One of the sky’s best-known dark nebulae is the Horsehead Nebula (B33), so named for its distinctive equine shape. Dark nebulae are composed of dust grains, making them visible only when backlit by brighter objects. The Horsehead lies in the constellation Orion, at a distance of about 1,400 light-years, and is a difficult object to spot visually in backyard telescopes.
Adam Block/Mount Lemmon Skycenter/University of Arizona