The star Alpha Herculis, an M5 supergiant or bright giant that is one of the largest stars known, with an estimated diameter some 400 times the Sun's. It is a semiregular variable that ranges between magnitudes 2.7 and 4.0 with a period of 100 days or so, although there are also longer-period variations lasting about 6 years. It is a binary, with a companion of magnitude 5.3 that appears greenish by contrast with the orange primary; this star is itself a spectroscopic binary. The orbital period of the visual pair is estimated at around 3600 years. Rasalgethi lies 360 l.y. away.