M17, also known as the Omega Nebula, NGC 6618, Sharpless 45, RCW 160, Gum 81, Collinder 377, LBN 60, Swan Nebula, Lobster Nebula, Horseshoe Nebula, Checkmark Nebula, is a famous star-forming nebula located in the southern constellation Sagittarius. The diffuse emission nebula lies near the constellation’s northern border with Scutum, at a distance of 5,000 to 6,000 light-years from Earth at right ascension 18h 20m 26s and declination -16°10’36”. It has an apparent magnitude of 6.0. The Omega Nebula is one of the brightest star-forming nebulae of the Milky Way. With a total mass of roughly 800 solar masses, it is also one of the most massive H II regions of our galaxy. The nebula occupies an area roughly 15 light-years in diameter and is part of a larger cloud, which is about 40 light-years in diameter. Messier 17 is located in the Sagittarius Arm of the Milky Way, the next inward spiral arm to our own, and maybe part of the same giant cloud complex as its close neighbour, Messier 16 (the Eagle Nebula), located in the constellation Serpens.
The Omega Nebula lies just to the south of the Eagle Nebula. Two other famous nebulae – the Trifid Nebula (Messier 20) and the Lagoon Nebula (Messier 8) are located in the same part of the sky. Messier 17 can also be found by star-hopping from the bright Altair in the constellation Aquila down the eagle’s back toward Gamma Scuti in the constellation Scutum. M16 and M17 lie just to the southwest of the star. The best time of year to observe the Omega Nebula and its neighbours is in the summer months when Sagittarius hovers over the southern horizon in the evening.
The Omega Nebula was discovered by the Swiss astronomer Jean-Philippe Loys de Chéseaux in 1745. It was independently discovered by Charles Messier on June 3, 1764. John Herschel, William Herschel’s son, was the one who noted the nebula’s resemblance to the Greek capital letter omega (Ω). He was the first to accurately draw M17 in 1833 and published his drawing as part of a series of sketches of nebulae in 1836.
Messier 17 contains an open cluster of 35 hot, young stars which are responsible for the nebula’s glow. The cluster is only 1 million years old, which makes it one of the youngest known clusters in our galaxy. It is home to up to 800 stars, including nine stars of spectral type O and about 100 of spectral type earlier than B9. Additionally, there are more than a thousand stars in the process of forming in the nebula’s outer regions. The young stars in M17 do not readily appear in visible light images but are hidden within the nebula’s dust and gas. Even the light of the 35 brightest stars is obscured by the surrounding nebulosity. Messier 17 has a similar geometry to Messier 42, the famous Orion Nebula, but unlike M42, which appears face-on, M17 is seen edge-on. The Omega Nebula is considerably richer in star-forming material and young stars than M42, and it is also much more distant. The Orion Nebula lies some 1,300 light away from Earth and is located in the Orion Arm of the Milky Way, the same spiral arm that is home to our solar system.
Two blue hypergiants are believed to be associated with M17: HD 168607, a luminous blue variable (LBV), has a visual magnitude of 8.25. The star forms a pair with another blue hypergiant and suspected luminous blue variable, HD 168625, that lies just southeast of the nebula. HD 168607 is 240,000 times more luminous than the Sun, while HD 168625 has 220,000 solar luminosities. The nebula’s O-type stars and blue hypergiants will likely end their life in Type II supernova explosions in the not-too-distant future, astronomically speaking. Credits: Messier Objects, NASA, Universe Today, Wikipedia.
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