The Omega or Swan Nebula

Degree of difficulty 1 (of 5)

Minimum aperture Naked eye

Designation NGC 6618

Type Galactic nebula

Class Emission nebula

Distance 5910 ly (K2005) 8160 ly (CMD, 2003)

Size 70 ly

Constellation Sagittarius

R.A. 18h 20.8min

Decl. –16° 11′

Magnitude 6.0

Surface brightness 21mag/arcsec2

Apparent diameter 40′×30′

Discoverer de Chéseaux, 1746

History M 17 was found in 1745 or 1746 by the Swiss observer Philippe Loys de Chéseaux. He described his discovery: “It has perfectly the shape of a ray, or of a comet tail, of 7′ length and 2′ width, its sides are exactly parallel and quite well defined, much like its ends. The middle is whiter than the edges.”

Charles Messier did not know of de Chéseaux's note, because it was not published in print. Hence, his first observation of M 17 on the 3rd of June 1764 was an independent discovery. He noted: “Train of light without stars, of 5′ to 6′ in size, in the shape of a spindle, & a little bit like that one in the girdle of Andromeda [M 31], but of a very faint light; there are two telescopic stars near & placed parallel to the Equator.”

William Herschel saw much more, when he enthusiastically wrote in 1783: “A wonderful nebula. Very much extended, with a hook on the preceding side; the nebulosity of the milky kind; several stars visible in it, but they seem to have no connection with the nebula, which is far more distant. I saw it only through short intervals of flying clouds and haziness; but the extent of the light including the hook is above 10′.” To Herschel, too, there was some resemblance to M 31. It took 100 years for it to become clear that M 17 is, in fact, an object within our Galaxy.

John Herschel studied M 17 in more detail. Two of his descriptions have been documented; in the first, he makes a remarkably accurate connection with M 42 and comments: “A most curious object, not unlike the nebula in Orion. There is in it a resolvable portion or knot distinctly separated from and insulated in the rest as if it had absorbed the nebula near it.”