The bubble nebula is an emission nebula found 7100 light-years from Earth in the star Cassiopeia. (Also known as NGC 7635)
The bubble formed around the nebula, which is about 7 light-years in diameter, due to the stellar wind emanating from the giant star SAO 20575, which shines brightly in the center.
The star in the center is brighter than our Sun and 45 times the mass of the Sun.
It was discovered in 1757 by William Herschel.