Rapid rotation physically distorts brightest star in Cepheus constellation

Alpha Cephei, also called Alderamin, rotates so quickly that its shape is compressed into an oblate form resembling a partially deflated ball rather than a sphere. Located approximately 49 light-years away, this white star is roughly twice the mass of our sun and about 17 times more luminous, classified as a Class A star that will eventually expand into a red giant. Astronomers used optical interferometry at Georgia State University to measure the star's extreme rotational velocity.

Alpha Cephei belongs to a stellar class known as Class A stars, which represent a distinct phase in stellar evolution. These stars will eventually exhaust their core hydrogen supplies and undergo dramatic transformations, expanding into red giants as their outer layers balloon outward. The star's current state—roughly double the sun's mass and significantly more luminous—positions it as a particularly instructive example of how stellar properties change across a star's lifecycle.

The phenomenon of gravity darkening observed in Alpha Cephei reveals fundamental physics about rotating celestial bodies. As rotation speed increases toward the equator, centrifugal forces push the star's material outward, simultaneously cooling this region. The poles, experiencing less outward force, remain hotter and therefore brighter, creating observable temperature gradients across the star's surface that telescope arrays can now detect and measure.