The Swan is a large area of emission nebulosity. It contains the red light of hydrogen-alpha emissions, as well as the blue-green light of Oxygen III emission. As with most bright emission nebulas, M17 is a large concentration of hydrogen gas and dust in space. These types of nebulas are the birthplace of stars, which form out of the gas and dust in the nebula's cloud. The brighter portion of the nebula is about 15 light years wide, but the entire complex is much larger. The nebula glows red in this image from the emission of light from the gas which is excited by the ultra-violet light of hot young stars forming from the nebula. About 40 stars are involved in the open cluster associated with the nebulosity of M17, the brightest being magnitude 9.3. P.L. de Cheseaux discovered M17 in 1764, and Charles Messier independently observed it and listed it in his famous catalog in the same year. In 1893, Isaac Roberts took the first known photograph of M17 with a 20 inch reflector and a two-hour exposure. Most DSLR cameras have a filter in front of the sensor that blocks most of the red hydrogen-alpha light that makes up these nebulas. This is done to make the camera's response more similar to normal human vision, which is not very sensitive at this wavelength. Unfortunately for astrophotographers, this makes taking pictures of red emission nebulas more difficult with a stock camera. Image Data
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