Hydrogen Trifid . ... Credit Copyright: David McDavid ( Limber Observatory ) Explanation: Clouds of glowing hydrogen gas mingle with dark dust lanes in the Trifid Nebula , a star forming region in the constellation Sagittarius. In this and other similar emission nebulae , energetic ultraviolet light from an embedded hot young star strips electrons from the surrounding hydrogen atoms . ... The relative strength of this emission can trace the densities of atoms within the gas cloud. ...