The
Omega Nebula, also known as the
Swan Nebula,
Checkmark Nebula,
Lobster Nebula, and the
Horseshoe Nebula (catalogued as
Messier 17 or
M17 or
NGC 6618) is an
H II region in the
constellationSagittarius. It was discovered by
Philippe Loys de Chéseaux in 1745.
Charles Messier catalogued it in 1764. It is by some of the richest starfields of the
Milky Way, figuring in the northern two-thirds of Sagittarius.The Omega Nebula is between 5,000 and 6,000
light-years from
Earth and it spans some 15 light-years in diameter. The cloud of
interstellar matter of which this
nebula is a part is roughly 40 light-years in diameter and has a mass of 30,000 solar masses. The total
mass of the Omega Nebula is an estimated 800
solar masses.It is considered one of the brightest and most massive star-forming regions of our galaxy. Its local geometry is similar to the
Orion Nebula except that it is viewed edge-on rather than face-on.The
open clusterNGC 6618 lies embedded in the nebulosity and causes the
gases of the nebula to shine due to
radiation from these hot,
young stars; however, the actual number of stars in the nebula is much higher - up to 800, 100 of
spectral type earlier than B9, and 9 of
spectral type O,[
citation needed] plus over a thousand stars in formation on its outer regions. It is also one of the youngest clusters known, with an age of just 1 million years.The
luminous blue variableHD 168607, in the south-east part of the nebula, is generally assumed to be associated with it; its close neighbor, the blue
hypergiantHD 168625, may be too.The Swan portion of M17, the Omega Nebula in the
Sagittariusnebulosity is said to resemble a
barber's pole.
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