Under a dark sky, the Milky Way is impressive – a milky band of light that outlines the disk of our home galaxy. But the band isn’t smooth. It’s lumpy, with some parts much wider and brighter than others. And a dark lane runs down its middle, bisecting the Milky Way into two bands. It, too, is lumpy.
That dark lane is known as the Great Rift, and it looks almost empty – as though a cosmic dragon has swallowed most of the stars. Instead, the darkness has simply swallowed the light of the stars behind it. That’s because the dark lane is made of giant clouds of dust. They block the visible light of the stars inside and behind them, just as a dark cloud here on Earth can block the Sun from view.
Perhaps the most famous cloud is the Coalsack, an especially big “hole” in the Milky Way. It’s in the Southern Cross, which is too far south to see from the continental United States. The Coalsack is about 600 light-years from Earth, and spans about 60 or 70 light-years.
There’s also the Northern Coalsack. It’s just about as big as the southern version, although it’s not quite as dark or well-defined. Still, there’s a definite gap in the stars there. It’s in Cygnus. As night falls now, it’s to the right of Deneb, the bright star that marks the swan’s tail.
The Northern Coalsack veils an especially bright region behind it – one of the biggest stellar nurseries in our part of the galaxy. More about that tomorrow.
Script by Damond Benningfield