Quoted from SEDS:
Discovered by Charles Messier in 1764.
The Trifid Nebula Messier 20 (M20, NGC 6514) in Sagittarius is a
remarkable and beautiful object as it consists of both a conspicuous
emission nebula and a remarkable reflection nebula component.
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, where the remark on nebulosity follows only
after the description of nearby
M21, and includes
that object.
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; this astronomer assigned only one
catalog entry to the whole object (h 1991, h 3718, GC 4355) which became
J.L.E.
Dreyer's NGC 6514.
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, a value which is also used by
Archinal and Hynes (2003), and which we
adopt here. 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, HD 196692 or HN 40 or 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 to O7. The Sky Catalogue 2000.0 even lists 4 more,
faint components of this "multiple star:" D: 10.7, E: 12.6, F: 14.0, and
G: 13.4 mag. This star is located on the west side of the Trifid Nebula
cluster. Situated on the northern edge is HD 164514 of visual magnitude
7.42, a supergiant of spectral type A5 Ia. The presence of these
considerably bright stars makes brightness estimates for the nebula
difficult. |