A large star-forming, emission and reflection nebula in the constellation Sagittarius (1.3° north of the ), famous for its three-lobed appearance. It was discovered by Charles Messier in 1764 and named the "Trifid" by John Herschel. The dark nebula which is the reason for the Trifid's appearance was catalogued by Edward Barnard as Barnard 85 (B 85).
The red emission portion of the nebula is illuminated by a young cluster of hot, massive O stars near its center including the triple-star system known as ADS 10991. Component A of this trinary is of spectral type O5 or O6 and has an apparent magnitude of 7.6. Components B and C have magnitudes 10.7 and 8.7, respectively. Surrounding the red emission portion of the Trifid is a blue reflection nebula which is particularly conspicuous to the northern end.
The bright central O stars illuminate a dense pillar of gas and dust, seen to the right of the center of the Hubble image (above), producing a bright rim on the side facing the stars. At the upper left tip of this pillar is a complex filamentary structure. This wispy structure has a bluish color, not because of reflection, but because of emission from ionized oxygen as it recombines.
Star formation is no longer occurring in the immediate vicinity of the central cluster of O stars, because their intense radiation has blown away the gas and dust from which new stars are made. In 2005 the Spitzer Space Telescope, observing at near infrared wavelengths, discovered 30 embryonic stars and 120 newborn stars in the Trifid Nebula which do not show up in visible light.