Achernar

Achernar, or Alpha Eridani, is a bright blue-white star at the southern end of Eridanus. Its rapid rotation makes it markedly flattened at the poles.

Stellar rotation: The spinning motion of a star around its axis. Achernar’s unusually fast spin drives its pronounced equatorial bulge.
Eridanus: A large constellation representing a river, extending across the southern sky. Achernar marks the southern end of this constellation.
Long-baseline interferometry: Interferometry that combines light collected by widely separated telescopes to achieve high angular resolution. This method resolved Achernar’s apparent outline at unusually fine detail.
Vega: A bright A-type star in Lyra that rotates rapidly and is viewed nearly pole-on. Vega’s orientation conceals much of the rotational flattening that is obvious in Achernar.
Oblate spheroid: A sphere compressed along its rotational axis and widened around its equator. This geometry describes Achernar’s strongly flattened shape.
Alpha Eridani: The Bayer designation of Achernar, the brightest star in Eridanus. This is Achernar’s standard designation in star catalogs.
VLTI: The Very Large Telescope Interferometer, an instrument combining light from telescopes at Paranal Observatory. VLTI observations measured Achernar’s extreme oblateness.
Altair: A rapidly rotating A-type star in Aquila with a measurable equatorial bulge. Altair provides another nearby example of rotation distorting a star’s shape.
Gravity darkening: The variation in a star’s surface brightness and temperature caused by differences in effective gravity. Achernar’s rapid rotation makes its poles hotter and brighter than its equator.
Bayer designation: A system naming stars with Greek letters followed by their constellation’s Latin genitive. Alpha Eridani follows this naming system for stars within Eridanus.

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Gamma CassiopeiaeRelated: Its rapid rotation offers a contrasting stellar example without Gamma Cassiopeiae’s defining variability.