The Hertzsprung–Russell Diagram (H-R Diagram) is a graphical tool used in astronomy to classify stars based on their luminosity and temperature. It plots stars on a two-dimensional graph, with the vertical axis representing luminosity and the horizontal axis representing surface temperature, which decreases from left to right. Stars are grouped into distinct categories on the H-R Diagram, including main sequence stars, giants, and white dwarfs. This classification helps astronomers understand stellar evolution, as stars change position on the diagram throughout their life cycles, providing insights into their age and development.