OPEN CLUSTERS AND GLOBULAR CLUSTERS
By David Lengyel
Go outside on a clear, moonless night and look at the stars all around you.
They seem to be more or less randomly placed against the celestial sphere.
However, you may notice a few closely-packed groupings of stars which stand
out even to the unaided eye. In northern hemisphere Fall and Winter, you
can easily spot the Pleiades and Hyades star clusters in the constellation
of Taurus. Here, the stars lie very close to one another and if you use
binoculars or a small telescope, dozens of stars are seen in a small area
of sky. If you then locate the constellation Hercules, and look within the
asterism known as the Keystone using binoculars or a small telescope, you’ll
spot a fuzzy, roundish group of stars known as the Hercules cluster, or M-13,
the 13th object that Charles Messier noted to avoid when searching for new
comets. But, instead of following Messier’s advice and avoiding this stange
fuzzball, I linger here often, as I do when viewing the Pleiades or Hyades.
For these are wonderful examples of star clusters, groupings of stars within
our galaxy which certainly merit a good, long look.
It turns out that star clusters come in two varieties, open clusters sometimes
called galactic clusters, and globular clusters, whose name strongly suggests
their shape. The shapes of open clusters vary widely. They are basically
loose collections of stars that seem to be randomly placed within the cluster.
Their common names can be quite curious, as astronomers have allowed themselves
to see various patterns in their arrangements. We have the Beehive in Cancer,
the Wild Duck cluster in Scutum and the Double Cluster in Perseus. Globular
clusters, though they vary in apparent brightness and relative size, all
are spherically shaped and so lack the colorful monikers of their more scattered
cousins.
Open clusters and globular clusters are quite different in many other ways.
There are a little over 1000 known open clusters. Astronomers have determined
that they are found chiefly within the flattened disk of our galaxy, among
its spiral arms (hence the name “galactic clusters) and the stars in open
clusters seem to be representative of the stars found in that part of our
galaxy. These clusters usually contain no more than a few hundred stars,
and some as few as 20. The true number of open clusters in our galaxy may
indeed be much higher, since their location in the plane of the galaxy means
that many of them are obscured from our view by interstellar dust and stars
along the galactic plane. Globular clusters are monsters by comparison. They
contain somewhere between ten thousand and a million stars. They are not
found within our galaxy’s disk, but rather they mostly occupy a region roughly
spherical in shape surrounding the center of the galaxy. This spherical “halo”
extends in three dimensions from near the galactic center out to a distance
of more than 100 000 light years. Their distribution is more concentrated
toward the center of the galaxy. The 150 known globular clusters are distributed
among only 43 of the 88 constellations in our sky. Their location in this
spherical galactice halo causes their distribution to be concentrated around
the galactic center from our point of view or roughly centered around the
constellation of Sagittarius.
When we observe either type of cluster, we are seeing stars that are relatively
the same distance from us. For example, the Pleiades cluster is found about
400 light years from Earth. Its estimated diameter is ten light years, which
means that all of the stars in the cluster are within 2.5% of being the same
distance from us, which is a reasonable margin of error. It is also reasonable
to assume that all of the stars in a cluster formed at about the same time,
since the process of the formation of the stars in the cluster requires much
less time than the overall lifespan of the cluster as a whole. It is also
reasonable to assume that since the stars in a cluster are relatively close
together that they formed out of pretty much the same ingredients.
The stars in an open cluster have about the same chemical distribution as
our Sun. They contain about 90% hydrogen, about 10% helium and less than
1% heavier elements (“metals”, in astronomical terms). These stars are termed
Population I stars. Globular clusters contain stars which are different.
These stars, termed Population II stars contain very little amounts of elements
heavier than helium. These “metal poor” stars are all older than the relatively
“metal rich” stars of Population I (in this case II comes before I). The
Population II stars are indeed more primordial in nature, having formed out
of the original stuff of the galaxy. Population I stars formed, at least
in part, out of the debris of those star which came before and have since
ended their lives, often in great drama and fireworks. It is in the last
stages of the life of an old star when the heavier elements are able to form.
The similarities in age, distance from Earth and chemical composition make
star clusters of both types very important to astronomers. But stars in
a given cluster are not identical. When they formed, they all had their own
unique mass. A few were really big, more were of the midsize variety, and
many,many more were small, at least as stars go. Astronomers like to look
at all of the stars in a given cluster as a kind of closed system. That
way, it is possible to see differences among clusters. One way to look at
a collection of stars is to first ascertain information concerning the individual
stars’ spectral type and absolute magnitude, that is, how bright it would
appear from a distance of ten parsecs from Earth. Spectral type is not difficult
to obtain since astronomers have been doing that for well over a hundred
years. Absolute magnitude is a bit more sticky. In order to determine this,
the star’s distance must be known to a reasonable degree. In the case of
open clusters, an estimate of a star’s distance can be made, if it is a main
sequence star, by knowing its spectral type, since main sequence stars of
a given spectral type have similar absolute magnitudes. In the case of globular
clusters, astronomers use special variable stars called RR Lyrae variables.
These stars, which have a unique light curve, all have absolute magnitudes
of about 0.6. So, if an RR Lyrae variable is found within a cluster, then
its absolute magnitude of 0.6 is compared to its apparent magnitude and the
distance can be computed using the inverse square law.
Once data about absolute magnitude and spectral type are collected for many
stars in the same cluster, a useful chart can be produced called the Hertzsprung-Russell
diagram. The often-used H-R diagram, which plots spectal type on the horizontal
axis and absolute magnitude on the vertical axis, shows most stars falling
on sort of “S” curve diagonally from the upper left to the lower right.
This is the main sequence. Most stars would fall on the main sequence when
a large sample of stars is used, generally from the same region of space.
In considering the stars on the main sequence, the ones on the lower right
are the smaller, cooler stars and those on the upper left are the larger,
hotter ones. Small, cool, red stars tend to use up their hydrogen more slowly
and therefore last a lot longer. Big, hot, blue stars tend to live fast
and die young. They fuse their hydrogen at a rapid pace.
The H-R diagrams produced from many different open clusters can be compared
with interesting results. What is found is that some open clusters, such
as NGC 2362, still have most of their stellar population on the main sequence.
These cluster cannot be too old. Other open clusters, like M-44 (the Beehive)
seem to have a main sequence that only goes about halfway to the upper left
of the chart. In other words, the main sequence seems to end with stars
of spectral type A and hotter. What this means is that the cluster is older.
It has been around long enough for the hotter stars to have evolved off
of the main sequence. Some open clusters, like NGC 188 are even older, with
no stars hotter than spectral type G on the main sequence any more. So,
by comparing H-R diagrams of open clusters, their individual ages can be
estimated.
Globular clusters are another story. All globular cluster show the same
“shape” H-R diagram. What does this mean? It indicates that they are all
approximately the same age, and they are all very old, with the entire upper
end of the main sequence gone. The H-R diagrams also show prominent horizontal
bands, indicative of stars which have evolved beyond the giant or supergiant
stage. These truly ancient clusters are estimated to be between 12 and 16
billion years old, which would put their formation in the earliest days of
the galaxy itself.
Our Sun itself probably formed as part of an open cluster long ago. Quite
possibly, that cluster was what we now call the Ursa Major cluster, which
is the closest star cluster to us at the present time. Our Sun and perhaps
a hundred other stars, including Sirius, may be part of a diffuse outer shell
of the Ursa Major Cluster, or maybe those stars, having escaped the cluster,
are still moving parallel with it without actually being a part of it. But
what if we lived on a planet revolving around a star that was part of a globular
cluster, if that was possible? Our night sky would be a lot brighter, with
perhaps a thousand stars as bright as the planet Venus, though they would
still appear as points of light. Even from the darkest desert, the sky would
appear as bright as a fully moonlit night does to us now.
Open clusters and globular clusters are indeed quite different. The rather
shapeless open clusters contain hundreds of mainly young stars, they are
found in the plane of our galaxy and they are all of different ages. Globular
clusters are mostly spherical, with up to a million stars, all very old and
they seem to be doing their own thing as they move around the galaxy’s center
without regard to the motion of the spiral arms. So, take a look at the
Pleiades and then a look at M-13. Think about how these clusters of stars
are so different in so many ways.
References
Abell, George O., Morrison, David, and Wolff, Sidney, 1987, Exploration
of the Universe, Saunders College Publishing, Philiadelphia
Zeilik, Michael, 1985,Astronomy, the Evolving Universe, Harper and
Row, Cambridge
Pasachoff, Jay M., 1998, Astronomy: From the Earth to the Universe
, Saunder College Publishing, Philadelphia
SEDS website, 2001, Globular Star Clusters, http://www.seds.org/messier/glob.html
SEDS website, 1998, Open Star Clusters, http://www.seds.org/messier/glob.html
Ottewell, Guy, 1988, The Astonomical Companion, Astronomical Workshop,
Greenville, SC.