31 Orionis

A fourth magnitude star near the line of Orion's Belt, close to the Belt's westernmost star Mintaka in the sky. Actually it is much closer to the Solar System than Mintaka - almost exactly half the distance - but nonetheless the more distant but luminous supergiant outshines 31 Orionis considerably.

Structurally 31 Orionis is an orange giant, with a spectral classification of K5III making it directly comparable with the more familiar Aldebaran in Taurus. At a distance of some five hundred light years from Earth, however, it is far less distinct in the sky than Aldebaran, which is seven times closer. 31 Orionis is nonetheless a huge and massive star by comparison with the Sun, having perhaps sixty times the Sun's diameter.

This star was at one time thought to be variable in nature, and hence carries the variable designation 'CI Orionis'. This variability has not been confirmed, and the original designation seems to have been made in error. 31 Orionis appears to have a faint white companion star, following a distant, millennia-long orbit around its giant orange primary star.