Cluster isochrones. Observed isochrones for star clusters, based on parallaxes and photometry obtained with the Hipparcos satellite. The ages of the clusters are indicated by the colours of the dots, with the youngest clusters (around 50 million years old) in blue, to the oldest (around 1 billion years old) in red. The background shows the data for 30000 stars with accurate parallaxes as obtained in the new reduction of the Hipparcos data.
Star clusters offer astronomers a very effective way of studying the effects of stellar evolution. A star cluster is group of stars that were formed at the same time and from the same cloud of gas and dust. All stars in a cluster are therefore of nearly the same age and composition. The stars in a star cluster cover a wide range of masses, typically ranging from less than 0.1 to more than 10 solar mass. The very massive stars evolve very quickly, while low mass stars evolve very slowly. As stars evolve, they will go through different phases of energy production. This reflects in the relation between the brightness of a star and its temperature. A star will typically spend most of its life “burning” hydrogen (known as the "main sequence" stage), producing helium. When hydrogen becomes depleted, a phase of helium burning starts. What happens next depends very much on the mass of the star.
When we observe stars in a particular star cluster, we observe a clear relation between the temperature and brightness. As these stars are all of the same age, we call this relation an isochrone, in this case it is an observed isochrone. We can recognize clusters of different ages by looking at the relation between brightness and temperature for the most massive stars of each cluster. For example, the most massive stars in the Hyades cluster have already evolved beyond their main sequence stage, while similar stars in the much younger Pleiades are still close to the main sequence.
Gaia is going to make a huge impact on such studies. It will determine accurate distances of a few hundred star clusters in our galaxy. For those clusters it will provide complete and accurate lists of stars that are cluster members. The photometry provided by Gaia will allow us to translate this information into a set of observed isochrones. A comparison of observed isochrones with similar isochrones as obtained through modelling will enable us to adjust the models and learn more about stellar evolution, stellar structure and stellar atmospheres.
Page last updated: 23 December 2013