The Hertzsprung–Russell Diagram: Physical Mechanisms

The main sequence runs diagonally from hot luminous stars to cool faint stars. Giants and supergiants lie above the main sequence because their large radii make them very luminous, while white dwarfs lie below it because their small radii keep luminosity low despite high temperature.

Stars occupy different regions because luminosity depends on radius and temperature, while stellar evolution changes both quantities over time. A main-sequence star's position is strongly tied to mass, whereas post-main-sequence stars move away as core fuel changes and their structures expand or contract.

Measured stellar temperatures and luminosities populate distinct sequences rather than a random scatter. Star clusters show clear main-sequence turnoff points that shift with cluster age, confirming predicted evolutionary tracks.

A star moves through H-R space when its interior fuel and structure change. Core hydrogen exhaustion can drive expansion toward the giant branch, while envelope loss can expose a hot remnant that later cools along the white-dwarf sequence.