genzel of the Max-Planck Institute for Extra-Terrestrial
Physics, near Munich, said he remained cautious about claiming
absolute proof that the black hole existed.
But "it was caution backed up by the best evidence that has yet
existed," he told Reuters.
Evidence for a massive black hole -- an object that sucks matter
towards it and is so dense that not even light can escape -- has been
accumulating for the past 20 years.
The only means of detecting a black hole is by observing its
gravitational effects on other objects. Line-of-sight velocity of
stars orbiting the galactic centre had suggested the presence of a
black hole but proof was missing. Since 1992, scientists at the
Max-Planck Institute have measured the "proper" motions -- at right
angles to line-of-sight velocities -- of 39 stars in the Milky Way,
they reported in Nature magazine.
Their observations backed up the assumption that the stars moved in
circular orbits around a large central mass with gravitational pull.
If orbits had been irregular, it would have shown the central mass was
much smaller.
"The fact that we were able to come so close to the central object and
to test the velocities of the stars -- that's what makes the
measurements unique," Genzel said.
The central dark mass was about 2.5 million times greater than the
sun, the research showed.
"Why do I hesitate (in claiming absolute proof)? Before we go too far,
we will give the information to colleagues around the world and they
will test it," he said.
"Our colleagues will take it as a very interesting new step but it has
to be confirmed. By early next year, if they don't find any problems,
then it will be a definite."
In a commentary published in Nature, Mark Morris from the University
of California said the German findings "show with little ambiguity
that about 2.5 million solar masses of dark matter lurks within a
relatively tiny region at the centre."
The probable black hole is situated very close to a bright, as yet
unresolved, radio source called Sagittarius A+. The German
measurements are consistent with the possibility that all the dark
mass is in Sagittarius A+ and fulfils the theory about how hot gas
sucked into a black hole would behave, Morris said.
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