Jimmy Westlake: A dark horse in the Milky Way
Once the lingering twilight of late spring fades, you can see the misty star clouds of the Milky Way arching across our summer sky, from the northeast, all the way to the south.
The band of the Milky Way that we see from Earth is only a small portion of our vast Milky Way Galaxy, a spiraling collection of several hundred billion suns. Its glowing star clouds are composed of millions of stars too distant and too faint for us to distinguish as individual stars without a telescope.
I compare it to flying over a stretch of white, sandy beach. From the air, you can see the band of white below stretching in both directions, and you know that it’s made of millions of individual sand grains, but from that distance, you can’t see the sand grains, just the white band that they form due to their sheer numbers.
If the night is dark and clear, you also can detect a network of dark clouds and tendrils meandering through the bright star clouds of the Milky Way. These dark patches are vast interstellar dust clouds thousands of light years away that gather in the space between the stars and effectively obscure the light of the more distant suns behind them.
Astronomers once considered these patches of missing stars to be giant voids — opened windows in the Milky Way — allowing us to peer beyond into the deep universe beyond our galaxy. It wasn’t until the opening years of the 20th century that American astronomer Edward Emerson Barnard recognized that these dark regions were clouds of obscuring matter.
The dark of the moon this week will allow us to see something truly wonderful — the elusive Great Dark Horse Nebula in the Milky Way. The Dark Horse is not made of stars, like other constellations, but is made from the absence of stars. He is made of a collection of dark nebulae that forms the silhouette of a black stallion, reared high on his hind legs, against the bright star clouds behind him.
You can spot the Great Dark Horse with your naked eye; in fact, optical aid renders the Dark Horse invisible. He is big — about as large as your fist held at arm’s length — and hides in the patch of Milky Way between the fishhook pattern of Scorpius the Scorpion and the teapot pattern of Sagittarius the Archer.
His prominent dark hindquarters, also known as the Pipe Nebula, stand out in stark contrast against the dense star clouds seen toward the Milky Way’s center. Look about one-third of the way up in the southern sky around midnight this week.
The bright planets Mars and Saturn are near the Great Dark Horse this year and can serve as a guide. Extending the line connecting Mars to Saturn about half of the distance between them places you right on the forehead of the Dark Horse.
If you don’t spot him at first, keep looking. Searching for dark constellations is much like an ink-blot test. Once you see him, you’ll wonder how you ever overlooked something so obvious. He only comes out during the darkest of nights, so now is your chance to see the sky’s best-known dark constellation, the Great Dark Horse.
Professor Jimmy Westlake teaches astronomy and physics at Colorado Mountain College’s Alpine Campus. His “Celestial News” column appears weekly in the Steamboat Today newspaper and his “Cosmic Moment” radio spots can be heard on local radio station KFMU. Check out Jimmy’s astrophotography website at http://www.jwestlake.com.
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