NGC 7635, the Bubble Nebula, is located in the constellation of Cassiopeia.
The Bubble nebula shines at magnitude 11. It lies 11,000 light-years from the Earth with an apparent size of 15 x 8 arcminutes. This corresponds to a real size of 10 light-years in space.
NGC 7635 is an emission nebula formed from a fast stellar wind of ionized gas coming from the massive O-type Wolf-Rayet star HD 220057, which is the brightest star inside the bubble at the upper left. HD 220057 shines at magnitude 6.9. As the wind comes off the star, it pushes the surrounding gas in the area into a giant sphere that is surrounded by a molecular cloud. The high-energy ultra-violet light from the star then ionizes the gas, causing it to fluoresce and glow.
The star that illuminates the Bubble is some 25 to 40 times as massive as our Sun and thousands of times brighter. The Wolf-Rayet star will probably end up exploding in a supernova.
The Bubble Nebula was discovered in 1787 by William Herschel.
North is to the top in the above image.