Describe the H-R diagram and explain its significance?

Answer 1

The Hertzsprung-Russell diagram plots the luminosity of stars (in magnitude) versus spectral type. Instead of spectral type on the abscissa, you sometimes see surface temperature (useful in astrophysics) or color index, B-V (because it is easy to measure).

Most stars fall in a roughly diagonal band called the main sequence. In main sequence stars, mass is directly correlated to luminosity - bigger stars are brighter. These are stars that are fusing hydrogen into helium in their cores. When all the available hydrogen in the core has been fused, stars depart from the main sequence and become some variety of giant or supergiant star. The larger stars burn faster and so leave the main sequence earlier. In very old star clusters, only the small, red stars are left on the main sequence. By looking at the length of the main sequence in a star cluster, astronomers can estimate the age of the cluster.

Answer 2

H-R (..Hertzsprung-Russell..) diagrams basically plot the energy output of stars against their luminosity. H-R diagrams mainly give astronomers a means to determine the life cycle of stars based on their spectral types.