The role of stellar clusters in empirically determining stellar evolution.

Key Concepts

Stars sometimes form in clusters.
Open clusters contain 10 to 3000 stars; globular clusters contain 100,000 to 1,000,000 stars.
Clusters are useful ``laboratories'' for testing our theories of stellar evolution.

(1) Stars sometimes form in clusters.

A giant gas cloud in the Galaxy may contain thousands of dense cores. These dense cores will be converted into a cluster of thousands of stars.
Open clusters contain 10 to 3000 stars. Lots of gas and dust is left over from the formation process.

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.

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 Orion Nebula

(3) Clusters are useful ``laboratories'' for testing our theories of star formation.

Laboratories are generally thought of as places where scientists can run controlled experiments to test their hypotheses and theories. 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

have the same age,
had the same initial chemical composition,
and are at roughly the same distance from Earth.

(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 apparent brightness, it must be because they have different intrinsic brightnesses. We don't have to undertake the tedious chore (e.g. running the parallax applet) 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 M_sun (L = 530 L_sun, 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 M_sun (L = 1 L_sun, 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.

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 youngest open clusters are only 1 million years old.
The oldest open clusters are about 4 Billion years old.

In class exercise.

Blue Image

Red Image

Click on the above two links to open two separate windows.

This is the CCD simulation applet that we used back in September and that you used for your first homework assignment.

On a separate piece of paper, make a reproduction of the star field that you see after you make an exposure. The link that says Blue Image represents an image of a star cluster taken with a blue filter (only blue light hits the detector).

The link that says Red Image represents an image of a star cluster taken with a red filter (only red light hits the detector).

Compare the two images, and on your star field that you have drawn on your piece of paper, indicate which stars that you think are the bluest ones and which ones are the reddest ones. (Draw a circle around the star and label it red or blue

Put your names on the piece of paper and give it to the instructor when done.

Note, the positions of each star are the same on the blue detector and the red detector. So your comparing image brightnesses on each of the detectors for the same star. Stars whose images appear approximately the same in each filter would not be either very red or very blue.