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NGC 6888 on August 16, 2004

NGC 6888 on August 16, 2004. Prime focus (f/8.0). Total integration time 565×8.53 sec = 80 min. This is the famous Crescent Nebula, also number 27 in the Caldwell list. The bright star in the lower right, HD192163, is a Wolf-Rayet star. It blew off its outer atmosphere, revealing a hot stellar core whose intense radiation drives a massive stellar wind. The shock front resulting from this high-speed material plowing into the previously blown-off (and slower moving) outer atmosphere causes the glow we now see as the nebula. The whole nebula (about half is shown here) measures about 25×16 light years, and it is about 4700 ly distant.