Our examples so far have focused on clusters that are dominated by main sequence and pre-main sequence stars, and we’ve taken care in the instructions to state that these are “young” stars which are fusing hydrogen into helium in their cores. Before moving on to collect your own observations of star clusters and trying to analyse them, you also need to understand what happens when stars evolve off the main sequence, and how this affects the HR diagrams of older clusters.
We’ve already discussed the main sequence turn-off point, and you saw in the above examples that this is important for determining the age of a cluster since stars at the turn-off point are those which have just run out of hydrogen in their cores, so they begin to evolve into old age. In the case of NGC 3766, the data you worked with contained two red giant stars, and you fit an isochrone model with a red giant branch that passed through those points. But our Galaxy also contains star clusters that are far older than the ones you’ve seen so far, and in these older clusters the red giant populations become far more important.
The oldest and largest clusters in our Galaxy are known as globular clusters. They contain ~105-107 stars and formed within the first few billion years of the Milky Way’s existence. Their orbits are oriented randomly within the galactic halo rather than being located in the disk of the Milky Way where we see open clusters and ongoing star formation, so we can see even very distant globular clusters because they are less obscured by interstellar dust.
Apart from the lowest mass stars in a globular cluster, the stars above roughly the Sun’s mass have all evolved to become giants. The sizes, ages and distances to globular clusters all amount to a distinctly different general HR diagram structure than the ones you’ve seen so far of young open clusters, for a couple of reasons:
- When star clusters form, it turns out that low mass stars form more often than high mass stars. Therefore, young open clusters contain relatively few stars massive enough to have evolved past the main sequence stage.
- High mass stars not only evolve from the main sequence more quickly than lower mass stars, but also have relatively short post-main sequence lifetimes, after which they die in supernova explosions and are soon no longer visible.
Therefore, young clusters tend to have very few giant stars in their HR diagrams, as these stars both die quickly and are less common in stellar populations. Eventually, more moderately-aged open clusters develop larger red giant populations as their turn-off points migrate down the main sequence and their giant populations suffer slower deaths. Relatively few of these old open clusters exist because the stars in lower mass clusters tend to disperse after ~109 years. Such clusters are said to “evaporate” as follows: the fastest-moving stars at the outskirts of the cluster eventually escape the cluster’s gravitational pull, which lowers the cluster’s overall mass, enabling more stars to escape the weakened gravitational field, which allows more stars to escape, etc.
In contrast to open clusters, globular clusters have enough mass that they’ve survived since the formation of the Milky Way, and contain the oldest populations of stars. In fact, when we look at globular clusters, they typically contain several thousand giant stars spanning wide ranges in magnitude, and it can actually be difficult to make out the main sequence stars because of the great distances to these clusters and the fact that only the dim, red low-mass main sequence stars remain.
Globular clusters therefore provide the best opportunity to study the full range of evolutionary stages that stars of moderate mass pass through after leaving the main sequence. There are three such stages, each occupying a distinct region of the HR diagram.
The Red Giant Branch (RGB)
When a star exhausts the hydrogen in its core, fusion there stops. With no outward radiative pressure to support it, the core begins to contract under gravity. As it does, the particles falling inward convert gravitational potential energy into kinetic energy — the core heats up. This rising temperature ignites hydrogen fusion in a shell surrounding the inert core, which actually increases the star’s total energy output. The result is counterintuitive: a star whose core has stopped fusing burns more brightly than it did before. The increased luminosity drives the outer layers of the star outward, causing the star to expand dramatically. As the star’s radius grows, its surface temperature drops — the same physics you encountered on the previous page, where increased luminosity without increased gravity leads to a larger, cooler star. These stars, now cool and enormously luminous, populate the red giant branch in the upper-right of the HR diagram. Their inert helium cores continue to shrink and heat while the hydrogen-burning shell deposits more helium onto them, and the stars climb the red giant branch — growing more luminous and cooler — until the core becomes hot enough to ignite helium fusion.
The Horizontal Branch
Once helium fusion ignites in the core, the star’s structure stabilises in a new equilibrium. Helium fuses into carbon and oxygen, generating energy at a lower rate than the preceding hydrogen shell burning did. As a result, the star contracts somewhat and its luminosity drops relative to the tip of the red giant branch. These stars — now fusing helium in their cores — populate the horizontal branch, which appears as a spread of moderately bright, relatively blue stars in the HR diagram. They are less luminous than the brightest red giants but considerably hotter, since the reduced luminosity allows the star to settle into a more compact configuration.
The Asymptotic Giant Branch (AGB)
Eventually, the helium in the core is exhausted. The star is now left with an inert carbon/oxygen core, surrounded by a helium-burning shell, which is itself surrounded by a hydrogen-burning shell. The same process that drove the star up the red giant branch now repeats: the inert core contracts, heats, and the energy output from the surrounding shells increases. The star’s luminosity rises again, its outer layers expand, and its surface temperature falls. On the HR diagram, these stars migrate back toward the upper-right — tracking a path that approaches the red giant branch from the left. This is why they are called the asymptotic giant branch: their evolutionary track runs roughly parallel to the red giant branch and converges toward it asymptotically, though now from stars with carbon/oxygen cores rather than helium cores.
In the next two examples, you will analyse both a moderately old open cluster and a globular cluster, to understand how to identify these populations when studying older clusters, as well as how to fit isochrone models to them.
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