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Achernar , also designated Alpha Eridani (α Eridani, abbreviated Alpha Eri, α Eri), is the brightest star in the constellation of Eridanus and the tenth-brightest star in the night sky. Of the ten apparent brightest stars in the nighttime sky, Achernar is the hottest and bluest in color, being of spectral type B. Lying at the southern tip of Eridanus, the star has an unusually rapid rotational velocity, causing it to become oblate in shape. It is actually part of a binary star system with the second star, known informally as Achernar B, being smaller, of spectral type A and orbiting Achernar at a distance of roughly 12  astronomical units (AU).