The Bubble Nebula or NGC 7635 is an emission nebula located about 8000 light years in the constellation Cassiopeia. This bubble is the result of the strong stellar winds from the star located at the centre. The star BD+60°2522, is an O type star- hottest and massive stars in the universe.The star at the centre is about 45 times more massive than the Sun and is responsible for the formation of bubble nebula. The stellar winds of gas from the star are flowing out in the space at speeds of about 4 million miles per hour. The star has lost its outer layer of hydrogen and is now burning helium. In about 10 to 20 million years, the star will explode as a supernova.The Bubble itself is 7 light years across. The Bubble Nebula was discovered in 1787 by the British astronomer William Herschel.