Giddyup! Time to gambol down the Great Rift of the Milky Way in search of the Galactic Dark Horse. He’s one of many ‘dark constellations’ we might fashion from the ebony clouds of interstellar dust that pool along the band of the Milky Way.
In late June, the Milky Way begins its rise across the eastern sky at nightfall. From the suburban fringes and countryside, you’ll have no problem seeing the soft band of hazy light that first-time observers often mistake for approaching clouds. Even a casual inspection will show that the band is not evenly-textured. In particular, there’s a large gap or split extending from the Northern Cross all the way down to Sagittarius in the south. Astronomers call it the Great Rift.
The Rift splits the Milky Way down the middle and runs all the way from the Northern Cross (Cygnus) through Sagittarius in the south. It consists of enormous clouds of interstellar dust and gas in the plane of the galaxy called dark nebulae that blot out the more distant stars. If you could suck it all up with a monster vacuum cleaner and expose the billions of stars otherwise hidden, the Milky Way would be bright enough to cast shadows.
Tiny dust particles spewed by older, evolved stars and exploding stars called supernovas settle in the plane of the galaxy. Over the eons, it gets re-compressed into new stars. While the dust is really sparse, it adds up over the light years to form a thick, dark band that appears to slice the Milky Way right in half.
Australian aboriginal peoples created dark constellations like the Emu by connecting a dark patches of the southern Milky Way into creature forms. One of the my favorite such dark patterns and what’s come to be known as the Galactic Dark Horse (GDH) lies about one outstretched fist to the left of the Scorpius’ brightest star Antares.
Astronomers catalog a number of individual dark nebulae that together form the horse. But to the human brain, which has a knack for seeing patterns in just about everything, the silhouetted outline of a horse prancing on its hind legs is unmistakable. The largest part of the, the hind leg, is also nicknamed the Pipe Nebula and lies 600-700 light years away.
For skywatchers at mid-northern latitudes, the GDH is best viewed under dark rural skies where the horse prances low in the south not far from Antares in Scorpius. Allow your eyes time to fully dark adapt and look for his dark rump, wispy head and prong-like legs. This is a fairly large but very dim naked eye object some 10 by 7 degrees across. Averted vision will be your friend. Wide-field binoculars will show it in greater detail against a fabulously rich star field. Best viewing during the current moon-free window starts tonight and continues through about July 3.
Many spiral galaxies like the Milky Way have dark clouds of interstellar dust striping their galactic equators. NGC 891 is just one example. Think of all the wild horses that must be out there!