Dark nebula in the Crux constellation (Southern Cross).
It reduces the brightness of stars beyond it by 11.5 magnitude s. Easily visible against a starry background, it is probably about 550600 light-years from Earth and 2030 light-years in diameter. It figures in legends of peoples of the Southern Hemisphere and has been known to Europeans since 0441; 1500. The Northern Coalsack, in the constellation Cygnus, is similar in nature and appearance but somewhat less obvious.