Orion the Hunter is probably the most recognized and easily identified constellation in the entire sky. As a bonus, the stellar stick-figure even somewhat resembles the mythological character it represents. We envision him standing facing us with either a bow, shield, or his quarry held in his outstretched left hand, while with his upraised right limb he wields a sword (or sometimes club). His belt of three bright stars is prominent and unmistakable. Hanging down from the belt are what appear to be three fainter stars, representing either his sword or its scabbard.
The sheath is adorned with a jewel like no other in the northern sky: the Great Orion Nebula, more mundanely referred to as Messier 42. If you look carefully at the central “star” of the scabbard, you’ll notice it appears indistinct and fuzzy. Binoculars easily reveal that it’s not a star at all, but a self-luminous cloud. Even a small telescope will give you the impression of bright swirls of glowing gas, while photography will show the gem in all its glory.
The Great Nebula is a stellar nursery, a region of gravitationally collapsed gas and dust from which hundreds of new stars are being formed. Currently the brightest and hottest of the newborns–most particularly a star with the catchy title of Theta-1 Orionis C–are throwing tantrums, wreaking havoc on their playpen by blasting their surroundings with ionizing radiation and ferocious stellar winds. In a few million years their destruction will be complete; the nursery will be in shambles, blown apart and dissipated–but leaving its cluster of newborn stars behind. In the meantime, we humans get to enjoy the spectacle.
Orion is a night hunter who does his stalking during the cold winter months, so we see him arising from his bed in the east at twilight at the beginning of winter season. By mid-January he is fully awake and standing upright, high in the southern skies in late January at 9 pm. That is the ideal time for you to head outside to learn the brightest stars in the constellation; seven are brighter than magnitude 2.3.
Some of these stars have figured into modern popular culture. There is Betelgeuse, whose name (often pronounced “Beetlejuice”) means “Armpit of the Great One” and is the eponymous character in the Tim Burton ghost spoof film. The left shoulder is Bellatrix, a name made familiar in the Harry Potter tales as the dark witch Bellatrix Lestrange. The left foot is brilliant Rigel, seventh-brightest star in the night sky, while the opposite appendage is Saiph, probably the most neglected of the Big Seven. Orion’s Belt (Alnitak, Alnilam, and Mintaka) figures prominently in the story of the alien comedy Men in Black. And who can forget the green Orion slave girl in the original Star Trek series?
An important thing to recognize about the bright stars of Orion (which tends also to be true for most visible stars): None of them appear bright merely because of their proximity; instead, they are brilliant because they are enormous. Every one is exceedingly huge, extraordinarily heavy, and intensely bright. The most mundane of the group is Bellatrix, which weighs “only” 8.6 times as much as our own sun, occupies 175 times more volume, and shines over 9,000 times brighter. By comparison to the others, that’s relatively sedate, and of the Big Seven, it is probably the only star that will not end up as a supernova.
Finally, all of them–with one exception–are amongst the hottest stars to be found in the sky; all are classified by astronomers as blue or blue-white O- and B-type stars ranging in surface temperatures from 22,000 to 50,000 degrees Kelvin. The lone oddity among oddballs is the cool, orange-red Betelgeuse, which is getting a whole lot of attention recently from the astronomical community by acting very strangely. We’ll talk about The Armpit next week.