Pi

Its story

Pi Cassiopeiae is a double-lined spectroscopic binary: two stars packed so close that the spectrograph picks out both chemical signatures while they chase each other in a circular two-day orbit. That proximity distorts them: they are no longer spheres but ellipsoids whose apparent brightness shifts slightly as they spin, on a 23.57-hour cycle that is exactly half the orbital period. The pair lies 175 light-years away, on the southern edge of Cassiopeia's W.