The rate at which an object rotates, often inferred from spectral line broadening.Rotation speed is the rate at which an object spins around its axis, typically measured in revolutions per minute or radians per second. In astrophysics, the rotation speed of stars, galaxies, and black holes is a key parameter that influences their evolution, stability, and observable properties. For stars, rotation affects their brightness, magnetic activity, and lifetime; for galaxies, the rotation curve (speed as a function of radius) provides evidence for dark matter.
The measurement of stellar rotation speeds began in the early 20th century with the development of spectroscopy. In the 1970s, Vera Rubin and Kent Ford measured galaxy rotation curves and found them to be flat at large radii, providing strong evidence for dark matter.
The rotation speed of a celestial object is often inferred from the Doppler effect: if the object is rotating, one side moves toward the observer and the other away, causing spectral lines to broaden. The amount of broadening gives the projected rotation speed. For galaxies, the rotation speed at different radii is measured by observing the Doppler shift of spectral lines from gas or stars, revealing how mass is distributed.
Rotation speed is used to classify stars, estimate their ages, and study stellar activity. In exoplanet research, the rotation of host stars helps determine planetary system architectures. For black holes, measuring spin is crucial for understanding accretion and jet formation.
The Sun rotates once every 27 days at its equator, but only once every 35 days near its poles — a phenomenon called differential rotation.