The Trifid Nebula is a familiar sight and an excellent example of an emission and reflection nebula. The red emission nebula contains a bright blue star cluster near its center: it glows red because the ultraviolet light of the stars ionizes the hydrogen gas, which then recombines and emits the characteristic red hydrogen-alpha light. Further out, when the radiation from these hot young stars becomes too weak to ionize hydrogen, the gas and dust instead glows by reflecting the original blue light. M20 is in the constellation of Sagittarius, at a poorly-known distance somewhere between 2200 and 7600 light-years.