Question

What are HR diagrams?

Answer 1

The Hertzsprung-Russell Diagram is a tool that shows relationships and differences between stars. It is something of a "family portrait." It shows stars of different ages and in different stages, all at the same time. But it is a great tool to check your understanding of the star life cycle

check this out for source
http://aspire.cosmic-ray.org/labs/star_life/hr_diagram.html