What characteristic is used to classify stars on the H-R diagram?

The classification of stars on the H-R diagram primarily relies on two key characteristics: brightness (or luminosity) and temperature. The H-R diagram, or Hertzsprung-Russell diagram, is a graphical representation that plots stars according to their absolute magnitude (brightness) and their surface temperature (often represented as color, where hotter stars appear blue and cooler stars appear red).

Brightness is critical because it allows astronomers to understand how luminous a star is compared to others, while temperature provides insight into its spectral type and evolutionary stage. The two parameters interact as they indicate where a star is located on the diagram, allowing researchers to categorize stars into distinct groups such as main sequence stars, giants, and white dwarfs. This classification helps in understanding stellar evolution and the life cycles of different types of stars.

In conclusion, the combination of brightness and temperature is fundamental to the organization and insights derived from the H-R diagram, making it a powerful tool in astrophysics.