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We have now seen how the mass, radius, luminosity and effective temperature depend on each other. So what of the other fundamental properties listed in table 1 - age and chemical composition? It is believed that mass, age and, to a lesser extent, the chemical composition, are the main factors determining the properties of a star and hence its position on the HR diagram. The only way of disentangling the effects of these three factors is to look at clusters of stars, of which there are two main types: galactic (or open) clusters and globular clusters, as shown in figure 4.

Figure 4:  Left - the globular cluster Omega Centauri. Right - the Pleiades galactic cluster.


Since all the stars in a particular cluster were presumably formed together at the same time and out of the same material, they must be of the same age and have the same chemical composition. The stars in a cluster hence differ from one another only in mass and the main factor which accounts for the differences in the observed properties of the stars in a cluster is that they have different masses. This assumption has formed the basis of most work on stellar evolution to date.

Figure 5:  Left - HR diagram for a typical globular cluster. Right - HR diagrams for a sample of galactic clusters.

The HR diagrams of globular and galactic clusters appear as in figure 5. The fact that the cluster HR diagrams are rather well defined adds to the hope that the cluster stars do form very homogeneous groups, with only mass varying from star to star in any cluster. An essential feature of all of these diagrams is that there is a turn-off point from the main sequence. Below this point the cluster has a well-defined main sequence, whereas above the point there are few stars on the main sequence. Other features of note in the cluster HR diagrams shown in figure 5 are:

globular clusters

galactic clusters

  1. They all have main sequence turn-off points in a similar position and a giant branch joining the main sequence at that point.
  2. They have a horizontal branch running from near the top of the giant branch to the main sequence above the turn-off point.
  3. In many clusters, there is a region of the horizontal branch which is populated only by stars of variable luminosity, known as RR Lyrae stars.
  1. There is considerable variation in the position of the main sequence turn-off point, with the lowest being in about the same position as those of the globular clusters.
  2. In many clusters there is a gap between the main sequence and the giant branch, known as the Hertzsprung gap.

The differences between the HR diagrams of globular and galactic clusters are interpreted as age differences, with the galactic clusters being younger but having a larger range of age. To see why the different HR diagram shapes arise, we first need to understand how stars evolve. We will discuss this in detail in the following sections.

©Vik Dhillon, 27th September 2010