In Orion’s Belt
Above and left of center in this image is Alnilam, the middle star in the belt of the constellation Orion, The Hunter. Shining at a visual magnitude of 1.69, it is the brightest of the three belt stars, despite being nearly 500 light-years more distant than its companions, Alnitak and Mintaka. Overall, it is the fourth brightest star in Orion, and the 30th brightest in the night sky.
Also known as Epsilon Orionis, Alnilam is a blue supergiant, roughly 40 times as massive as the Sun. It has a surface temperature of 25,000 Kelvin, almost five times hotter than the Sun. It is so hot that it illuminates its own faint nebula (NGC 1990) of interstellar gas. Bright? The average estimate is around 375,000 times brighter than the Sun, but a new calculation (Dec 2015) estimates it could be more than 850,000 times brighter. Stellar winds? They move at 2,000 kilometers per second, carrying away solar mass at a rate 20 million times greater than the Sun.
Born in the rich molecular clouds of Orion, this supergiant boasts some stunning stats, but they come at a price. At some 4 million years old, it is less than one-tenth of one percent of the age of the Sun. But while our own star will burn for another 5 billion years, Alnilam is nearing the end of its life. It may have already burned through all of its hydrogen, and over the next few million years it will burn through successively heavier elements. It will briefly turn into a red supergiant, outshining fellow Orion member Betelgeuse, before the inward force of gravity wins out over the outward pressure of nuclear fusion. Alnilam’s core will collapse, blasting its outer layers into space in a cataclysmic supernova.
Image credit and copyright: Owl Mountain Observatory