A large bag or sack used for storing or transporting coal, or in astronomy, a dark nebula visible in the southern sky.
Coal + sack. The compound refers to containers for coal, but the astronomical meaning refers to the Coalsack Nebula, a dark dust cloud near the Southern Cross constellation, named for its appearance.
The Coalsack Nebula is one of the few dark nebulae visible to the naked eye—ancient sailors in the Southern Hemisphere used it to navigate, showing how even cosmic darkness can light the way!
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