Conference warhed::astronomy

From:	VBORMC::"[email protected]" "MAIL-11 Daemon" 27-MAY-1997 04:09:01.90
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Subj:	[ASTRO] Black Holes Really Are Black!

Black Holes Really Are Black!

Black holes get their name because -- at least according to the theory -- they
are so dense that no light can escape from them. However, there has so far
been no direct demonstration of the fundamental property that they have no
surface, and are thus genuinely black. Now research at Leicester University
done by Professor Andrew King, with co-workers Dr Ulrich Kolb and Dr Ewa
Szuszkiewicz, has shown for the first time that black holes really are
black! Professor King will talk about this conclusion on Wednesday 9th April
at the UK's National Astronomy Meeting at the University of Southampton.

There is strong indirect evidence that several binary star systems in our
Galaxy contain black holes closely orbiting a normal companion star. The
binary systems in question are unusual in that they emit huge amounts of
X-rays in sporadic outbursts. These outbursts last for months, but may take
as long as 50 years or more to recur. The outbursts are powered by the
gravitational energy released when huge masses of gas from the companion
star fall on to the star astronomers believe is a black hole. The gas is
being steadily torn off the companion star by the black hole's gravitational
pull. However it does not fall directly on to the black hole, but orbits it
in the form of a large flat disc of gas, rather like Saturn's rings. The
outbursts occur when this disc becomes unstable, and its gas suddenly falls
towards the black hole.

Gas can build up in the disc and cause periodic outbursts only if the disc
is normally fairly cool. Professor King's team has shown that, if there is a
hot star at the centre of the disc, it will keep the disc hot. Under these
circumstances, the instability never gets a chance to build up because
material can flow steadily in towards the central star. The system then
appears as a steady source of X- rays instead of having outbursts. There are
many systems like this known: the central star in them is not a black hole
but a neutron star. Neutron stars are almost as dense as black holes, but,
crucially, they are just extended enough to have a real stellar surface.
This surface is heated by the steady infall of gas from the disc, keeping
the disc hot and stable, which in turn maintains the steady inflow heating
the stellar surface in the first place.

Yet in every case where astronomers suspect for independent reasons that the
central star is not a neutron star but a black hole, the system has
outbursts rather than being steady. This must mean that the central star
cannot heat the disc, i.e. that this star has no hot surface.

The inevitable conclusion is that black holes really are black. And this
reasoning dispels any remaining doubt about whether the suspect black holes
are the real thing.

This work was performed in the Department of Physics and Astronomy at the
University of Leicester. The group is led by Professor Andrew King, with
co-workers Dr Ulrich Kolb and Dr Ewa Szuszkiewicz.

Contact

     Prof. Andrew King, Astronomy Group, University of Leicester, Leicester
     LE1 7RH
     Telephone 0116-252 2072; fax 0116-252 2070
     email: [email protected]



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From:	VBORMC::"[email protected]" "MAIL-11 Daemon" 27-MAY-1997 04:06:34.78
To:	[email protected]
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Subj:	[ASTRO] Astronomer Royal Puts The Case For Supermassive Black Holes

Astronomer Royal Puts The Case For Supermassive Black Holes

When the Astronomer Royal, Professor Sir Martin Rees of Cambridge
University, launches the 1997 National Astronomy Meeting at the University
of Southampton as the first invited speaker on Tuesday morning (8th April),
he will share with his audience his personal conviction that supermassive
black holes do exist in the universe. He will explain how observations over
the last two years have transformed astronomers' understanding of these
fascinating objects.

'Though theorists have predicted for three decades that black holes should
exist, the evidence has, until recently, been substantial but not
overwhelming' Rees admits. But he goes on, 'The case has, however, greatly
strengthened in the last two years. The most convincing evidence pertains to
huge dark masses, millions (or even as much as a billion) times heavier than
the Sun, that lurk in the centres of most big galaxies. The mass at the
centre of our own Milky Way Galaxy amounts to two and a half million Suns.
These 'dark objects' are the outcome of catastrophic gravitational collapse
when the Universe was 10 to 20 percent of its present age, and the galaxies
themselves were young.'

'The formation of these central black holes, and its immediate aftermath,
was signalled by the violent events that we call 'quasars'. We can now
directly observe galaxies so remote we are looking back that far in cosmic
history, because of the great period of time it has taken the light to
travel to us. In so doing, we can study the population of quasars and black
holes in our Universe.'

'According to Einstein's general theory of relativity, the properties of
black holes depend only on their mass and their spin. The drastic
distortions of space and time close to a black hole, predicted by Einstein's
theory, are now being directly observed: such measurements offer the first
test of Einstein's theory under circumstances where the effects are truly
dramatic, rather than mere small deviations from Newton's theory, as in our
solar system. An even more interesting prospect is that we may sometime
observe two black holes in collision.'

Future prospects for learning more about supermassive black holes depend on
ever better optical and radio observations from the ground, as well as data
from x-ray telescopes in space.

Contact

     Professor Sir Martin Rees, Institute of Astronomy, University of
     Cambridge.
     Phone (0)1223 337520; fax (0)1223 337523;
     e-mail: [email protected]

Note

The 1997 National Astronomy Meeting runs from 9.00 a.m. on Tuesday 8th April
to 12.30 p.m. on Friday 11th April in the Physics Department of the
University of Southampton. Press room facilities are available and media
representatives are welcome to attend. Press Room phone nos. for the
duration of the meeting are (0)1703 593173 and (0)1703 593174.



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