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Eagle and Omega nebulae. Optical image of the Eagle (red, upper right) and Omega (red, bottom centre) nebulae. They are emission nebulae, clouds of gas that glow due to the ionisation of hydrogen by ultraviolet radiation emitted from hot, young stars within them. The Eagle Nebula, also known as M16 or NGC 6611, lies around 7000 light years away, within the constellation of Serpens. The Omega Nebula, also known as M17, NGC 6618 or the Horseshoe or Swan Nebula, lies at a distance of around 4800 light years, in the constellation of Sagittarius. (KEYSTONE/SCIENCE PHOTO LIBRARY/JOHN SANFORD)