An H-R Diagram (HRD) is a plot of Effective Temperature (Teff) versus Luminosity (L) (or Absolute Magnitude (M), effectively the log of luminosity) on a log-log scale. The modern H-R diagram is an outgrowth of tables and charts in the work of Ejnar Hertzsprung and Henry Norris Russell.
H-R diagrams are used to summarize the characteristics of stars by plotting each of a number of stars, but a plot point might also be used for whole groups, such as galaxies to characterize the differences in the stars of different galaxies.
H-R diagrams are a starting point in applying stellar theory to individual stars. The diagram clearly shows the different characteristics of various phases of stellar evolution, such as Main Sequence Star, the Red Giant branch, the Asymptotic Giant Branch (AGB), White Dwarves.
H-R diagrams are often plotted of just the stars of a specific Open Cluster or Globular Cluster (GC) to display the cluster's characteristics. In this context, the term Isochrone (which means "same age") indicates a line (undoubtedly curved) that traces the positions of stars of a specific age.
Asymptotic Giant Branch (AGB)
Color-Magnitude Diagram (CMD)
Galaxy Main Sequence
Globular Cluster (GC)
Horizontal Branch (HB)
Main Sequence Fitting
Stellar Distance Determination
Stellar Mass Determination
Stellar Parameter Determination
Turn-off Point (TO)