Ages and Evolution

Globular clusters are ancient. Estimates of the ages of the globular clusters around our galaxy are typically 12-14 billion years. Since the best estimate of the age of the Universe is about 12 billion years (from redshifts and the Hubble constant; see Chapter 24), the globulars are about as old as the Universe itself!

HR Diagram for a Globular Cluster
The HR diagram for a globular cluster has substantial differences from the HR diagram for stars near the Sun or for those in an open cluster. An HR diagram for the globular cluster M5 in the constellation Serpens is shown in the adjacent image. The horizontal axis is the B - V color index and the vertical axis is the absolute visual magnitude MV.

The primary difference of this diagram from that for stars near the Sun is the absence of a main sequence for an absolute magnitude lower than about 4, the peculiar structure above that point, and the absence of supergiant stars. Click on the "Show Labels" button to toggle on annotation for features in the diagram that are described below.

Components of the HR Diagram
The main components of a globular HR diagram for an evolved cluster are summarized as follows (refer to the annotated figure above):

  • Main Sequence: where stars spend most of their lifetime producing energy from hydrogen fusion in their cores. For a globular cluster, there are few main sequence stars above the turnoff point, described below.
  • Turnoff Point: As the hydrogen fuel in a star's core is depleted the core contracts and the star moves away from the main sequence. At the turnoff nearly all the central hydrogen fuel has been depleted. For M5 shown above, the turnoff point is at about absolute visual magnitude 4.
  • Red Giant Branch: When the central fuel is gone, hydrogen burns in a shell around a dense helium core (hydrogen shell burning). The star's outer envelope expands and the star ascends the Red Giant Branch (RGB) of the HR diagram.
  • Horizontal Branch: The helium-rich core ignites when the star reaches the tip of the RGB branch and core helium fusion begins. The star moves down the HR diagram to the Horizontal Branch (HB), where it burns helium to carbon in its core.
  • Asymptotic Giant Branch: When the central helium is depleted, shell burning is initiated in two shells: an inner one burning helium and and outer one burning hydrogen. The star moves up the Asymptotic Giant Branch (AGB) and a strong stellar wind blows off its outer layers, forming a planetary nebula and leaving behind the core of the star as a hot white dwarf.

    • The location of the turnoff point is a measure of the age of the cluster: younger clusters have more luminous main sequence stars than older ones. We shall discuss these evolutionary stages for stars more substantially in the chapter on star death (Chapter 21). This animation illustrates the evolution of the HR diagram for a star cluster over time.

    White Dwarf Stars in the Globular Cluster M4
    White dwarf stars are especially interesting because they are a late stage in the evolution of low-mass stars, so the number of white dwarfs present in a globular cluster provides a measure of the age of the cluster. Unfortunately, white dwarfs are intrinsically very faint, so it has been difficult to observe them within the very crowded interiors of globular clusters.

    Recently the Hubble Space Telescope has been used for this purpose, and a population of white dwarfs has been observed in the relatively nearby globular cluster M4 in the constellation Scorpius (figure above). Future detailed studies of the white dwarf population in globular clusters may provide an accurate estimates of their ages.

    Technically Speaking: The Fate of Globular Clusters

    From the viewpoint of theoretical physics, globular clusters are an interesting problem. They can be treated mathematically using the kinetic theory of gases as if they were simply balls of gas, held together by the gravitational force between "atoms"--but in this case, the individual "atoms" are stars!

    This mathematical description of globular clusters leads to the result that globulars are not stable indefinitely. First, the gravitational force is not strong enough to hold the faster stars, so the globular cluster will slowly "evaporate". Second, the individual stars gradually interact to form binary stars, which then interact with other passing stars to become more and more tightly bound; these close pairs are known as hard binaries. The two stars of the hard binary may eventually merge, which is one possible source of the hot young stars known as blue stragglers that are found in globular clusters beyond the turnoff point.