The Bubble Nebula really is a bubble. It is being blown into this shape by the bright star known as SAO20575, which sits just to the left of centre in this image. This is a giant star of 10–20 times the mass of the Sun.
The star is pumping out a fearsome torrent of ultraviolet radiation, causing the surrounding gases to glow like a fluorescent light. But it is not this ultraviolet radiation that is blowing the bubble. Instead, it is being created by SAO20575’s stellar wind.
A stellar wind is a high-speed flow of particles streaming away from the star. As they collide with the gas atoms and molecules in the surrounding cloud they push them away, creating this luminous bubble.