Most bubbles in space are interstellar gas pushed aside by a star. This one is different: the shell is the star’s own material. EZ Canis Majoris, the bright point near the top, is a Wolf-Rayet star that has already shed its outer layers through an enormous stellar wind, and the faster wind of its current phase is now catching up and slamming into that older debris from inside. The colour comes almost entirely from oxygen, because the hydrogen went out with the layers that were lost. Around 5,000 light-years away in Canis Major. Stars in this state don’t last long, and when this one goes, the supernova will hit the shell you’re looking at from the inside out.
