Image Centered on Ra.18 : 20.8 (h:m) Dec. -16 : 11 (deg:m) Distance 5000 Ly
Diffuse Nebula M17 (NGC 6618), an emission nebula, in Sagittarius
Information ( from the seds online website)
Discovered by
Philippe Loys de Chéseaux in 1745-46.
The Omega Nebula M17, 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.
The color of the Omega Nebula is reddish, with some graduation to pink. This
color comes from the hot hydrogene 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.
As for many diffuse nebulae, the overall brightness of this object is difficult
to estimate, and is given discordantly in the sources. While older sources give
estimates around 7.0 magnitudes, probably because these were performed at
northern observatories, modern compilations list its visual magnitude brighter:
Don Machholz lists it at 6.6 mag, the Sky Catalogue 2000.0 at 5.0 mag, and the
Deep Sky Field Guide to Uranometria 2000.0 gives a value of 6.0 mag (which we
adopt here); anyway, it is visible to the naked eye under good observing
conditions from not too northern geographic latitudes!
The discovery of M17 by De Chéseaux didn't get widely known, so Charles Messier
independently rediscovered it and cataloged it on June 3, 1764.
The Omega or Swan Nebula M17 can be found quite easily, and similar and
simultaneously to its apparent neighbor, M16. The first way to find it is
locating the white giant star Gamma Scuti, of magnitude 4.70 and spectral type
A2 III, e.g. from Altair (Alpha Aquilae) via Delta and Lambda Aql; M16 is
slightly more than 2 degrees to the southwest of this star. Alternatively, in
particular with a pair of binoculars, locate star cloud M24 and move northward
via a pair of stars of 6th and 7th mag in the north-eastern edge of M24,
followed by small open cluster M18 1deg north, and M17 another 1deg to the
north.
Under very favorable conditions, M17 is just visible to the naked eye at its
apparent visual brightness of 6.0 magnitudes
Optics - Exposure information
Telescope, Vixen R200ss 8 inch Newtonian at F4 (Fl 800mm) with a Televue Coma Corrector and aftermarket Moonlite accessories focuser.
Mount, Losmandy G11 with Gemini control electronics
Imager, Starlite-Xpress SXV-h9 using Schuler Ha and OIII filters
Exposure data, Ha 90 minutes, OIII 90 minutes Combined using Steve Cannistra's Synthesized Green channel technique
Images acquired with Astroart and aligned then combined in Maxim Dl. Final RGB composite processed with Photoshop Cs
Images acquired from my backyard - " Dirt Clod Observatory" in Antelope California
