Trifid Nebula — Discovered by Charles Messier in 1764.
Charles Messier discovered this object on June 5, 1764, and described it as a cluster of stars of 8th to 9th magnitude, enveloped in nebulosity.
The Trifid Nebula M20 is famous for its three-lobed appearance. This may have caused William Herschel, who normally carefully avoided to number Messier’s objects in his catalog, to assign four different numbers to parts of this nebula: H IV.41 (cataloged May 26, 1786) and H V.10, H V.11, H V.12 (dated July 12, 1784).
That he numbered this object at all may have its reason in the fact that Messier merely described it as `Cluster of Stars’.
The name `Trifid’ was first used by John Herschel to describe this nebula.
The dark nebula which is the reason for the Trifid’s appearance was cataloged by Barnard as Barnard 85 (B 85).
The red emission nebula with its young star cluster near its center is surrounded by a blue reflection nebula which is particularly conspicuous to the northern end. The nebula’s distance is rather uncertain, with values between 2,200 light years (Mallas/Kreimer; Glyn Jones has 2,300) and about 7,600 light years (C.R. O’Dell 1963).
The Sky Catalog 2000 gives 5,200 light years, the WEBDA database has 3140, the Hubble Press Release of Jeff Hester (STScI-PRC99-42) gives “about 9000″ light years.
As often for nebulae, magnitude estimates spread widely: Kenneth Glyn Jones gives 9.0, while Machholz has estimated 6.8 mag. This may partly come from the fact that the exciting star, ADS 10991, is a triple system of 7th integrated magnitude (with components A: 7.6, B: 10.7, C: 8.7 mag).
All are extremely hot; component A is of spectral type O5 or O6. The presence of this considerably bright triplet makes brightness estimates for the nebula difficult.
In the sky, the Trifid nebula M20 is situated roughly 2 degrees northwest of the larger Lagoon Nebula M8, so that both nebulae form a nice target for wide field photographs, as these images of the M8 and M20 region, or the big DSSM image of this region.
It is even closer to the open cluster M21 and shows up in the upper left edge of our M21 image.