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 M_sun up to 2,000,000 M_sun. 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 M_sun protostars take only 1 million years to become a fusion-fueled star; the 0.5 M_sun 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 M_sun star runs out after 1 million years; a 6 M_sun 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 M_sun have not had time to become stars.
Stars with M > 40 M_sun have already run out of fuel.
Only stars with M between 4 M_sun and 40 M_sun 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 M_sun have not had time to become stars.
Stars with M > 6 M_sun have already run out of fuel.
Only stars with M between 0.5 M_sun and 6 M_sun 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 M_sun, so the Pleiades are estimated to be just under 100 million years old. The most massive star in the Orion Nebula is 40 M_sun, so it must have formed less than a million years ago.

Prof. Barbara Ryden (ryden@astronomy.ohio-state.edu)

Updated: 2003 Jan 23