A Orionis, or Betelgeuse, is a bright, yellowish-red star of varying magnitude (0.5 to 1.4, generally 0.9).
[3 Orionis or Regel is a 1st magnitude star.
Y Orionis or Bellatrix, and x Orionis are stars of the 2nd magnitude.
Three collinear stars ?', c and 3 Orionis constitute the "belt of Orion"; of these E, the central star, is of the ist magnitude, 3 of the 2nd, while Orionis is a fine double star, its components having magnitudes 2 and 6; there is also a faint companion of magnitude io.
A Orionis, very close to Orionis, is a very fine multiple star, described by Sir William Herschel as two sets of treble stars; more stars have been revealed by larger telescopes.
0 Orionis is a multiple star, situated in the famous nebula of Orion, one of the most beautiful in the heavens.
In 1920 he was able to demonstrate by means of light-interference that the diameter of Alpha Orionis was 260,000,000 miles.
Of multiple stars the most famous is 0 Orionis, situated near the densest part of the great Orion nebula.
Thus it is found that Procyon gives about three times as much light as the sun in proportion to its mass, Sirius about sixteen times, and Orionis more than ten thousand times.
The Orion stars have the highest temperature of all and have admittedly the greatest surfaceluminosity, but the extreme brilliancy of i Orionis in proportion to its mass must be mainly due to a small density.