Altair

Altair is a rapidly rotating A-type main-sequence star in the constellation Aquila. Its rotation produces a measurable equatorial bulge and makes its equator cooler than its poles.

Stellar rotation: The spinning motion of a star around its axis. Altair spins rapidly enough for rotation to flatten its shape.
CHARA Array: An optical and infrared interferometer operated by Georgia State University on Mount Wilson, California. Its long baselines resolved Altair's shape and surface-brightness pattern.
A-type star: A star with a blue-white spectrum, strong hydrogen lines, and a surface temperature typically around 7,500–10,000 kelvin. Altair's spectrum places it among hot, blue-white A-type stars.
Stellar interferometry: The use of optical or infrared interferometers to resolve angular details on stars. This method made Altair's angular shape measurable despite its tiny apparent size.
Oblate spheroid: A sphere-like shape flattened at the poles and widened around the equator. This geometry describes Altair's rotationally produced equatorial bulge.
Vega: A bright A-type main-sequence star in the constellation Lyra. Like Altair, Vega rotates rapidly, but its nearly pole-on orientation conceals much of its flattening.
Main-sequence star: A star that generates most of its energy by fusing hydrogen into helium in its core. Altair is a main-sequence star rather than an evolved giant.
Angular diameter: The apparent angle subtended by an object's width as seen from an observer. Combining Altair's angular dimensions with its distance yields its physical size.
Gravity darkening: A variation in a star's surface brightness and temperature caused by uneven effective gravity. Altair's lower equatorial gravity makes its equator cooler and dimmer than its poles.
Regulus: A rapidly rotating B-type star and the brightest star in Leo. Regulus offers another interferometrically studied example of rotational flattening.

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