|M20 "Trifid Nebula": Emission and Reflection Nebulae (Sagittarius) RA: 18h 02.4m / DEC: -22° 59'.0|
Instrument: 10-inch Starfinder
The Trifid Nebula, M20, is a wonderful combination of two different types of nebulae. The brighter southern portion is a classic emission nebula. Ultraviolet radiation from a young, hot star excites the enveloping gas cloud. As a result the gas fluoresces, emitting light seen by us as a nebulous glow surrounding the star. The fainter northern section of the Trifid is a reflection nebula. As the name suggests, light is scattered by the nebulae much in the same manner as Earth's atmosphere scatters sunlight. Reflection nebulae are typically not as bright and are blue in color. This color is typically not discernible by the eye but is recorded in long exposure photographs.
The drawings are based on two different views. The drawing above is from a low power, wide field view in my 10-inch Newtonian. I used a 32-mm Plossl producing 36X and a 1.5 degree true field. Fitted with an oxygen III (OIII) filter, this eyepiece reveals a richly detailed web of three main intersecting dark rifts centered about a bright double star. The sketch at left was made at 129X without a filter. The pair of 7th magnitude stars at the juncture of the dark lanes is the source of the radiation producing this beautiful object. Notice the outlying faint nebulous patches. Under a dark sky, the Trifid is a genuine showpiece. You'll find it 1.3 degrees northwest of M8, the Lagoon Nebula.
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Revised: February 10, 2002 [WDF]