- Photo courtesy of "Mouser," 25 minute exposure. GNU Free Documentation License
- Orion is our night-sky companion in the south on winter evenings. Note the nebulosity in the sword, visible to the naked eye, but much better through binocs.
Winter is Orion the Hunter's season as he strides across the evening sky, trusty sword hanging below the three stars of his belt. Orion is the most recognizable of all the "connect the dots" shapes created by our brains when we look up at night. Although occupying less than 1 percent of the area of the entire sky, the constellation looks deceptively large due to its brightness — six of the 50 brightest stars in the night sky are found there. Why so bright? When we look at Orion, we're looking at a vast active zone of star formation. All seven stars comprising his shoulders, belt and feet are less than 20 million years old, babies compared to our own sun's five billion years. Let's take a closer look at some of Orion's features:
Betelgeuse, Orion's orange-tinted left shoulder star, is a red "supergiant," 20 times the mass of the sun. It's "burning" its fuel — that is, fusing hydrogen nuclei — at a prodigious rate. Although only about 10 million years old, Betelgeuse is destined to explode in a violent supernova sometime in the next million years (assuming it hasn't already done so, since we see it as it was 600 years ago, about the time Gutenberg was figuring out mechanical printing). Betelgeuse was the first star to be resolved as an actual disk, by the Hubble telescope — quite a feat, given that the star appears to us 40,000 times smaller than the full moon.
The three stars of Orion's Belt are even younger, less than six million years old. Looking deceptively close to each other, they're actually separated by great distances, lying between 700 and 1,400 light years from us. They look bright to us because they're so intrinsically luminous. Compare them to Sirius, brightest star in the night sky — easily found, because Orion's Belt points to it, 20 degrees down and to the left. Sure, Sirius appears a lot brighter, but that's because it's practically next door, just nine light years away.
The three main stars of Orion's sword are worthy of attention. Even to the naked eye (more so through binoculars) the middle one looks fuzzy. That's because it sits in a vast glowing cloud of dust and gas known as M42, the brightest nebula in the night sky. It's an active "stellar nursery" containing at least 700 newly formed or very young stars. Curiously, Galileo, who reported seeing many nebulae through his telescope, didn't mention M42, leading to speculation that it has brightened during the past 400 years. It won't last forever. 100,000 years from now, after most of its dust and gas have been ejected, M42 will probably resemble the Pleiades star cluster.
Orion's distinctive hunter shape has been, and will be, a fixture in the night sky for millions of years, since any relative movement between its constituent stars appears small to us at such great distances. So there's no hurry, but I do recommend braving the cold of the next clear winter's evening to visit our local galactic neighborhood as photons from Orion's stars and nebulae end their long journeys as they slam into your retinas.
Barry Evans (barryevans9@yahoo.com), whose Field Notes anthologies are sold in local bookstores, considers the stars his friends.
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