Trifid Nebula in Sagittarius (Messier 20).
Caption: Measuring some forty light years across, the Trifid nebula contains enough gas to make many thousands of stars. Within it a number of young hot stars have already formed. They cause the surrounding gas, mostly hydrogen, to emit its characteristic red light. However, one side of the nebula contains many dust grains that reflect the stars' light, producing a bluish color. In some parts of the nebula the dust grains are so numerous that they hide the glowing gas, producing three dark lanes, which give this beautiful object its name.
Copyright: (c) 1977 Anglo-Australian Telescope Board, photograph by David Malin
Credit: D. F. Malin