Dark nebula in the Crux constellation (Southern Cross). It reduces the brightness of stars beyond it by 1–1.5 magnitudes. Easily visible against a starry background, it is probably about 550–600 light-years from Earth and 20–30 light-years in diameter. It figures in legends of peoples of the Southern Hemisphere and has been known to Europeans since circa 1500. The Northern Coalsack, in the constellation Cygnus, is similar in nature and appearance but somewhat less obvious.
Learn more about Coalsack with a free trial on Britannica.com.