Kappa Orionis - Saiph

At magnitude 2.06, Kappa Orionis is the sixth brightest star in Orion. To an observer in the northern hemisphere, it is the lower left of the four bright stars that compose Orion's main quadrangle. Kappa Orionis also has the traditional name Saiph, from the Arabic "saif al jabbar", literally "sword of the giant". The name was erroneously transferred to the lower left star of Orion's figure, and it stuck.

Properties

Saiph is a hot class B1 bright supergiant that shines with a sparkling blue-white light. It is one of the hotter stars in the constellation, with a surface temperature of 26,000 K. At 720 light years away, κ Ori is at about the same distance as Rigel. But Saiph looks fainter because its higher temperature makes it radiate much of its light in the invisible ultraviolet. Saiph pours this radiation into space with 65,000 times the Sun's luminosity, implying a large mass of around 15-17 suns.

Massive stars such as Saiph are destined expand into red supergiants, fuse their interior elements into iron, then collapse and explode as supernovae. From its spectrum, Saiph is classed as a "bright supergiant," implying that it is well along in its evolution, having entirely stopped fusing hydrogen. However, its luminosity and temperature place it close to the region of hydrogen-fusion stability, as if it were just in the process of developing into a supergiant. Whether true supergiant or not, it is still large: the luminosity, temperature, and a direct measure of its angular diameter agree on a size about 11 times the Sun's.

Saiph seems to be single. It has a slightly variable spectrum, with only about a tenth the Sun's carbon abundance, though its chemical composition seems otherwise normal. These few peculiarities make it a good background source of light with which we can study the matter in interstellar space.

[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]