THE UNIVERSE
multiples, novae, open and globular clusters, and gaseous
and planetary nebulae. There has even been one recent
supernova, 1987A, in the Large Cloud. Telescopically they
are magnificent; look, for example, at the Tarantula Nebula,
30 Doradûs, in the Large Cloud, beside which the much-
vaunted Orion Nebula seems very puny indeed.
M31, the Andromeda Spiral, is the senior member of
the Local Group, and is considerably larger and more lumi-
nous than our Galaxy. It too contains objects of all kinds,
and there has even been one supernova, S Andromedae of
1885, which reached the fringe of naked-eye visibility.
Unfortunately its true nature was not appreciated at the
time, and it was not then generally believed that M31 was
an independent galaxy.
M31 is a typical spiral, but it lies at a narrow angle to
us, and its full beauty is lost. It is frankly rather a dis-
appointing sight in a small telescope (or even a large one),
and photography is needed to bring out its details, together
with its halo and its 300 globular clusters. The present
accepted value for its distance – 2.2 million light-years –
may have to be revised slightly upwards if, as now seems
possible, the Cepheids are rather more luminous than has
been thought. At the moment M31 is actually approaching
us. The two main satellites, M32 and NGC 205, are easy
telescopic objects; both are elliptical.
M33, the Triangulum Spiral, is often nicknamed the
Pinwheel. It is very close to naked-eye visibility, and is
not difficult in binoculars, though users of small telescopes
often find it elusive because of its low surface brightness.It is a looser spiral than M31, but lies at a more favourable
angle; it too contains objects of all kinds. Its diameter is
about half that of our Galaxy. Unlike M31, it does not
seem to have been known in ancient times, and in fact it
was discovered in 1764 by Messier himself.
Most of the remaining members of the Local Group
are dwarfs whose faintness makes them rather hard to
identify, particularly if they lie almost behind bright fore-
ground stars of our Galaxy – as with Leo I and Leo II,
which are very close to Regulus. Incidentally, no dwarf
spiral has ever been found, and it is not likely that any
dwarf spirals exist.
Finally there is Maffei 1, discovered by the Italian
astronomer Paolo Maffei in 1968. It is probably a giant
elliptical, though it has been suggested that there are signs
of spirality; it lies in Cassiopeia, not far from the plane of
the Milky Way, and is so heavily obscured that we know
little about it. A second galaxy discovered by Maffei at the
same time was once thought to be a Local Group member,
but is now thought to be a spiral at a distance of around
15 million light-years.
It is now known that there are many ‘intergalactic
stars’, presumably ejected from their original galaxies.
Observations with the Hubble Space Telescope have iden-
tified 600 of these stars in the Virgo Cluster, around
60,000,000 light-years away. There is also a globular clus-
ter, NGC 5694, which seems to be moving in a path which
will lead to its escaping from the Galaxy, and becoming
what may be called an intergalactic tramp. The Tarantula Nebula,
round 30 Doradûs, in the
Large Magellanic Cloud,
imaged with the Kueyen
mirror of the Very LargeTelescope (VLT). If the
Tarantula Nebula were as
close as the Orion Nebula,
it would cast strong
shadows. M33, also known as the
Triangulum Spiral.This
image was obtained by the
National Science
Foundation’s 0.9-metre
telescope on Kitt Peak. The
reddish areas are regions of
star formation. In 2005
astronomers using the VLBA
equipment succeeded in
measuring the proper motion
of M33. It is not exactly racing
along against its background.
The annual motion amounts
to 30 micro-arc seconds,
which is 1/100 the apparent
speed of a snail, crawling on
the surface of Mars, as
observed from the Earth!192-217 Atl of Univ Phil'05 7/6/05 2:07 pm Page 199