The sky is never clearer than on a cold, starry midwinter night. It is on such occasions, thanks to winter’s exceptional atmospheric transparency, that fainter stars can be seen throughout the sky. And the dazzling array of bright stars and constellations which dominate our late-evening skies are led by the brightest of all the star patterns: Orion the Mighty Hunter.
As darkness falls this month, we can watch Orion with giant strides climbing across the southeast part of the sky. He then arrives in full prominence — in pursuit of the Bull which backs steadily away from him — high toward the south at around 8 p.m. local time and then he descends to below the western horizon at around two the following morning, only to make the trip again the next night.
Belt stars were renowned
Orion is the brightest constellation but is not unique in this respect. Along the southern Milky Way, from Orion to Scorpius, there are several other lustrous groupings, of which the Southern Cross (Crux) and the Centaur (Centaurus) each contain two stars of the first magnitude, just like Orion.
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However, Orion is unique in the fascinating trio of second magnitude stars that form his belt. Indeed, there is probably no catchier star pattern in the entire sky. All three are second-magnitude stars quite similar in brightness and are nearly equally spaced in a line, spanning an angular size of 2.3 degrees.
Those three belt stars also have poetically catchy names: going from left (east) to right (west): Alnitak, Alnilam, and Mintaka. To some old cultures, these stars were a mini constellation by themselves. The people of Greenland, for example, considered them to be bewildered seal hunters who were lost at sea; the Chinese saw them as a weighing beam; Australian aborigines believed they were three young men dancing to the music played by nearby maidens (the Pleiades).
They are even mentioned in the Bible in Job 38:31: “Canst thou bind the sweet influences of the Pleiades, or loose the bands of Orion?” This is the King James version: some of the later translations clearly mention “the belt of Orion.”
Dazzling components of Orion
These three stars are all blue supergiants, rare stars that are among our galaxy’s most luminous members: hot young bluish objects, physically like most of the prominent ones in Crux and Centaurus. Together they form the most conspicuous part of Gould’s Belt, named after Benjamin Apthorp Gould (1824-1896), who studied this array and called attention nearly 150 years ago, to a band of bright blue-giant stars that roughly coincides with the Milky Way.
When we look at the winter sky, we see some of the brightest stars in our part of the galaxy. The distances of Orion’s belt stars range from 900 to 2,000 light-years from us, and shine with average luminosities roughly estimated to be more than 200,000 times that of our sun! These blue stars optically dominate our galaxy’s disk, in which lie its spiral arms. Our solar system is within one of the arms.
When we gaze at the summer sky, however, our view is in the opposite direction, where such dazzling galactic beacons are fewer. Yet, the center of our galaxy is in the direction of Sagittarius, whose innumerable stars, together with those of adjacent constellations, pile up along our line of sight to create the magnificent summer Milky Way. In winter, even though we see brighter stars, we are gazing directly away from the galactic center; thus, the band of the Milky Way here is much thinner and less conspicuous.
Along with Orion’s other blue beacons, the belt stars form a loose stellar grouping called an association. These young stars are literally burning the candle at both ends, furiously fusing their elements into heavier ones. Indeed, such bright blue stars as these can live only for a few million or possibly tens of millions of years. In contrast, our sun has an expected lifetime of at least 10 billion years. Stars that are cooler and fainter can live far longer; the dimmest red dwarf stars theoretically can live on for as much as a trillion years or more.
Innocent bystanders
Two of Orion’s stars are not members of the Orion association and are much nearer to us. Betelgeuse, in the hunter’s right shoulder, is approximately 500 light years away; it is a red supergiant irregular variable star equal in luminosity up to 15,000 suns. In Orion’s other shoulder is Bellatrix, about 250 light years from us. It is fortunate, of course, that these two luminaries happen to be in the same direction as the Orion association, for without them we could not envision a mighty hunter in our winter sky!
Incidentally, in the far distant future, Orion will not appear as prominent as he does now, for our solar system is moving away from it at about 12 miles per second. The apex of the sun’s motion through space is near the summer star Vega, almost opposite in the sky from Orion.
Joe Rao serves as an instructor and guest lecturer at New York’s Hayden Planetarium. He writes about astronomy for Natural History magazine, Sky and Telescope, The Old Farmer’s Almanac and other publications.
