Bubble Nebula

NGC 7635 in Cassiopeia is an emission nebula around 7,100 light-years away, famous for the strikingly spherical bubble at its heart — a shell of gas nearly seven light-years across being inflated by the ferocious stellar wind of the massive, hot star SAO 20575. The star, some 45 times the mass of the Sun, sits noticeably off-centre within its bubble because the surrounding molecular cloud is denser on one side, slowing the shell’s expansion unevenly. That same cloud both confines the bubble and glows under the star’s intense ultraviolet radiation, producing the nebula’s characteristic red and blue hues. The central star is burning through its fuel so rapidly that it is expected to end its life in a supernova within the next ten to twenty million years, ultimately shredding the bubble it spent its life blowing.