Black Holes
ATLAS OF THE UNIVERSE
The Vela supernova
remnant from the 3.9-m
(153-inch) Anglo-Australian
telescope. The red, glowing
filaments are due to
hydrogen. The Vela pulsar
was the second to be
identified optically; its
magnitude is 24, one of the
faintest objects ever
observed. Its rate of slowing
down indicates that the
supernova outburst occurred
about 11,000 years ago. The
green line across the
photograph is the path of a
satellite that traversed the
field of view during the
exposure of the green-
sensitive plate.I
n many ways, stellar evolution is now reasonably well
understood. We know how stars are born, and how they
create their energy; we know how they die – some with a
whimper, others with a very pronounced bang. But when
we come to consider stars of really enormous mass, we
have to admit that there are still some details about which
we are far from clear.
Consider a star which is too massive even to explode as
a supernova. When its energy runs out, gravity will take
over, and it will start to collapse. The process is remarkably
rapid; there is no outburst – the star simply goes on becom-
ing smaller and smaller, denser and denser. As it does so,
the escape velocity rises, and there comes a time when the
escape velocity reaches 300,000 kilometres (186,000
miles) per second. This is the speed of light, so that not
even light can escape from the shrunken star – and if light
cannot do so, then certainly nothing else can, because light
is as fast as anything in the universe. The old star has sur-
rounded itself with a ‘forbidden area’ from which absolute-
ly nothing can escape. It has created a black hole.
For obvious reasons, we cannot see a black hole – it
emits no radiation at all. Therefore, our only hope of locat-
ing such an object is by detecting its effects upon some-
thing which we can see. A typical example is Cygnus X-1,
so called because it is an X-ray source; it lies near the star
Eta Cygni (Map 8). The system consists of a B-type super-giant, HDE 226868, which is of the ninth magnitude. It
seems to have about 30 times the mass of the Sun, with a
diameter of perhaps 18 million kilometres (11.25 million
miles); it is associated with an invisible secondary with 14
times the Sun’s mass. The orbital period is 5.6 days as we
can tell from the behaviour of the supergiant;the distance
from us is 5000 light-years. What seems to be happening is
that the black hole is pulling material away from the
supergiant, and swallowing it up. Before this material dis-
appears, it is whirled around the supergiant, and is so
intensely heated that it gives off the X-ray radiation which
we can pick up.
The size of a black hole depends upon the mass of the
collapsed star. The critical radius of a non-rotating black
hole is called the Schwarzschild radius, after the German
astronomer who investigated the problem mathematically as
long ago as 1916; the boundary around the collapsed star
having this radius is termed the ‘event horizon’. Once mater-
ial passes over the event horizon, it is forever cut off from
the rest of the universe. For a body the mass of the Sun, the
Schwarzschild radius would be about 3 kilometres (1.9
miles); for a body the mass of the Earth the value would be
less than a single centimetre (less than half an inch).
There is now no doubt that massive black holes exist
in the centres of many galaxies, and are also the power-
sources of quasars. Movements of stars near the centre of152-191 Atl of Univ Phil'05 7/6/05 1:24 pm Page 184