Star Facts: Saiph

Saiph Star

Saiph (Kappa Orionis) is the 6th most luminous star in the constellation Orion with an apparent magnitude of +2.09, which also makes it the 54th brightest star in the entire night sky. Interestingly, while Saiph is significantly hotter than the constellation’s brightest star Rigel, it is smaller and also appears to be much less luminous because its high temperature causes it to radiate much of its light in ultraviolet wavelengths that are not visible to human eyes.

Quick Facts

• Constellation: Orion
• Coordinates: RA 05h 47m 45.38884s|Dec. -09° 40′ 10.5777″
• Distance: 650 light years
• Star Type: B0.5 Ia
• Mass: 15.50 sol
• Radius: 22.2 sol
• Apparent Magnitude: +2.09
• Luminosity: 56,881 sol
• Surface Temperature: 26,500K
• Rotational Velocity: 83 km/sec
• Age: 11.1 million years
• Other Designations: Kappa Orionis, 53 Orionis, 141 G. Orionis, BD–09 1235, FK5 220, HD 38771, HIP 27366, HR 2004, SAO 132542


It is hard to miss the constellation Orion from either side of the equator, so when Orion is fully upright above the horizon in the northern hemisphere, look for the star Saiph at the lower left corner of the main quadrangle of the constellation formed by the stars Saiph and Rigel at the bottom, and Betelgeuse and Bellatrix at the top. Note that an observer in the southern hemisphere looking north will see an inverted quadrangle, with Saiph in the upper-right-hand corner of the quadrangle.

Physical Properties

Saiph’s is a blue supergiant (B0.5 Ia) located 650 light years from Earth that shines with an apparent magnitude of +2.09. According to its spectrum, hydrogen fusion has ceased in its core, but its high luminosity and temperature, on the other hand, place it in a region of the Hertzsprung-Russell diagram where stars are either in or close to the region where hydrogen-fusion stability exists. In practical terms, its position on the H-R diagram shows that the star is only starting to evolve into a giant, even though it is known that it has exhausted its hydrogen fuel.

Nonetheless, and despite the confusion regarding its evolutionary state, Saiph provides an excellent source of background illumination with which to examine the interstellar medium, or the “cocktail” of matter that fills the spaces between stars. Saiph also appears to be a single star, despite slight variations in its spectrum, and although it also seems to have a “normal” spectrum as derived from its chemical composition, it has only about 10% of the carbon abundance of the Sun.

In addition, Saiph is losing mass at a frenetic pace, with studies having revealed that its very energetic solar wind is blasting material into space at the rate of the about 1 solar mass every 1.1 million years. This finding suggests that at its present age of 11.1 million years, Saiph must once have been about 31.8 times more massive than the Sun. At about this time in its extreme youth, Saiph also reached its point of closest approach to the Sun, when it came within about 56 light years of the Sun during one of its orbits around the galactic core that takes it to within between 24,000 and 26,000 light years of the core.


Although the traditional name “Saiph” derives from the Arabic phrase “saif al jabbar” meaning “sword of the giant”, this name originally applied to Eta Orionis, which is a different star altogether. However, in the 17th century catalogue Al Achsasi al Mouakket, the name Saiph was mistakenly listed with under the name “Rekbah al Jauza al Yemeniat”, which translates into Latin as Genu Dextrum Gigantis or “[the] right knee of the giant”, which is how Saiph is known today.

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