201409_OkieTex -- m17_1_1d.jpg

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The Omega Nebula, also known as the Swan Nebula, Checkmark Nebula, Lobster Nebula, and the Horseshoe Nebula (cataloged as Messier 17 or M17 and as NGC 6618) is an H II region in the constellation Sagittarius. The Omega Nebula is between 5,000 and 6,000 light-years from Earth and it spans some 15 light-years in diameter. An open cluster of 35 stars 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, plus >1000 stars in formation on its outer regions. It's also one of the youngest clusters known, with an age of just 1 million years.