Globular
clusters are spherical clusters of stars that typically contain a few hundreds
of thousands of stars, or even as many as a million. They are not galaxies, but
are associated with galaxies in that they orbit around the galactic plane in a
spherical halo.
About 150
globular clusters have been detected around our own galaxy, at distances of
60,000 light years (or more) from the galactic plane. A few can be detected
with the naked eye, looking like faint, fuzzy stars. One of the best-known has
been given the designation M13 (pictured above) and is visible on summer evenings in the
constellation of Hercules. Seen through binoculars it has the appearance of a
mothball, and it is only when seen through a large telescope that its true
nature is revealed. The best images have come from the Hubble Space Telescope
which has shown it to be a whirling mass of tightly packed stars.
Density
However,
although the density of stars towards the centre of a globular cluster is up to
100 times greater than in our own region of space, that does not mean that the
stars are constantly crashing into each other. If you fired a gun at a point
near a cluster’s centre, the chance of hitting a star would be less than 10 to
the power of 11 (i.e. 1 with 11 zeroes following, which is 100 billion) to one.
Seen another
way, if you lived on a planet that circled a star at the heart of a globular
cluster, the other stars would appear as points of light just as neighbouring
stars do to us on Planet Earth. They would be light months away as opposed to
light years, and there would be many more of them, but they would be points of
light nonetheless. The night sky would always have a glow to it, similar to
that of faint moonlight on Earth, because of the thousands of stars in the sky
that would make it extremely difficult to see any objects that did not belong
to the cluster. A Hubble telescope in orbit round our fictional planet would
have a much harder task in aiding our understanding of the Universe than the
real one does!
Age
Observations
of stars in globular clusters show that they are very old, and that the
clusters would have formed between 12 and 20 billion years ago, which predates
the age of the Milky Way (our own galaxy). Their orbits tend to be highly
elliptical and not in the same plane as the galaxy. Such an orbit is therefore
similar to that of a comet within our own Solar System, and might take as much
as 100 million years to complete.
Star ages are
determined by calculating their mass, luminosity and temperature, which in turn
shows what stage they have reached in their life cycle. Depending on their
original mass, stars go through different evolutionary sequences, but a typical
process for a star of similar mass to our Sun is for it to turn into a red
giant when the hydrogen at its core has been exhausted. Before this point, a
Sun-like star is said to be in the “main sequence” of stars which are stable
and have plenty of fuel left to continue the thermonuclear reactions that
supply their energy output.
At a later
stage in a red giant, gravitational collapse may provide the conditions for the
helium at the core to ignite, which is termed the “helium flash” stage. A star
that has reached this stage will be both smaller and hotter at the surface than
a red giant.
Globular
clusters are known to be old because they contain no relatively high-mass main
sequence stars, these having evolved into red giants. The main sequence has
become shorter because many stars have reached the “turnoff point”. There will
also be post-helium flash stars, many of which will have started with
relatively low mass, and some “blue stragglers” which are stars that are hotter
than main sequence stars but have not turned into red giants, possibly because
they are gaining hydrogen fuel from a neighbouring star.
Metal poor stars
Another
characteristic that typifies members of globular clusters is that they tend to
be “metal poor”. This means that their spectra do not reveal the presence of
heavy elements which are found within stars such as our Sun. This is believed
to be because such elements were created by the supernova explosions of massive
ancient stars, the matter from which then condensed to form later star
generations. The stars within globular clusters were formed from material that
was available when there was no supernova detritus around.
Globular
clusters have, in the past, been very difficult to study because of the problem
of distinguishing individual stars. However, tools such as Hubble have made
this much easier, with the result that astronomers and astrophysicists are now
able to learn much more about how the Universe evolved.
© John
Welford
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