(catalog numbers NGC 6514 and M 20), bright, diffuse nebula in the constellation Sagittarius, lying several thousand light-years from the Earth. It was discovered by the French astronomer Legentil de La Galaisière before 1750 and named by the English astronomer Sir John Herschel for the three dark rifts that seem to divide the nebula and join at its centre. Of about the ninth magnitude optically, the Trifid is also a radio source.