M17 (NGC 6618) (Swan, Omega, Horseshoe, Lobster Nebula) (W38)
M17 is one of the largest HII region-molecular cloud complexes in the inner part of our galaxy and one of the closest to our sun. Its location within the Sagittarius arm of the galaxy places it near another famous HII region, the eagle nebula (M16). M17 is considered a prototypical example of triggered star formation where an HII region expands into an adjacent molecular cloud, triggering fragmentation and core collapse within the cloud, and ultimately initiating the process of new star formation. Although M17 is known mostly for its bright HII region, it is situated at the edge of a massive and dense molecular cloud. The visible nebula is illuminated by the massive stellar cluster NGC 6618. The core of the cluster is exceedingly rich in massive young stars and may contain up to 100 "O" and "B" type stars which illuminate the nebula although the stars are heavily obscured by dense and dusty foreground clouds. At the center of the cluster core and situated just west of the bright rim of the optical nebula is a ring of 7 hot O-type stars invisible at optical wavelengths due to obscuring foreground dust. Within this ring is the major source of illumination for the optically bright nebula, a double O4V star system known as Kleinmann's star which is so heavily obscured by intervening dust that it suffers nearly 9 magnitudes of extinction.
Geometrically the visible portion of M17 represents a 40 light year bowl-shaped ionization front projected like a blister along the eastern surface of a dense molecular cloud core known as M17SW. M17SW is itself a component of one of the largest molecular cloud complexes in the galaxy. The immense cloud complex is split into two basic bar-like structures, a northern bar associated with the bright HII region and a southwest bar which includes the fragmented and collapsing molecular core M17SW. Most of the exciting stars of the HII region lie in a central region of the cloud complex between the north and southwest bars. The radiation field from the exciting stars is eroding the edge of M17SW causing it to collapse into fragmented cores. The cores in turn are initiating the process of star formation which astronomers know to be occurring in M17SW by the presence of energetic young protostars.
Historical evidence points to sequential star formation in M17 which has likely proceeded away from the optically bright nebula towards M17SW and into the adjacent giant molecular cloud. Many massive (5 to 20 solar masses) young stellar objects have been detected within the M17SW complex suggesting a very young age for the cluster of about 1 million years. The oldest stars of M17 are the young OB stars which illuminate the nebula. Strong winds from these stars have generated shock waves which have propagated into the surrounding molecular clouds, fragmenting them and initiating the birth of a new generation of stars.
The central hot stars of M17 lie in an area depleted of gas, most likely carved out by the UV flux of the most massive stars. The optical component of this cavity has produced the "horseshoe" shape of M17, a description which has become one of its several monikers.
The nebula is also the second brightest radio source in the sky (after Orion A). Radio wave surveys have detected water vapor in the form of "water-masers" close to the center of the M17. A maser is an acronym for "Microwave Amplification by Stimulated Emission of Radiation". Similar to lasers they represent amplified energy emission phenomena and can be created artificially on earth. However when occurring naturally in space a maser is usually a sign of shocked gas in star forming regions. Astronomers believe the masers may be powered by the energetic outflows of early protostars and provide further evidence for ongoing active star formation in M17.