The expanse of bright gas known as the nebula of the bubble, excited by a hot star inside, image in the dense Milky Way Cassiopeia. Credit: Adam Block
The extreme northern sky holds its richest region of Cassiopeia, a splendid area of brilliant stars and milk gas. One of the largest nebulae in the far north is NGC 7635, the bubble nebula, so called for its remarkable spherical bubble visible in the images.
This object is a nebula of emission with a reasonably high surface brightness, which makes it visible in areas of moderate dimensions under a dark sky.
The bubble lights up at about 10th magnitude and extends 15 ‘per 8’. It is 8,000 light years away.
The weak bubble that gives this object its popular name, a stimulating part of the brightest nebula, is illuminated by a hot star and magnitude 8.7: Sao 20575, a young sun whose fierce radiation is ionizing the bubble.
The surrounding field is rich: the neighbors find open clusters (M52 and others) and a series of nebulae including NGC 7538, Sharpless 2–157 and IC 1470.
