As a stellar astrophysicist myself, I can’t help but falling in love with this diagram, which is the key in the understanding of stellar evolution and stellar population analysis.
This tool was developed independently by Ejnar Hertzsprung and Henry Norris Russell in the early 1900s. What it does is pretty cool — it plots stars’ temperature against their luminosity, in order to show how star color (spectral type) relates to their absolute brightness, which we call a Color-Magnitude Diagram (CMD).