Bubble Nebula

A bright nebula in the constellation of Cassiopeia, lying directly to the west of Cassiopeia's 'W' shape, along the band of the Milky Way. The Bubble Nebula lies less than a degree from the open cluster M52 in the sky, but actually these two objects are not related in space. Based on an estimated distance of some 8,800 light years, the Bubble Nebula actually lies more than twice as far from the Sun as the intervening star cluster.

This is an emission nebula, a cloud of ionised hydrogen that is visible because its gases are illuminated by the stars embedded within it. One of these is particularly notable: a highly luminous star designated SAO 20575. The intense stellar wind from this star has created a near-spherical pocket some six light years across within the main nebula. This pocket, with its delicate boundary standing out against the glowing hydrogen clouds, is the source of the Bubble Nebula's name.

There are in fact two nebulae in the sky named 'Bubble Nebula', with the same name being given to a relatively similar object beyond the Milky Way. This second Bubble Nebula is far less prominent than the Bubble Nebula in Cassiopeia, lying within Barnard's Galaxy, a small irregular galaxy in Sagittarius.