Astronomy 162: Professor Barbara Ryden
Thursday, January 23
STAR CLUSTERS
``There is one glory of the sun,
and another glory of the moon,
and another glory of the stars, for
one star differeth from another star in glory.''
- 1 Corinthians, 15:41
Key Concepts
- Dark nebulae are often found within
giant molecular clouds.
- Young stars are often found in
open clusters of 10 to 3000 stars.
- Clusters are useful `laboratories' to
study theories of star formation.
(1) Dark nebulae are often found within
giant molecular clouds
Giant molecular clouds are cold, massive clouds of
gas within a galaxy. How cold are they? They are
cold enough for molecules to form. How massive are
they? They have masses which range from 100,000
Msun up to 2,000,000 Msun.
A giant molecular cloud may contain thousands
of individual dark nebulae embedded within it.
Because dark nebulae are found clustered within
giant molecular clouds, star formation can
propagate through a giant molecular cloud, creating
large numbers of stars, fairly close together,
within a limited period of time.
Suppose that the edge of a giant molecular cloud is
compressed by a shock wave (generated by a nearby
supernova, perhaps). A cluster of
stars forms from the compressed dark nebulae
at the edge of the giant molecular cloud. Hot, luminous
stars in the cluster (of spectral type `O' and
`B') heat the surrounding gas, causing a
shock wave to expand outward. The shock wave
compresses more dark nebulae, further inside the giant
molecular cloud. A new cluster of stars is formed. The hot
stars in the cluster create a new shock wave,
which compresses more dark nebulae, which
form more hot stars, which create a new shock
wave, which compresses more dark nebulae, which....
Well, you get the picture. Once stars start to
form at an edge of a giant molecular cloud, they
trigger a `domino effect'; a wave of star formation
propagates through the cloud. An example of
this effect can be seen in the vicinity of
the Orion Nebula. The Orion Nebula is on
the edge of a giant molecular cloud. When we look
straight at the Orion Nebula at visible wavelengths,
as in the picture below, we see four very hot, luminous
stars within a glowing emission nebula. These stars
are very young -- only a million years old, at most.
However, when we look at the Orion Nebula at
infrared wavelengths (as in the picture below),
we are seeing deeper into the dark and dusty
giant molecular cloud. What we see in this picture
is a large number of protostars in the process of
forming RIGHT NOW.
The protostars began forming when they were shocked
by the hot young stars in the Orion Nebula. The
hot young stars in the Orion Nebula began forming
when they were shocked by the slightly older stars
in Orion's belt (which are about 8 million years old).
(2) Young stars are often found in
open clusters of 10 to 3000 stars.
Stars which form in the giant molecular cloud at
the same time tend to form a loosely bound cluster,
called an open cluster. Typical
open clusters contain between 10 and 3000 stars.
The stars in an open cluster are not very tightly
packed together (hence the name ``open''). A
randomly chosen star in an open cluster will be
about 1 light year from its nearest neighbor.
(Compare that to the Sun, which is not in a cluster;
the Sun is about 4 light years from its nearest neighbor.)
The most familiar example of an open cluster is
the Pleiades, 117 parsecs (380 light years)
away from us in the constellation Taurus. Because
the Pleiades are so close to us, they are easily
visible to the naked eye. The Pleiades are an
open cluster of some 500 stars, in a region 4
parsecs (13 light years) across. (The Pleiades,
being quite young, are still surrounded by the
gas and dust from which they formed, and hence
are in the midst of a reflection nebula.)
An example of a particularly large open cluster is
the Wild Duck cluster, about 1600 parsecs (5200
light years) away from us. The Wild Duck cluster (also
known by its catalog number, M11) contains about
3000 stars.
Open clusters, since the stars they contain are
so loosely packed, are not strongly glued together
by gravity. From time to time, a star within the
cluster is accelerated to the cluster's escape
speed, and is lost to outer space. The open
cluster gradually ``evaporates'', as the
textbook states the matter. (Just as a glass of water
evaporates by losing high-speed water molecules into
the air, so an open cluster of stars ``evaporates''
by losing high-speed stars into outer space.)
The Sun was probably formed as part of an open
cluster of stars. However, since open clusters only
last a billion years or so before evaporating, the
Sun has long since lost touch with its littermates.
(3) Clusters are useful `laboratories' to
study theories of star formation.
The stars within an open cluster all formed at
the same time from the same giant molecular cloud.
Thus, the stars within an open cluster all
have
- the same age,
- the same (initial) chemical composition,
- the same distance from Earth.
Thus, stars within an open cluster give astronomers
a rare chance to perform a ``controlled experiment'';
within a cluster, the only thing varying from one
star to another is the mass. One thing we know,
both from observation and experiment, is that
high-mass protostars evolve more rapidly than
low-mass protostars. (Trigger star formation in
a giant molecular cloud. The 4 Msun
protostars take only 1 million years to become
a fusion-fueled star; the 0.5 Msun
protostars take 100 million years.) Moreover,
we also know that high-mass stars use up their
hydrogen `fuel' much more rapidly than low-mass
stars. (A 40 Msun star runs out after
1 million years; a 6 Msun star will
last for 100 million years.)
The fact that high-mass stars form more rapidly
- and die more rapidly - gives us a method of
determining the age of a star cluster.
Consider a very young open cluster,
1 million years old.
- Protostars with M < 4 Msun have
not had time to become stars.
- Stars with M > 40 Msun have already
run out of fuel.
- Only stars with M between 4 Msun and 40
Msun will lie on the main sequence of the
Hertzsprung-Russell diagram.
Now, consider an older open cluster,
100 million years old.
- Protostars with M < 0.5 Msun have not
had time to become stars.
- Stars with M > 6 Msun have already
run out of fuel.
- Only stars with M between 0.5 Msun and
6 Msun will lie on the main sequence of the
Hertzsprung-Russell diagram.
Thus, we can tell the age of a cluster from the length
of its main sequence on the Hertzsprung-Russell diagram.
The most massive main sequence stars in the Pleiades are
just over 6 Msun, so the Pleiades are estimated
to be just under 100 million years old. The most massive
star in the Orion Nebula is 40 Msun, so it
must have formed less than a million years ago.
Prof. Barbara Ryden
(ryden@astronomy.ohio-state.edu)
Updated: 2003 Jan 23
Copyright © 2003, Barbara Ryden