Globular and Open Star Clusters

Location of globular star clusters and open star clusters in the Milky Way
Image credits (clockwise from top center): NASA/HST/STScI/AURA
(M80), HST/NASA/ESA (M5), Subaru Telescope (NAOJ)/DSS/Robert
Gendler (NGC 2264), Robert Gendler (Pleiades)


H-R Diagrams of Star Clusters

H-R Diagram of a Globular Cluster
An H-R Diagram of a globular star cluster


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. 

Spectral Class Luminosity Time on Main Sequence
O 104 LSUN  < 107 years
B 103 LSUN 10< t < 109 years
A 101 LSUN  109 years
G 100 LSUN    1010 years 
K 10-1 LSUN  1011 years


H-R Diagrams of Individual Star Clusters

Open Star Cluster M67, Stassun et al., 2002, A&A 382, 899
Open Star Cluster Pleiades, Kharchenko et al., 2004 A&A 438 1163  
fig 1
Globular Star Cluster M71, Hodder et al. AJ 1992, 103 460 
Globular Star Cluster M2, Lee & Carney, ApJ 1999 117 6


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".