Globular and Open Star Clusters

H-R Diagrams of Star Clusters

We can use the HR Diagram of a Star Cluster to approximate its age.  This is done using the turn off point (TP), or the point in which stars start evolving off the main sequence after exhausting their main source of fuel.  This evolution will start with the brightest stars, which have the shortest lifetimes.  See the diagram below for more detail on this process over time.

Now using this knowledge, examine the HR diagram for the globular cluster M3 and try to identify the main sequence (MS), the main sequence turnoff point (TP), the giant branch (GB), the horizontal branch (HB)

Spectral Types as a Function of Color

The temperature or spectral type of a star can be calculated as a function of its B - V color index. Astronomers use Ballestero's Formula to do this. But for our purposes, we can infer the spectral type from the B - V color index using the plot below, which serves as a good "by eye" approximation. This plot also indicates the turn off point (TP), or the B-V magnitude at which each spectral type will evolve off the main sequence. 

Age of a Star Cluster

The main sequence turn off age tells us how old the cluster is. The mass of a star sets the luminosity, the temperature, the size, and how fast it will evolve off of the main sequence: stars of the same mass will evolve at the same rate. Listed below are the main sequence luminosities and life times of stars. 

H-R Diagrams of Individual Star Clusters

A Final Word on Reddening

Reddening (interstellar extinction) is the preferential scattering and absorption of "bluer" EM radiation by dust and gas along the observer's line of sight to an object. This can be quantified in color index by independent methods and used to calculate the intrinsic (i.e. unaltered) color of the astronomical source via the equation:  

(B-V)_intrinsic = (B-V)_observed - E(B-V)

Notice that a positive extinction makes the intrinsic color of the source "bluer".