In the H-R diagram, what do the letters "H" and "R" stand for?

The letters "H" and "R" in the H-R diagram stand for Hertzsprung and Russell, respectively. This diagram is a crucial tool in astrophysics that plots stars according to their absolute magnitude (or luminosity) against their spectral types (or temperatures). The H-R diagram helps astronomers understand the life cycles of stars and their characteristics based on their positions within the diagram.

The H-R diagram was developed independently by two astronomers, Ejnar Hertzsprung and Henry Norris Russell, in the early 20th century, which is why their names are associated with the terminology used. Their work illustrated the relationship between a star's brightness and its temperature, leading to significant insights about stellar evolution.