Messier 20, Trifid Nebula
The Trifid Nebula is one of the main attractions of the Sagittarius constellation. It is located about 1.5 ° north-northwest of the Lagoon Nebula and possibly belongs to the same wide cosmic nebula complex.
M 20 was probably first spotted by LeGentil in 1747 while observing M 8 and later rediscovered by Charles Messier on June 5, 1764. Messier only saw the star cluster, because the light-gathering power of his telescopes was too low to be able to see the nebula. [4, 281]
Sir William Herschel observed the cluster on July 12, 1784 and cataloged three very large nebulae (V 10, V 11, V 12) and described them with: «Three nebula, faintly joined, form a triangle. In the middle is a double star. Very faint and of great extent.»  On May 26, 1785 he revisited the nebula and added with IV 41 another entry: «A double star with extensive nebulosity of different intensity. About the double star is a black opening resembling the nebula in Orion in miniature.» 
Herschels son John cataloged the object as h 1991 (= h 3718, GC 4355) and noted in his Slough catalogue of 1833: «Very large, trifid, three nebulae with a vacuity in the mindst, in which is centrally situated the double star SH 379, nebula = 7' in extent. A most remarkable object.» 
The Trifid Nebula combines the characteristics of an emission nebula and those of a reflection nebula. Excited hydrogen in the central region emits photons in the Balmer wavelength and therefore glows in a pink light. The outer region contains a lot of dust and reflects the light of the hot blue stars and forms the blue part of the Trifid Nebula. The bright star HD 164514 in the north is surrounded by a pronounced blue reflection nebula. The star is not hot enough to stimulate the surrounding gas to glow and so all light is reflected or scattered by the cold gas and dust.
The brightest area of the nebula measures around 20×15 arc minutes, but weaker outer regions allow the size of the nebula to grow to around 25 arc minutes. The «trifid pattern» can be found in the southern bright part of the nebula, where the three dark, approximately 45 arc seconds wide bands of dust lead out from the center and stand out against the glowing background. The mass of the nebula is sufficient to form several thousand suns. The nebula is stimulated to glow by the strong ultraviolet radiation of hot, young stars.The multiple central star is designated HN 40 or GC 24537 and its age is estimated at around 7 million years.These are O7 stars with a calculated absolute Brightness of -5.2 mag. With the 36-inch refractor from the Lick Observatory, SW Burnham even found six stars in this system, whose brightness, distances and position angles are shown in the following table: 
Pair Magnitude Sep. PA A-B 7.0-10.6 5.4" 23° A-C - 8.8 10.6" 212° A-F -13.8 22.1" 106° C-D 8.8-10.5 2.2" 282° C-E -12.4 6.2" 191°
As with most galactic nebulae, the published distances are spread over a wide area. There are values from 2200 to 7700 light years. The latest values are around 5000 light years, which corresponds roughly to the distance from the Lagoon Nebula (M 8). The distance from M 20 is estimated to be about 1500 light years farther than that of M 8. The two nebulae are possibly of a gigantic H-II association, as Figure 3 suggests, but this association has not yet been proven. Like M 8, this nebula is also a source of strong radio and infrared radiation, which is typical for star formation areas. 
|Right Ascension||18h 02m 42.0s|
|Declination||-22° 58' 18"|
|Diameter||20.00 × 20.0 arcmin|
|Photographic (blue) magnitude||8.5 mag|
|Dreyer Description||!!! vB, vL, trifid, D * inv|
|Identification||M 20, OCL 23, ESO 521-N*13, LBN 27, Trifid nebula|
The Trifid Nebula is located in the constellation Sagittarius. If you extend the axis from φ Sagitarii to λ Sagitarii and position the outer ring of the Telrad there, the trifid nebula should already be visible in a large field eyepiece. It can best be observed in the months of June to August. About 0.7 ° to the northeast is the small open star cluster M 21.