Achernar (Alpha Eridani ) is the brightest star in the constellation Eridanus and the eighth-brightest star in the nighttime sky. It lies at the southern tip of the constellation. The extreme rotation speed has flattened Achernar. It is the least spherical star in the Milky Way studied to date. Achernar spins so rapidly that its equatorial diameter is more than 50% greater than its polar diameter.

In Star Trek, at least one planet in the system, Alpha Eridani II, is known to be populated. In 2156, the Redjac entity was responsible for ten women being knifed to death in the city of Heliopolis on this planet. (TOS: "Wolf in the Fold").

In reality, Achernar is a bright, blue, B-type star of six to eight solar masses lying approximately 144 light-years (44 pc) away. Although classified as a main-sequence (dwarf) star, it is about 3,000 times more luminous than the Sun. Achernar is in the deep southern sky and never rises above 33°N. Achernar is best seen from the southern hemisphere in November; it is circumpolar below 33°S.

Until about March 2000, Achernar and Fomalhaut were the two first-magnitude stars furthest in angular distance from any other first-magnitude star in the celestial sphere. Antares, in the constellation of Scorpius, is now the most isolated first-magnitude star.