Stellar rotation

Stellar rotation is a star’s spin about its axis, described by its angular velocity or rotation period. It varies with stellar age, mass, and internal structure.

Angular momentum: A conserved physical quantity measuring rotational motion and resistance to changes in that motion. A star’s spin is its angular momentum expressed through its mass distribution and rotation rate.
Gyrochronology: Estimating a star’s age from its rotation period and color. Spin-down makes rotation periods useful as age indicators for many cool stars.
Star formation: The process by which dense regions of molecular clouds collapse to form stars. The collapsing cloud’s angular momentum sets the initial conditions for stellar spin.
Radial velocity: The component of an object's velocity directed toward or away from an observer. Bulk motion shifts a whole spectral line, whereas rotation broadens it across the stellar disk.
Differential rotation: Rotation in which different parts of an object turn at different angular velocities. Many stars rotate faster near the equator than near the poles.
Asteroseismology: The study of stellar oscillations to infer stars’ internal structures and properties. Rotation splits oscillation frequencies, exposing internal spin rates.
Protostar: A young forming star that has not yet begun sustained hydrogen fusion in its core. Accretion and magnetic interactions can strongly change a protostar’s rotation.
Projected rotational velocity: The component of a star’s equatorial rotation speed along the observer’s line of sight, commonly written as v sin i. Spectroscopy usually measures this projection rather than the star’s true equatorial speed.
Magnetic braking: Loss of rotational angular momentum when a magnetized outflow carries it away. Stellar winds coupled to magnetic fields gradually slow many stars.
Starspot: A magnetically cooler, darker region on a star’s photosphere. Recurring brightness changes as spots rotate into and out of view yield rotation periods.

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