The Bubble Nebula (NGC 7635, Sharpless 162, or Caldwell 11), is an emission nebula in the constellation Cassiopeia. The "bubble" is created by the stellar wind from a massive central star,SAO 20575 (estimated mass 10-40 Solar masses). The adjacent giant molecular cloud contains the expansion of the bubble nebula while itself being excited by the hot central star, causing it to glow. Right of the Bubble Nebula is the emission nebula Sharpless 159. This H II region, contains within it a very compact source of radio waves, which coincides with the densest part of the cloud, and surrounds a star of spectral type O9, which is the main source of ionization of gases in the region. At the upper left corner is another emission nebula - NGC 7538. This is one of the regions of intense star formation easier to observe, due to the fact that it is obscured by dust and dark clouds.The main source of gas ionization in this cloud is the biggest yet discovered protostar which is about 300 times the size of our Solar System.