The Bubble Nebula (NGC 7635), found in the constellation of Cassiopeia, is an emission nebula discovered in 1787 by Friedrich Wilhelm Herschel. It resembles a "giant bubble" in outer space – and one might wonder what created this huge and impressive formation. The answer is a massive star at the center, constantly emitting a fierce stellar wind of ionized gas that stimulates the inner layers of the nebula cloud causing them to glow.

     NGC 7635 is part of the larger complex of three bubbles around the massive star and also belongs to a larger network known as Sharpless 162 or S162. S162 lies near the galactic plane in the Perseus arm at a distance of about 3.6 kpc and has been created with the help of other stars as well. 

     The core of the nebula is an 8.7 magnitude blue star known as SAO 20575 or BD+60 2522. It is almost 40 times more massive than our Sun, emitting a wind so hot and intense that it pushes any surrounding sparse gas into a shell. The energetic radiation emitted from the star ionizes this shell, causing it to glow. The star is so energetic that it is constantly shedding material into space at an incredibly fast pace. This material is being shaped into a bubble due to the resistance of the hot and dense gas that surrounds the stellar core. Its irregular shape is a result of encounters with gases of different thickness in different directions. 

          Brighthub.com.