The Bubble Nebula (NGC 7635) lies in Cassiopeia, about 8,000 light years away. The near-perfect sphere that gives it its name is seven light years across, hollowed out of the surrounding gas by the stellar wind of SAO 20575, a star some forty times the mass of the Sun.
It is not a soap bubble: it is a shock front. The star's wind, blowing at millions of kilometres per hour, pushes the gas outwards until it meets the denser molecular cloud, which slows and compresses it — and that is where the shell lights up. The star is not at the centre of the bubble precisely because the cloud is not uniform.
In the same field, lower down, stretches Sh2-157, the Lobster Claw Nebula, with its two curving arms; and between them sits the open cluster M52.
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