Stellar Trifecta

Finding the frosty Winter Triangle.

Brave the clear air on a frigid night, and you’ll be rewarded with the bright stars of the winter sky. These sparkling jewels provide ample opportunities to make one of the simplest star patterns: a triangle. And one colorful and prominent stellar trio that forms the so-called “Winter Triangle” is well worth finding.

Look for another geometric shape to start: the bright rectangle of stars—with a line of three bright stars inside—that forms the constellation of Orion the hunter. Orion rises just after dark in mid-December and sits high south by midnight. By mid-January, it’s up in the southeast at nightfall and southward by 10pm; in mid-February, these same positions by 8pm.

The upper-left-hand star of Orion’s rectangle marks his shoulder and shines distinctly orange. This is Betelgeuse, the first of our Winter Triangle stars.

Now trace Orion’s belt of three stars down to the left and you’ll come to blazing white Sirius in Canis Major, the Great Dog. Also called the Dog Star, it’s the brightest-appearing star in the sky.

Then look up and left of Sirius to find another bright star that makes an equal-sided triangle with Betelgeuse and Sirius, and you’ll find yellow-white Procyon in Canis Minor, the Lesser Dog, completing the Winter Triangle.

Procyon is Greek for “before the dog,” so-named because it rises just ahead of Sirius by about 45 minutes at our latitude. But Sirius was also a herald. Called “Sopdet the Preparer” by the ancient Egyptians, it rose with the sun just before the annual flooding of the Nile, which watered and fertilized their riverside fields.

To the ancient Greeks and others, that “heliacal” rising of Sirius also marked the start of the hottest days of the year, called the “Dog Days,” when it was believed that the star combined its rays with the sun’s to make things particularly oppressive. No wonder they named it Sirius, which in Greek means “scorcher.”

Sirius is so bright because it’s one of the nearest stars to us in space—as is Procyon. This also made it easier for early 20th-century scientists to find their white-dwarf-star companions, among the first to be detected. These compressed stellar cores were once stars more massive than either Sirius or Procyon, which made them evolve faster, lose their outer layers, and collapse to their current dwarfy states, taking a back seat to their still-vital partners.

Betelgeuse, whose name is anatomically correct Arabic for “giant’s shoulder,” also has claims to fame. This massive, fast-living star has bloated in its old age to become one of the largest stars we know of in our galaxy, releasing envelopes of gas and generally preparing to blow itself up as a supernova sometime in the next 100,000 years or so, scientists estimate.

The star made news in 2019-20 by dimming dramatically to one-third its usual brightness, causing some to speculate that it might be about to explode. But then it began to brighten back to normal. Subsequent studies suggest the dimming was caused by hot gas cooling and becoming dusty, which blocked some of the star’s light until the dust cloud dissipated.

Sirius is especially fun to watch shortly after it’s risen, around 9pm in mid-December and 7pm in mid-January, when jumpy air near the horizon momentarily bends all but one color of its spectrum of light away from your eye, letting it flash vivid red, blue, and yellow. Binoculars can enhance the view.

While you have the binoculars out, look below Orion’s belt for the fuzzy patch that marks the Orion Nebula where new stars are being born, and below Sirius where a lovely small star cluster called M41 twinkles—more rewards for venturing out into the universe stretching above our winter landscape on a frosty night.

Jim Manning is the former executive director of the Astronomical Society of the Pacific. He lives in Bozeman.