If you venture out under clear and dark Western Slope winter skies, you’ll notice a diffuse glow, extending from the northwestern horizon across the zenith to the southeast. This is the winter view of our home galaxy, the Milky Way. While not as bright as the Milky Way we see during summer evenings, the winter Milky Way has a subtle beauty all its own.
Our galaxy is a flattened disk of spiral form that contains upwards of hundreds of billions of stars, plus glowing gas clouds called nebulae, and dark dust clouds. We live about halfway between our galaxy’s center and its outer edge, roughly 26,000 light years from the center. If you want that in miles, it’s a 153 followed by 15 zeroes! The outer limits of our galaxy are poorly known, but they are at least 25,000 light years distant from us opposite the center. A halo, rich in enigmatic dark matter, likely extends even farther beyond. Because we live within this flattened disk of distant stars, gas and dust, the Milky Way appears as an elongate, diffuse glow, about 20 to 30 apparent Moon diameters wide. It encircles the entire sky from the north, to well beyond our southern horizon.
During summer evenings, we can gaze southward toward the center of our Milky Way Galaxy in constellation Sagittarius. Here, star clouds, nebulae, and dark dust clouds are so concentrated that they render the true galactic center invisible to our eyes. During winter, sunlight hinders our view toward the center, but we can gaze toward the galaxy’s outer reaches, where stars, gas, and dust are not so highly concentrated. Spanning the sky from constellations Cygnus, Cassiopeia, and Perseus in the northwest, to Auriga overhead, through Gemini, Monoceros, Canis Major, and Puppis in the southeast are numerous star clusters and nebulae. Some of these clusters, like the Perseus Double Star cluster, are particularly striking, because there is little intervening dust to block our view.
So, on one of these crisp evenings, enjoy the view of the winter Milky way, our galaxy’s outer reaches. Binoculars will enhance your experience.
Western Slope Skies is produced by the Black Canyon Astronomical Society. This episode was written and recorded by Art Trevena.