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Starry, Starry Night



Alnilam is the middle star in the belt of Orion. It is a bright star shining at magnitude 1.7. It appears to be as bright as the other stars in the belt but it is about 50% further away than the others. Its designation is Epsilon Orionis and it's a solid member of the B stellar spectral class. Much of its light is in the ultraviolet. It shines with the luminosity of 375,000 suns and contains 40 solar masses. Stars of this size are headed for a spectacular end - a supernova.

Alnilam is only 4 million years old but it is already running out of hydrogen and will start fusing larger atoms and then turn into a red supergiant more luminous than Betelgeuse. A star of this size sends out a large amount of material at speeds up to 2000 kilometers per second. The total amount of mass that it sends out is 20 million times what our sun does. Alnilam is a single star unlike the majority of the brightest stars in Orion including the other belt stars.

When we look at the stars in Orion we are looking outward from the center of the galaxy, out along the (aptly named) Orion arm. Aldebaran is one of the two major stars easily found from the belt of Orion. If you follow the line formed by the belt to the right (or up if Orion is just rising) you get to Aldeberan, a bright red star sometimes called the eye of the bull (Taurus). To me it is more accurate to follow a line that runs parallel to the belt but actually starts at Bellatrix (the other shoulder of Orion besides Betelgeuse). If you follow the belt line to the left (or down), you get to the brightest star in the sky, Sirius.

Many of the stars in Orion, including Alnilam, are created from something called an OB association. The O and the B refer to the kinds of stars created in these associations, the kind that live fast and die young. This is an association that, because of the strong stellar winds and large masses, breaks down into substructures with the stars in each structure sharing the same (relative) birthdate. The stars in the Belt of Orion are part of one such substructure. The sword in Orion is another substructure and there the Orion nebula is still producing stars. The stellar winds in the nebula can be seen evaporating the dust around newborn stars thus making planetary formation unlikely.

The other stars in the belt are called Alnitak (to the left of Alnilam) and Mintaka. All three stars incorporate the name of the whole asterism. Alnitak and Mintaka both mean something like "the belt of the Central One". Alnilam means "string of pearls". All three names are Arabic.

Information for this article comes from kaler. Substitute another star name for alnilam and you will probably get some interesting information on that star. If you want to see where a constellation is relative to the Milky Way, try this website: website. And if you want a more technical description of the Orion OB1 association see here.


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