The above image is of a fairly rich open cluster, named NGC 3293. (Image credit: Anglo-Australian Observatory)
A giant molecular cloud may contain thousands of dense cores. As star formation propagates through the cloud, as discussed in a previous lecture, these dense cores will be converted into a cluster of thousands of stars.
The above image is of a fairly rich open cluster, named NGC 3293.
(Image credit: Anglo-Australian Observatory)
A typical open cluster is 20 parsecs or less in diameter. The stars within an open cluster are not too closely packed, typically being spaced about 1 parsec apart. It is estimated that there are 20,000 open clusters within our galaxy.
As seen from Earth, open clusters tend to lie near the Milky Way. This is an indication that open clusters lie in the central plane of our galaxy. (Remember: the galaxy looks like a pair of fried eggs placed back-to-back. Open clusters are located between the eggs.)
The central plane of our galaxy is where gas and dust are densest. Some of the open clusters are still embedded within great dusty gas clouds (for instance, the Trapezium, in the Orion Nebula) -- this is an indication that these particular open clusters are very young, having just formed from the gas cloud which surrounds them.
The above image is of a globular cluster named 47 Tucanae. (Image
credit = Anglo-Australian Observatory)
A typical globular cluster is 20 parsecs or more in diameter. The stars within a globular cluster are more closely packed than in an open cluster. Typically, stars are about 0.3 parsecs, or 1 light-year, apart, but in the globular cluster's central region, the stars may only be a few light-months apart. There are about 200 globular clusters associated with our galaxy.
Globular clusters, unlike open clusters, are NOT confined to the central plane of our galaxy. Instead, they form an extended spherical ``halo'' around the galaxy.
Globular clusters contain no dust, no gas, and
no massive main sequence stars. This is an indication that they
are not currently in the process of forming stars and, moreover,
did not form in the recent ( Other galaxies, in addition to our
own, have globular clusters. The picture below is of a globular
cluster associated with the Andromeda Galaxy (also known as M31),
2.2 million light years away.
Laboratories are generally thought of as places where scientists can run controlled experiments to test their hypotheses and theories. Unfortunately, astronomers cannot create stars in their home workshops. However, the galaxy has helped us out to some extent by creating stars in clusters, instead of creating them one by one in random places, at random times, under wildly varying conditions of temperature, density, and chemical composition.
Stars in a cluster formed at the same time, in
the same molecular cloud.
Therefore, stars in a cluster
(The fact that all the stars in a cluster are at the same distance is a great convenience. If two stars in a cluster have different {\it apparent} brightness, it must be because they have different {\it intrinsic} brightnesses. We don't have to undertake the tedious chore of determining the individual distance to each of the many stars in a cluster.)
Thus, when stars form within a cluster, they differ only in their mass. The more massive stars evolve more rapidly, so to find the AGE of a cluster of stars, we need merely determine the mass of the stars which have just now exhausted the hydrogen in their cores and are turning into red giants.
For instance, look at the three Hertzsprung-Russell diagrams shown below, derived from mathematical models of stellar evolution.
The first diagram is of a cluster which is only
1 million years old. The cool K & M stars have not yet
settled down onto the main sequence; they are still contracting
protostars, and have not yet ignited hydrogen fusion in their
cores. On the other hand, the hottest O star has already been
converted to a red supergiant.
The next diagram is of a cluster which is 100
million years old. The main sequence lifetime of a 6 solar mass
star is 100 million years, so stars with M = 6 Msun (L
= 530 Lsun, spectral type A) are just turning off the
main sequence.
The final diagram is of a cluster which is 10
billion years old. The main sequence lifetime of a 1 solar mass
star is 10 billion years, so stars with M = 1 Msun (L
= 1 Lsun, spectral type G) are just turning off the
main sequence. 
Fortunately, the mathematical models provide a good fit to the Hertzsprung-Russell (H-R) diagrams which are actually observed for clusters of stars. (We don't have to trash our computer programs for computing the interior properties of stars.)
For instance, the H-R diagram of the Pleiades (an open cluster in the constellation Taurus) indicates that the cluster is 76 million years old. When this technique is applied to other open clusters, they are found to have a fairly wide range of ages.
The same technique for finding cluster ages can
be applied to globular clusters. (A typical H-R diagram for a
globular cluster is shown below.)
The hottest stars on the main sequence are only 5000 Kelvin or so, cooler (and less luminous and less massive) than the Sun.
If you carefully scrutinize the H-R diagram for a globular cluster, you will note that the main sequence stars are displace downward and to the left from the Zero-Age Main Sequence defined by stars in our own neighborhood. This displacement is best explained by the hypothesis that stars in globular clusters contain few elements heavier than helium (hence fewer absorption lines, hence a lower opacity, hence a different -- hotter -- temperature at a given luminosity.)
Globular clusters all formed when the universe
was young, 10 billion years ago or more.
Open clusters formed more recently, out of ``recycled'' material
that contained heavy elements (such as carbon & oxygen) that
had been manufactured in previous generations of stars.
