Hertzsprung–Russell diagram

A graph that plots stars by luminosity against surface temperature or spectral type. Its distribution reveals stellar groups such as the main sequence, giants, and white dwarfs.

Stellar luminosity: The total energy a star radiates per unit time. It supplies the brightness coordinate, often expressed relative to the Sun.
Main sequence: The band of stars that generate energy mainly by fusing hydrogen in their cores. Core hydrogen fusion produces the prominent diagonal band in the diagram.
Ejnar Hertzsprung: A Danish astronomer who studied relationships between stars’ colors, spectra, and luminosities. His early plots revealed distinct groups of stars with different luminosities.
Color–magnitude diagram: A plot of stellar brightness against color, commonly used to study star populations. It applies the diagram’s logic to observed cluster brightness and color.
Color–color diagram: A plot comparing two color indices of astronomical objects. It compares spectral shape rather than luminosity with temperature.
Effective temperature: The temperature of an ideal blackbody that emits the same energy per unit area as a star. It sets the temperature coordinate, which increases toward the diagram’s left.
Red giant: A luminous, expanded star that has exhausted hydrogen in its core and burns fuel in surrounding layers. Expansion raises luminosity while lowering surface temperature, moving stars above and right of the main sequence.
Henry Norris Russell: An American astronomer whose work linked stellar spectral types with absolute magnitudes. His independently developed diagram established the form now associated with his name.
Star cluster: A group of stars bound together or sharing a common origin and motion. A cluster’s stars are at nearly the same distance, making its diagram especially informative.
Color index: The difference between an object’s measured magnitudes in two wavelength bands. Color indices can estimate temperature when a spectrum is unavailable.

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