Much of the time, we know something is out in the universe because it is glowing, emitting its own light or reflecting the light of something else. But glowing matter isn’t all there is in the universe. Sometimes we know something is there only by the shadow it casts. These are the Dark Nebulae.
Imagine that instead of just looking at this image, you’re actually looking through a very, very long pipe and seeing this image. You see thousands of beautiful points of light, but you have no idea how far down the pipe they are. You try to reach into the pipe to see if you can grab one of those points, and you can’t, so you know they’re farther away than your arm. You try a broomstick, but come up empty. Everything you try only tells you that those points of light are farther away than that.
This is the situation with the universe. We have developed methods of determining distances to some things in the universe, but in most instances, the distance to a particular star is still a mystery. We only know it is vastly distant. Where dark nebulae are present, though, we have a new clue. Here, with the Cocoon Nebula, we see a sinuous streak of darkness primarily because there seem to be fewer stars in that location than in its surroundings. In fact, that’s not the case. The star field is quite uniform in this area of the sky. The dark nebula is in front of most of the stars.
Here we have a hint at the three-dimensional nature of what we’re looking at. The dark nebula is associated with the beautiful emission and reflection nebula at its end (thus the moniker) and we have techniques for determining how far away the bright part of the nebula is. In fact, it is about fifteen light years across and some 2,500 light years distant. Since the dark nebula is associated with it, we know that most of the stars in this are are farther than 2,500 light years distant.
Try to imagine this image in three dimensions. Some of the stars are just a few hundred light years distant, maybe even less than that. Others are thousands of light years away. There are stars at every distance in between. Somewhere in there, a river of obscuring matter disrupts the view. The stars behind it can’t shine through it, and we are standing in the shadow it casts.