Latest studies lend credence to the theory that the Milky Way has a large black hole at its center
THE centre of the Milky Way, a wispy
band of faint white light, has long been
an object of speculation. There have been
conjectures that there is a huge black hole
at the centre. The latest reports, however,
for the first time seem to confirm the
existence of a compact, massive object, a
few million times more massive than the
Sun, at the centre. A Eckart and R Genzel
of the Max Plank Institut, Garching,
Germany, have reported the first direct
observations of the proper motions of
stars near the galactic centre (Nature,
Vol 383, No 6599).
Their observations, which have been
under way since 1991, were carried out
by the 3.5 m-long New Technology
Telescope in La Silla, Chile. Using specially-designed cameras and other
instrumentation, they have determined
the proper motions of 39 stars located
between one and a tenth of a light year
from the centre.
The Milky Way galaxy is lensshaped, with its centre towards the constellation Sagittarius, almost 33,000
light years away. The galaxy is about
15,000 light years thick at the centre and
about 100,000 light years across. It contains more than 100 billion stars. The
observations of the motion of stars in
any system give us information regarding the mass contained in the system. If,
for instance, the cluster of stars is bound
(that is, if they are not moving apart),
then their velocities are determined by
the total mass, just as the earth's orbital
velocity is calculated by the mass of the
Sun.
However, the measurement of
velocities of stars is not easy and we only
have knowledge of what is called the
line-of-sight velocity, since this is what
we measure by using Doppler shift of
the light emitted by the star. The normal
assumption has been that other components of the stellar velocity are the
same as the one observed, but this had
never been proved.
Genzel and Eckart have measured
the proper motion of the stars against a
distant backdrop and measured for the
first time the missing components of the
stellar velocities. Their results indicate
almost incontrovertibly the existence of
a sizeable mass at the
galactic centre, which is
not visible. The mass is
estimated to be about
2.5 million times the
mass of the Sun. What
is more intriguing is the
fact that this mass is
very close to a bright
radio object called
Sagittarius A*.
This is consistent
with the hypothesis that
the radio source is actually a disk of hot gas
going into a black hole.
If correct, this would
make Sagittarius A* the
largest black hole in the
galaxy and confirm
what theorists have believed for a long
time, namely, the existence of a black
hole at the galactic centre. High resolution measurements are underway and
should provide us with more evidence
in support of this tantalising idea.
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