Achernar (α Eri, α Eridani, Alpha Eridani), sometimes spelled Achenar, is the brightest star in the constellation 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.[nb 1] Lying at the southern tip of Eridanus, the star has an unusually rapid rotational velocity, causing it to become oblate in shape. Achernar is actually a binary star system, with the second star known as Achernar B. The second star is smaller and orbits Achernar A at a distance of roughly 12 astronomical units (AU). Achernar B is of spectral type A.