NGC 7635, also called the Bubble Nebula, is a H II region[2] emission nebula in the constellation Cassiopeia . It lies close to the direction of the open cluster Messier 52. The "bubble" is created by the stellar wind from a massive hot, 8.7[2] magnitude young central star, the 15 ¡À 5 M¨‘[4] SAO 20575 (BD+60 2522).[7] The nebula is near a giant molecular cloud which contains the expansion of the bubble nebula while itself being excited by the hot central star, causing it to glow.[7] It was discovered in 1787 by Friedrich Wilhelm Herschel.[6] The star SAO 20575 or BD+602522 is thought to have a mass of 10-40 Solar masses.