THE UNIVERSE
the Hubble Space Telescope show a dark X-structure
silhouetted across the nucleus, due to absorption by dust.
It has been suggested that it may indicate the presence
of a black hole at least a million times as massive as the
Sun, and in fact it is widely believed that most active
galaxies are powered by black holes deep inside them,
though on this point there is still disagreement between
astronomers.
Certainly there are some galaxies which are very ener-
getic at radio wavelengths; such is NGC5128 (C77) in
Centaurus (Map 20), also known as Centaurus A, which is
crossed by a broad dust lane giving it a most remarkable
appearance. There are also Seyfert galaxies, named after
Carl Seyfert, who first drew attention to them in 1942.
Here we have small, very brilliant nuclei and inconspicu-
ous spiral arms; all seem to be highly active, and most of
them, such as the giant M87 in the Virgo cluster, are
strong radio sources. Other galaxies emit most of their
energy in the infra-red. There are also galaxies with low
surface brightness, so that they are difficult to detect even
though they may be very massive indeed.
We must admit that our knowledge of the evolution of
galaxies is not nearly so complete as we would like. It is
tempting to suggest that a spiral may evolve into an ellip-
tical, or vice versa, but the situation does not seem to be
nearly as straightforward as this. Because giant ellipticals
are so massive, it has been suggested that they may havebeen formed by the merging of two spirals, but here too
opinions differ. At least we have observational evidence
that collisions between members of a group of galaxies do
occur. The Cartwheel Galaxy A0035, which is 500 million
light-years away, is a splendid example of this. It is made
up of a circular ‘rim’ 170,000 light-years across, inside
which lie the ‘hub’ and ‘spokes’ marked by old red giants
and supergiants; apparently the Cartwheel was once a
normal spiral, but about 200 million years ago a smaller
galaxy passed through it, leading to the formation of very
massive stars in the ‘rim’ region. The invading galaxy
can still be seen. Then there are galaxies with double
nuclei, probably indicative of cosmic cannibalism, and
even the Andromeda Spiral seems to have a double centre,
due perhaps to a smaller system swallowed up long ago.
One profitable line of research open to professionals
and non-professionals alike is the search for supernovae in
external galaxies. Because these outbursts are so colossal
they can be seen across vast distances, and are very useful
as ‘standard candles’, because it is reasonable to assume
that a supernova in a remote system will be of approx-
imately the same luminosity as a supernova in our own
Galaxy. It is important to study a supernova as soon as
possible after the flare-up, and amateur ‘hunters’ have a
fine record in this field. One never knows when a normally
placid-looking galaxy may be transformed by the dramatic
outburst of a brilliant newcomer.▲ Type Sa.NGC7217, spiral
galaxy in Pegasus. The
nucleus is well defined and
the arms are symmetrical
and tightly wound.▲ Type SB.M81 (NGC3031)
in Ursa Major. Seen at a
narrower angle than
NGC7217, its arms are
‘looser’; magnitude is 7.9.▲ Type Sc.M33 (NGC598),
the most distant member
of the local group, has a
less-defined nucleus, and the
spiral arms are not so clear.▲ Type SBa. NGC3504 in
Leo Minor. The ‘bar’ through
the centre of the system is
noticeable.▲ Type SBb.NGC7479 (C44)
in Pegasus, magnitude 10.3.
The bar formation is much
more pronounced.▲ Type SBc.Galaxy in the
Hercules cluster. Here the
bar formation is dominant,
and the arms are secondary.F Atl of Univ Phil'03stp 3/4/03 5:42 pm Page 201