As we discussed in Lesson 4, stars are not born in isolation; they are born in clusters containing many thousands of stars. Moreover, the timescale for a cluster of stars to be born from a dense interstellar gas cloud is a few million years -- far less than the typical lifetimes of most stars, which range from ten million years for the most massive stars to billions of years for stars like the Sun.
Before reading this section, it might be a good idea to review the observations of star clusters, which are discussed in in Lesson 4. Now we are ready to address the last question in the summary of Lesson 4: Why are the types of stars found in globular clusters so different from those in open clusters? We now know the answer: globular clusters are very old, while open clusters are relatively young. As you will see, our understanding of this fact gives us a powerful tool to determine the ages of astronomical systems, even the universe itself.
The key to understanding the difference between the two types of clusters (and exploiting this difference to our advantage!) is the fact that different stars have very different lifespans. The most massive stars remain on the main sequence (i.e., burn hydrogen in their cores) for only about 107 (10 million) years, while stars like the Sun remain on the main sequence for about 1010 (10 billion) years. That means that in a newborn star cluster, of age less than 107 years, the blue giant stars will still lie on the main sequence, as is the case with the Pleiades. But after several tens of millions of years the most massive blue giants will have consumed all the hydrogen in their cores and will have become red giants (while the less massive stars will still be on the main sequence).
As time goes on, less massive stars also become red giants, and some of the stars that were previously red giants have evolved to become almost invisible (we'll learn about their final fates later in this lesson and in the next lesson). Finally, after about 10 billion years, all stars more massive than the Sun have depleted the hydrogen in their cores and have evolved to become red giants, horizontal branch stars, or invisible. On the color-magnitude diagram of such a cluster one only sees main sequence stars less massive than the Sun, red giant stars, and horizontal branch stars. A good example is the color-magnitude diagram of the globular cluster M3.
This sequence of events is illustrated by this HR diagram simulator, from the University of British Columbia. Select "Add stars: 100" stars and "Evolve". Watch the time indicator in the lower left. The simulator does not follow the evolution of stars beyond the first red giant stage, though, so you won't see a horizontal branch.
We define the main sequence turnoff of a star cluster as the position on the H-R diagram of the most luminous star that remains on the main sequence. (This point is labeled "TO" on the color-magnitude diagram of M3). As the HR diagram simulator shows, this point moves to lower luminosity and temperature as age of the star cluster increases. Since we have a theory that tells us how long stars of a given mass (or luminosity) can remain on the main sequence, we can infer the age of a star cluster from the location of the main sequence turnoff of its H-R diagram.
The globular clusters are the oldest star clusters known. By comparing the main sequence turnoff points of their H-R diagrams with the theory of stellar evolution, astronomers find that the oldest globular clusters are 12.6 billion years old with an uncertainty of +/- 2.1 billion years (data are shown here). This tells us something important about the universe itself. Of course, the universe must be at least as old as any star cluster that it contains -- i.e., at least 10.5 billion years old.
(Return
to course home page)
Last modified April 14, 2002
Copyright by Richard McCray