H-R Diagrams of Star Clusters

We can plot apparent magnitude and color for a star cluster -- we wouldn't know the actual luminosities (unless we knew the distance), but the trends between luminosity and color should be visible, since the stars all lie at the same distance. These plots are called "color-magnitude diagrams".

What do they look like? For open clusters (like h and chi persei) we see a clear main sequence:

We can define a main sequence turnoff -- a point where stars start to leave the main sequence. What does this tell us?

For globular clusters (like M15) we see a very different kind of color-magnitude diagram:

Stellar Isochrones

We can use theoretical calculations of aging stellar populations -- called stellar isochrones -- to fit cluster HR diagrams and derive distance, ages, and metallicities of star clusters:
This is an example of a stellar isochrone, labelled by color:
  • 10 million years
  • 100 million years
  • 1 billion years
  • 10 billion years
Each curve shows the properties of stars as a function of mass for the given age.

Don't confuse this with an evolutionary track, which shows the properties of a star as a function of age for a fixed mass!


Interesting Questions: