Stellar rotation

Article By:

Soderblom, David Space Telescope Science Institute, Baltimore, Maryland.

Last reviewed:January 2021

DOI:https://doi.org/10.1036/1097-8542.654200

The spinning of stars, due to their angular momentum. Stars do not necessarily rotate as solid bodies, and their angular momentum may be distributed nonuniformly, depending on radius or latitude. However, it is nearly impossible to resolve the surfaces of stars, or see their interiors, and the limited ability to observe them means that their rotation is generally expressed as a single number, ν_eq sin i. In this measured quantity, ν_eq is the star's equatorial rotational velocity (in kilometers per second), and i is the angle between the star's rotation axis and the line of sight to the star. In other words, ν_eq sin i is the component of a star's rotation that is projected onto the line of sight between the star and the observer. Measurements of ν_eq sin i in stars range from as little as 1 km s⁻¹ (0.6 mi s⁻¹) up to 400 km s⁻¹ (250 mi s⁻¹) or more. A more physically useful measure of rotation is Ω, a star's angular velocity, or P_rot, the rotation period (the inverse of Ω). In some cases it is possible to measure P_rot directly. See also: Angular momentum; Star

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