Besides for the fact that they are beautiful, star clusters are very important to astronomers for two reasons: (1) they are cosmic archives of stellar evolution; and (2) we can infer their distances without measuring their parallaxes.

You can readily see these distinguishing characteristics in the images in the above links (except for the white dwarfs, which are very difficult to see in any case).

 With the telescopes at the Sommers-Bausch Observatory, you can see a very different type of star cluster, called a globular cluster. The figure above is a color-magnitude diagram for the globular cluster M3. Note that M3 has no luminous blue main sequence stars. In fact, the only main sequence stars (labeled MS) are solar type or redder. M3 has many red giant stars, labeled RGB, and stars that lie on a horizontal strip, labeled HB, called horizontal branch stars. The HB stars have roughly the same temperature as the Sun and so have spectral types in the range A - G, but they are almost 100 times as luminous as the Sun. Their luminosity class is II or III - e.g., a star with spectral type F0III would be a horizontal branch star.

Until recently, white dwarf stars were too faint to see at the typical distances of globular clusters (5000 - 15000 parsecs). But the Hubble Space Telescope changed that. For example, here is a description of HST observations of white dwarf stars in the globular cluster M4.

Distances of star clusters: Most star clusters in the Milky Way are at distances too great to measure by means of parallax, even with the Hipparcos satellite. But still, we can infer the distance of a star cluster because we know what to expect for the luminosity of its main sequence stars. Astronomers call this method "spectroscopic parallax"; but it really has nothing to do with parallax. The argument goes as follows: when we plot the stars of a given cluster on a color-magnitude diagram, such as the one shown above for the Pleiades, we can recognize a familiar pattern - the main sequence. If we know that a star in the Pleiades (or any other cluster) is a main sequence star (luminosity class V) and we know its spectral type, we can say what its luminosity should be. (For example, the Sun has spectral type G2V. Therefore, we expect that every G2V star has roughly the same luminosity as the Sun. Likewise, we expect that all type A1V stars will have roughly the same luminosity as Sirius: L = 23.5 Solar units.) But if we know its luminosity (L), and we can measure its brightness (B), we can calculate its distance (D) from the inverse square law, which can be written in the form D = (L/4pB)^1/2.

This method of inferring distances from the spectral type and brightness of a star depends on two assumptions: (1) all main sequence stars of a given spectral type have the same luminosity; and (2) the star really is a main sequence star. Assumption (1) is another example of the principle of universality of physical laws. It says that the underlying physical rule that makes most stars fit on the main sequence when plotted on the H-R diagram should be true in every group of stars, everywhere in the universe. [In fact, the "rule of the main sequence" is not an exact rule, as you can see from the fact that the star a -Cen has spectral type G2V - the same as the Sun - but luminosity L = 1.56 Suns. Evidently, there is some uncertainty in the main sequence relationship, as indicated by the scatter of the points in the in the above color-magnitude diagrams. Assumption (2) can be verified by measuring the magnitudes and spectral types of many stars in the cluster and plotting them in a color-magnitude diagram. Then one can see which stars in the cluster belong to the main sequence.

There's a wonderful logic to this technique that might appear circular but is not. Astronomers first measured the distances (D) of nearby stars by parallax methods and their brightness (B). Then they used the inverse square law to infer their luminosities (D & B Þ L), and discovered that most stars had a relationship between spectral type and luminosity called the main sequence (Spectral Type Þ L). Assuming that the relationship is universal, astronomers can use it to infer the luminosities of more distant stars from their spectra, and then use the inverse square law to infer their distances from their luminosity and brightness (L & B Þ D). It's almost like pulling yourself up by your own bootstraps!

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Last modified February 18, 2001
Copyright by Richard McCray