|Designation|| Messier 17|
|Right ascension||18h 20m 26s|
|Declination||-16° 10′ 36″|
|Type of object||Emission nebula|
|Magnitude|| Apparent Mag: +6.0|
Absolute Mag: -5.1
|Distance from Earth||5,000-6,000 ly|
|Radial velocity||-44.577±0.682 km/s|
|Proper motion|| RA: 1.79 mas/yr|
Dec.: -1.96 mas/yr
The Omega nebula (M17, NGC 6618) is famous an emission nebula in the constellation of Sagittarius. Sometimes called the Swan nebula, the Horseshoe nebula or the Lobster nebula, it is named for its resemblance to the Greek letter omega, Ω. Like many emission nebulae, it is illuminated by a star cluster within the nebula but unlike most nebulae, the nebulae obscures these stars in the visible. It is found not far from the Trifid nebula, the lagoon nebula and the Eagle nebula.
The first observation of the Omega nebula was made in 1745 by Jean-Philippe Loys de Chéseaux. It was independently discovered by Charles Messier on 3 June 1764. William Herschel's son, John, first noticed its resemblance to the letter omega. Admiral William Henry Smyth decided it appeared like a horseshoe and named it so in 1836.
Properties and Structure
The nebula has a diameter of 15 light years and is part of a larger cloud that is at least 40 light years across. One of the larger H II (cloud of ionized hydrogen) in the galaxy, it is believed to have a mass of 800 solar masses. The open cluster housed within the nebula of 35 stars which illuminate the cloud. The nebula itself contains at least 800 stars. The gas an dust present in the nebula prevent event the brightest of these stars being visible.