a dark nebula in the Crux constellation (Southern Cross). Easily visible against a starry background, it is perhaps the most conspicuous dark nebula. Starlight coming to Earth through it is reduced by 1 to 1.5 magnitudes. The Coalsack is about 500 light-years from Earth and 50 light-years in diameter. It figures in legends of peoples of the Southern Hemisphere and has been known to Europeans since about 1500. The Northern Coalsack, in the constellation Cygnus, is similar in nature and appearance but somewhat less prominent.