THE STARS
evolved at different rates. Globular clusters are huge sym-
metrical systems containing up to a million stars.
The most famous open clusters are the Pleiades and
the Hyades, both in Taurus (Map 17). The Pleiades are
very conspicuous, and have been known since early times
(they are mentioned in the Odysseyand in the Bible), and
on a clear night anyone with normal eyesight can make out
at least seven individual stars – hence the popular nick-
name of the Seven Sisters. Keen-eyed people can see more
(the record is said to be 19), and binoculars show many
more, while the total membership of the cluster is around- Alcyone or Eta Tauri, the brightest member of the
cluster, is of the third magnitude. The Hyades are more
scattered, and are overpowered by the brilliant orange
light of Aldebaran, which is not a true member of the
cluster at all, but simply happens to lie about midway
between the Hyades and ourselves. The Hyades were not
listed by Messier, presumably because there is not the
slightest chance of confusing them with a comet.
Another open cluster easily visible with the naked eye
is Praesepe in Cancer (Map 5), nicknamed the Beehive or
the Manger; binoculars give an excellent view. It is much
older than the Pleiades cluster. Also in Cancer we find
M67, which is on the fringe of naked-eye visibility; it is
probably the oldest known cluster of its type, and is well
away from the main plane of the Galaxy, so that it moves
in a sparsely populated region and is not badly disrupted
by the gravitational pull of non-cluster stars. In Perseus
there is the double cluster of the Sword-Handle, and in
the far south there is the Jewel Box, round Kappa Crucis,
which has stars of contrasting colours – including one
prominent red giant. Telescopic clusters are common
enough and dozens are with the range of a small telescope.
Globular clusters lie round the edge of the main
Galaxy; over 100 are known, but all are very remote.
Messier listed 28 of them. The two brightest are so far
south that they never rise over Europe. Omega Centauri
(Map 20) is truly magnificent and is prominent even
though it is about 17,000 light-years away. The condensed
core is about 100 light-years across, and in it the stars are
so closely packed that they are not easy to see individually.
47 Tucanae, also in the far south (Map 21), rivals Omega
Centauri; by sheer chance it is almost silhouetted against
the Small Cloud of Magellan, but the Small Cloud is an
external system far beyond our Galaxy, while 47 Tucanae
is about the same distance from us as Omega Centauri.
The brightest globular in the northern sky is M13 in
Hercules (Map 9), which is just visible with the naked eye.
Globular clusters are very old, so that their leading stars
are red giants or supergiants; there is virtually no nebulosity
left in them, so that star formation has ceased. They are rich
in short-period variables, and this is how their distances
were first measured, by Harlow Shapley in 1918. By ob-
serving the ways in which the stars behaved, he could find
their real luminosities, and hence the distances of the glob-
ular clusters in which they lay. Shapley also found that
the globulars are not distributed evenly all over the sky;
there are more in the south than in the north, particularly
towards the constellation of Sagittarius. This is because
the Sun lies well away from the centre of the Galaxy, so
that we are having what may be called a lop-sided view.
Surprisingly, some hot blue giants are also found in
globular clusters and are known as blue stragglers.
Logically they ought not to be there, because high-mass
stars of this age should long since have left the Main
Sequence. What seems to happen is that because stars near
the core of the cluster are so close together, relatively
speaking, they may ‘capture’ each other and form binary
systems. The less massive member of the new pair will
then draw material away from the more evolved, less
dense companion, and will heat up, becoming blue again;
in a direct collision, two stars will merge. In this case
the resulting stars would be of greater than average mass,
and would tend to collect near the centre of the cluster,
which is precisely what we find.
If there are any inhabited planets moving round stars
near the core of a globular cluster, local astronomers will
have a very curious sort of sky. There will be many stars
brilliant enough to cast shadows; moreover, many of these
stars will be red. An astronomer there will be able to
examine many stars from relatively close range, but will
be unable to learn a great deal about the outer universe.▼ NGC 1818, star cluster in
the Large Magellanic Cloud,
170,000 light-years away.
The cluster is young – age
about 40 million years – and
is the site of vigorous star
formation. The cluster
contains over 20,000 stars.
The circled star is a young
White Dwarf, which has only
recently formed from a Red
Giant. The progenitor star
was between seven and
eight times as massive
as the Sun. Photo taken in
December 1995 with the
Hubble Space Telescope.E152-191 UNIVERSE UK 2003mb 7/4/03 5:50 pm Page 187