The constellation Orion rolls across the southern sky on winter evenings. The hunter's brightest stars are Betelgeuse, which represents his shoulder, and Rigel, at his foot. Three moderately bright stars form his belt. All five of these stars are far larger and heavier than the Sun, and are likely to end their lives with titanic explosions. Alnilam, at the center of the belt, is the most impressive of them all. Orion's Sword, which includes the star-making Orion Nebula, is below the belt.
If you could catch the hot gas that the star Alnilam is blowing into space, in a million years you’d have enough to make two stars as heavy as the Sun. But you wouldn’t be able to tell any difference in Alnilam itself because it’s one of the monsters of the Milky Way.
Alnilam is the center star in Orion’s Belt, a compact line of three bright stars that rolls high across the south on February evenings.
Alnilam is the most impressive member of the belt. It’s the brightest of the three stars, even though it’s hundreds of light-years farther than the others. And while each of the others consists of two stars, Alnilam moves through the galaxy alone.
Alnilam is a supergiant. It’s roughly 40 times as massive as the Sun, tens of thousands of degrees hotter, and hundreds of thousands of times brighter. Its radiation is so intense, in fact, that it pushes a dense “wind” of charged particles from the surface of Alnilam. The wind races into space at millions of miles an hour.
Although Alnilam is only a few million years old, it’s nearing the end of its life. It’s probably consumed its original hydrogen fuel, so over the next few million years it’ll burn through a series of heavier elements forged in its core.
Eventually, though, it won’t be able to sustain that process any longer. Its core will collapse, while its outer layers blast into space as a supernova — briefly outshining the combined light of most of the galaxy’s other stars.
Script by Damond Benningfield, Copyright 2010, 2014
For more skywatching tips, astronomy news, and much more, read StarDate magazine.