Trifid Nebula

Messier 20 (NGC 6514), the Trifid Nebula, is one of the most visually striking nebulae in the sky — a rare single object that displays emission nebula, reflection nebula, and dark nebula side by side, all in one compact field. It lies in the constellation Sagittarius, close to the Lagoon Nebula (M8). The Trifid's distance is poorly determined, with estimates ranging from about 2,200 to 7,600 light-years. Charles Messier catalogued it in 1764, and its common name comes from the three lobes visible in telescopes, carved out by dark lanes of dust that seem to divide the nebula into thirds. John Herschel coined the name "Trifid" in the nineteenth century.

The southern reddish glow of NGC 6514 is a classic HII emission region: ultraviolet radiation from a bright central star cluster ionizes the surrounding hydrogen gas, which then emits the characteristic red light of hydrogen-alpha recombination. Farther out, where that radiation becomes too weak to ionize hydrogen, a striking blue reflection nebula appears — the same starlight scattered off dust grains rather than absorbed and re-emitted. The dark lanes that trisect the emission region are cool molecular clouds silhouetted against the bright gas behind them, and careful examination reveals that they too are sites of active star formation, with young stellar objects embedded in their densest cores.

A small telescope shows the three-lobed emission region clearly against a rich Sagittarius starfield; the blue reflection nebula to the north requires a dark sky and a moderate aperture to appreciate fully. This image was made at Kitt Peak National Observatory.