Achernar /ˈeɪkərnɑːr/ (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. 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.