The Structure of the Universe
L
ook up into the sky on any dark, clear night, and you
will see the glorious band of the Milky Way, stretching
from one horizon to the other. It must have been known
since the dawn of human history, and there are many leg-
ends about it, but it was not until 1610 that Galileo, using
his primitive telescope, found that it is made up of stars –
so many of them that to count each one would be impossi-
ble. They look so close together that they seem in immi-
nent danger of colliding, but, as so often in astronomy,
appearances are deceptive; the stars in the Milky Way are
no more crowded than in other parts of space. We are
dealing with a line of sight effect, because the star system
or Galaxy in which we live is flattened. Its shape has been
likened to that of a double-convex lens or, less romantical-
ly, two fried eggs clapped together back to back. But does
it make up the whole of the universe?
When Messier drew up his catalogue of nebulous
objects, he included nebulae of two different types; those
which were fairly obviously gaseous (such as M42 in
Orion’s Sword) and those which were starry (such as
M31 in Andromeda, which is dimly visible with the naked
eye). In 1845 the Earl of Rosse, using his great 183-cm
(72-inch) reflector at Birr Castle in Ireland, found that
many of the starry nebulae are spiral in form, so that they
look like Catherine wheels, and it was suggested that they
were outer systems ranking as galaxies in their own right
(in fact William Herschel had considered this possibilitymuch earlier). The main problem was that whatever their
nature, the spirals were too far away to show measurable
parallax shifts, so that their distances were very hard even
to estimate. As recently as 1920 Harlow Shapley, who had
been the first man to make a good estimate of the size of
our Galaxy, was still maintaining that the spirals were
minor and relatively unimportant features.
It was left to Edwin Hubble to provide an answer. In
1923 he used the Hooker telescope at Mount Wilson,
then much the most powerful in the world, to detect
Cepheid variables in some of the spirals, including M31.
He measured their periods, and worked out their distances.
The results were quite clear-cut: the Cepheids, and hence
the systems in which they lay, were much too remote to
be members of our Galaxy, so that they could only be
external systems.
Hubble’s first estimate of the distance of M31, the
nearest of the large spirals, was 900,000 light-years, later
reduced to 750,000 light-years. This later proved to be an
underestimate. In 1952 Walter Baade, using the then-new
Palomar reflector, showed that there are two types of
Cepheids, and that one type is much more luminous than
the other; the variables used by Hubble were twice as
powerful as he had thought, and therefore much more
distant. We now know that M31 is over two million light-
years away, though even so it is one of the very closest of
the outer galaxies.ATLAS OF THE UNIVERSE
This map of the sky
in supergalactic coordinates
shows the distribution of
over 4000 galaxies with an
apparent diameter greater
than one arc minute. The
concentration of galaxies
along the supergalactic
equator, particularly in
the north (right half), is
evident, as is the clustering
of galaxies on many different
scales, from small groups,
such as the Local Group,
to large clusters, such as the
Virgo Cluster near longitude
100–110°, slightly south of
the equator.▼ M31, the Andromeda
spiral, as seen by the
0.9-metre (36-inch) telescope
at the Kitt Peak National
Observatory. This is
the nearest of the really
large galaxies; it lies at
an unfavourable angle to
us, so that the full beauty
of the spiral is lost. It is
considerably larger than
our Galaxy.F Atl of Univ Phil'03stp 3/4/03 5:41 pm Page 194