Stellar age

Stellar age is the time elapsed since a star formed. It is inferred from a star’s properties and evolutionary stage rather than measured directly.

Main sequence: The long-lived phase when a star fuses hydrogen into helium in its core. A star’s position and evolution along this phase constrain its age.
Stellar evolution: The changes in a star’s structure and properties over its lifetime, driven chiefly by nuclear fusion and gravity. Age dating interprets a star’s present properties as points along an evolutionary track.
Open cluster: A gravitationally bound group of stars that formed from the same molecular cloud. A cluster’s shared age makes its turnoff point a powerful population-wide clock.
Stellar rotation: The spinning motion of a star around its axis. Rotation-based dating becomes less reliable when spin-down departs from calibrated patterns.
Isochrone: A curve on a stellar diagram showing the predicted properties of stars of one age and composition. Matching a star to model isochrones is a standard way to estimate its age.
Hertzsprung–Russell diagram: A plot of stars’ luminosities against their surface temperatures or spectral types. A star’s location on this diagram helps identify its evolutionary stage.
Globular cluster: A dense, roughly spherical cluster containing old stars bound by gravity. Its oldest stars constrain the early history and minimum age of the Milky Way.
Magnetic braking: The loss of stellar angular momentum through magnetized winds carrying matter away from a star. Uncertainty about braking at old ages limits gyrochronology’s reach.
Gyrochronology: A method of estimating a star’s age from its rotation period and color. Magnetic braking links rotation slowdown to age for many cool stars.
Stellar mass: The amount of matter contained in a star. Mass strongly determines how quickly a star evolves and how long it lives.

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