Chances are you’ve seen Orion’s Belt this winter. It’s hard to miss the striking symmetry of those three equally-spaced bright stars in a row. The Belt appears due south around 8 p.m. local time in mid-February. Not far from it another bright star-pattern calls for our attention, the Winter Triangle. It’s big, bright and faces us every clear night in February and March.
No surprise, three stars compose the triangle: red-hued Betelgeuse in Orion’s shoulder, Sirius, a.k.a. the “Dog Star” in Canis Major the Greater Dog and Procyon in Canis Minor the Lesser Dog. The triangle’s sides measure about 25° (two and a half fists marked off against the sky). Sirius is the brightest star in the entire sky and one of the few with a negative magnitude. Star brightness is measured on the magnitude scale. Traditionally, the brightest stars are first magnitude, the next level fainter, second magnitude and so on. The average person can detect stars of sixth magnitude from a dark sky site. The Hubble Space Telescope can dig as deep as 31st magnitude.
Not all first magnitude stars are equal. When you spy the Winter Triangle, you’ll immediately notice that Sirius is much brighter than Procyon. To accommodate the differences, astronomers had to extend the scale “backwards” into negative numbers. The next magnitude brighter than first is zero (0), then negative one (–1), –2, –3 and so on. For precision’s sake, magnitudes are often measured down to the hundredth.
So … Betelgeuse, a variable star, shines around magnitude 0.50, Procyon 0.38 and Sirius –1.46. A half-moon radiates at magnitude –11, the full moon –13 and the sun a blistering –27.
After all this negative talk, let’s briefly turn to what lies inside the Winter Triangle. From a dark sky, the winter Milky Way cuts directly across it. If you’ve never seen this section of the galaxy, the triangle’s a good place to start. From a suburban sky however, all you might notice is a smattering of faint stars inside the figure. But if you connect the dots and salt them heavily with imagination, you’ll see — of all things — a unicorn. This is the realm of Monoceros the Unicorn. Monoceros (pronounced mun-OSS-ser-oss) is a compound word — mono is Greek for “one” and ceros means “horn.” Our familiar unicorn is derived from the Latin uni (one) and cornu (horn).
If you have a 3-inch or larger telescope, one of the unicorn’s finest objects is Beta Monocerotis, a fantastic triple star. The A and B stars are 7.4 arc seconds apart and easy to split at 50x, but B and C are separated by 2.8 arc seconds, so you’ll need 100x to see all three clearly. An arc second is 1/3,600 of a degree, a tiny angle. The full moon is ½° wide or 1,800 arc seconds across.
Have fun with the Winter Triangle, and if you skies are dark enough you might even be able to picture a unicorn running through it. It’s a fantastic universe up there 😉