Messier 17 NGC 6618 The Swan Nebula
Imaged by Martin S. Ferlito copyright
Film Photography
C8 SCT on Vixen GP mount Stepper Driven.
Prime focus @ f/6.3. 25 minutes exposure Kodak E200 asa
Information provided by seds.org
Discovered by Philippe Loys de Chéseaux in 1745-46.
The Omega Nebula Messier 17 (M17, NGC 6618), also called the Swan Nebula,
the Horseshoe Nebula, or (especially on the southern hemisphere) the Lobster
Nebula, is a region of star formation and shines by excited emission, caused by
the higher energy radiation of young stars. Unlike in many other emission
nebulae, however, these stars are not obvious in optical images, but hidden in
the nebula. Star formation is either still active in this nebula, or ceased
very recently. A small cluster of about 35 bright but obscurred stars seems to
be imbedded in the nebulosity.
This object was discovered by Philippe Loys de Chéseaux and is one of only
six "nebulae properly so called" in his catalog. De Chéseaux's
discovery didn't get widely known, thus Charles Messier independently
rediscovered it and cataloged it on June 3, 1764.
The color of the Omega Nebula is reddish, with some graduation to pink. This
color comes from the hot hydrogen gas which is excited to shine by the hottest
stars which have just formed within the nebula. However, the brightest region
is actually of white color, not overexposed as one might think. This phenomenon
is apparently a result of a mixture of emission light from the hottest gas,
together with reflections of the bright star light from the dust in this
region. The nebula contains a large amount of dark obscuring material, which is
obvious in its remarkable features. This matter has been heated by the hidden
young stars, and shines brightly in infrared light.
The mass of the gas has been estimated to amount about
800 times that of the Sun, enough for forming a conspicuous cluster, and a good
deal more than that of the Orion nebula M42. While the bright nebula seems to
be roughly 15 light years in extension, the total gaseous cloud, including
low-luminosity material, seems to extend to at least 40 light years. Distance
estimates are spread over a wide range, but modern values are between 5,000 and
6,000 light years, thus little less than that of its apparent neighbor, M16
with the Eagle nebula - apparently, these two star forming regions are indeed
close together, in the same spiral arm (the Sagittarius or Sagittarius-Carina
arm) of the Milky Way galaxy, and perhaps part of the same giant complex of
cosmic clouds of interstellar matter.
