Thanks to James B. Kaler. These contents are the property of the author and are reproduced from original without the author's express consent because of fair use and valid educational purposes.


With brilliant Betelgeuse and Rigel dominating great Orion, we pay little heed to the individual stars of the Hunter's belt except as a group, the trio the Arabs called the "string of pearls." All second magnitude, Johannes Bayer seems to have named the stars Delta, Epsilon, and Zeta from right to left. The name of the left hand star, Alnitak (Zeta Orionis), stands in for the whole string, and comes from a phrase that means "the belt of al jauza," "al jauza" the Arabs female "central one." Separate Alnitak from the belt and it becomes a most remarkable star in its own right, the brightest class O star in the sky, a hot blue supergiant. Tucked right next to it is a companion, a blue class B hydrogen-fusing star about three seconds of arc away, the pair orbiting each other with a period estimated to be thousands of years long. The region around Alnitak is remarkable as well, containing several dusty clouds of interstellar gas, including the famed "Horsehead Nebula" to the south. Alnitak approaches first magnitude even though at a distance of 800 light years. To the eye (ignoring the companion), it is 10,000 times more luminous than the Sun. However, its 31,000 Kelvin surface radiates mostly in the ultraviolet where the eye cannot see, and when that it taken into account, Alnitak's luminosity climbs to 100,000 times solar. A planet like the Earth would have to be 300 times farther from Alnitak than Earth is from the Sun (8 times Pluto's distance) for life like ours to survive. Such brilliance can only come from a star of great mass, Alnitak's estimated to be about 20 times solar (its dimmer companion's about 14 times solar). Like all O stars, Alnitak is a source of X-rays that seem to come from a wind that blows from its surface at nearly 2000 kilometers per second, the X- rays produced when blobs of gas in the wind crash violently into one another. Massive stars use their fuel quickly and do not live very long. Alnitak is probably only about 6 million years old (as opposed to the Sun's 4.5 billion year age) and it has already begun to die, hydrogen fusion having ceased in its core. The star will eventually become a red supergiant somewhat like Betelgeuse and almost certainly will explode as a supernova, leaving its companion orbiting a hot, madly spinning neutron star. (Thanks to Monica Shaw, who helped research this star.)