The Bubble Nebula, NGC 7635, is an emission nebula roughly 7,100 light-years away in Cassiopeia. Its famous spherical shell is about seven light-years across, but the bubble is only the most dramatic part of a much larger region of glowing hydrogen, oxygen, dust, and cold molecular gas.

The bubble is being carved by BD +60°2522, an extremely hot, massive O-type star. Its stellar wind races outward at more than four million miles per hour, sweeping surrounding material into a thin expanding shock front. Where that fast wind slams into denser interstellar gas, the material is compressed, heated, and made to glow.

The star is noticeably offset from the apparent center of the bubble. That is not a perspective trick: the surrounding cloud is uneven. The expanding wind encounters denser material on one side and is slowed there, while it pushes farther into lower-density gas elsewhere. The result is the remarkably round but textured shell seen in deep images, surrounded by ridges, knots, and ionized gas.

The Bubble Nebula lies inside Cassiopeia, the unmistakable W-shaped constellation of the northern sky. In Greek mythology Cassiopeia was the queen of Ethiopia and the mother of Andromeda. Her boast that she or her daughter surpassed the sea nymphs in beauty angered the gods and set the larger Andromeda legend in motion.

Poseidon sent the sea monster Cetus as punishment, and Andromeda was chained to a rock as a sacrifice before Perseus rescued her. Cassiopeia, Andromeda, Perseus, Cepheus, and Cetus still occupy neighboring regions of the sky, turning a large portion of the autumn heavens into a single connected myth.

The Bubble itself has no ancient mythological name; it is far too faint to have been seen without telescopes. Its modern nickname comes directly from its appearance. In that sense, the object layers a very modern astrophysical story—a massive star literally blowing a cavity into space—inside one of the oldest recognizable constellation figures.