The Lake County Astronomical Society

Dark Matters

Jack Kramer

When Edward Emerson Barnard turned the instruments of Yerkes Observatory toward star-filled areas of the sky, he took note of a strange phenomenon. There were dark spots of various sizes and shapes. His predecessors going back to William Herschel had concluded that these were simply areas where there weren't any stars -- "holes in the sky". But Barnard decided that the opposite was true. He believed that instead of being empty areas, these were, in fact, concentrations of matter that blocked our view of what lay beyond. Knowledge gained in the ensuing years has proved that he was correct.

The CCD image below shows an area of the sky in southern Ophiuchus centered on "The Snake" -- one of the more recognizable dark nebulae. Each of the prominent dark nebulae is designated with a Barnard number. "The Snake" is B72, while the nebula immediately to the left is B77 and the areas above are B261 and B262.

These dark nebulae (sometimes called "coal sacks") are visible in amateur-sized instruments. The important thing is to use a telescope with low power and a wide field. The most prominent of these objects may be detectable even in binoculars. The best hunting is in a part of the sky with an ample supply of background stars, such as the Milky Way. Very seldom do these objects jump right out at you, but in a dark sky you'll note areas that have fewer stars, and it's helpful if the dark nebula has a definite shape. Note the emphasis on observing from an area with dark skies. As with everything else, light pollution seriously compromises the view of dark nebulae. In my 10" scope I could not detect the entire extent of "The Snake" -- just the most sharply-curved section, which also happens to be the darkest portion.

If you set out to find some dark nebulae, one problem you'll encounter is that the majority of star atlases don't show any of these objects. The Uranometria 2000 is about the only popular star atlas that identifies a large number of them. A large-scale photograph of the Milky Way shows many such objects, but seldom are the photos captioned so you can tell exactly which Barnard object is which. For locating purposes in the image here, the brightest star at the bottom is 44b in Ophiuchus.

Research has shown that many galaxies are full of this dusty primordial matter. Even our own telescopes will show absorption features -- dust -- in some of the other bright galaxies. Considering that this dust is often the progenitor of new stars yet to be formed, we are indeed seeing the real stardust, despite what songwriters and romantics may say!

Some Dark Nebulae in Ophiuchus and Sagittarius from the Saguaro Database

Ophiuchus

Sagittarius