Trifid nebula (M20), optical image. This is a starbirth region, where gas coalesces into stars. Radiation from the hot young stars formed here ionises the hydrogen gas left over from their formation, causing it to emit pink light. The blue nebula to the right of the emission nebula is a reflection nebula. This does not emit its own light, but instead simply reflects the light of the bright stars nearby. Reflection nebulae typically appear blue as blue light is scattered more effectively than red light. Several dense dark lanes of dust are seen silhouetted against the nebulae. The nebula lies in the constellation Sagittarius, although its distance from Earth is unclear.