Color–magnitude diagram

A plot of stellar brightness against color that reveals patterns in star populations, including how stars evolve and how old a cluster may be.

Apparent magnitude: A measure of how bright an astronomical object appears from Earth. It is the observed brightness quantity plotted on one axis.
Main sequence: The band of stars that generate energy primarily by fusing hydrogen in their cores. It forms the diagram’s dominant sequence for hydrogen-burning stars.
Globular cluster: A dense, gravitationally bound collection of old stars orbiting a galaxy. Its well-defined turnoff makes its diagram useful for estimating cluster age.
Color–color diagram: A plot of one astronomical color index against another, used to study stellar temperatures and reddening. It compares colors without placing brightness on an axis.
Absolute magnitude: The apparent magnitude an object would have at a standard distance of 10 parsecs. It replaces observed brightness with intrinsic luminosity when distance is known.
Red giant branch: The region occupied by stars that have exhausted core hydrogen and expanded while burning hydrogen in a shell. Evolved stars climb this branch toward greater brightness and redder colors.
Open cluster: A relatively loose group of stars formed from the same molecular cloud and bound together by gravity. Its shared distance and origin make its plotted sequence comparatively easy to interpret.
Luminosity–temperature diagram: A graph of stellar luminosity against effective temperature, often used to display theoretical stellar models. It uses intrinsic physical quantities rather than observed brightness and color.
Stellar color index: The difference between a star’s measured magnitudes through two wavelength filters, used as a color measure. A color index commonly supplies the diagram’s horizontal coordinate.
Main-sequence turnoff: The point where a star cluster’s stars begin leaving the main sequence as their core hydrogen runs out. Its location provides a key estimate of a cluster’s age.

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