Hertzsprung–Russell diagram might be a difficult thing to say or remember,
but it's really just a very useful thing to help you remember
the things we've been learning about stars.
Since the name is so long, some people just call it a H–R diagram or even just HRD.
We learned about stellar classification, absolute magnitude and luminosity class
The HRD is a picture map you can use
to help understand how all of those things fit together.
(from: wikipedia - hertzsprung-russell diagram)
It was invented about a hundred years ago by two scientists,
named Ejnar Hertzsprung and Henry Norris Russell.
