Cassiopeia was the target this session, and specifically the object that gives this nebula its name: a shell of gas nearly seven light-years across, blown outward by a single furious star. NGC 7635 — Caldwell 11 to backyard observers, Sharpless 162 in the catalogues — sits around 7,100 light-years away, close enough for the Dwarf 3 to start resolving real structure in just over two hours of integration from here in Edinburgh.

The bubble itself is the work of SAO 20575, a star roughly 45 times the mass of the Sun, driving a stellar wind so powerful it has hollowed out a near-perfect sphere in the surrounding gas. Look closely, though, and the star isn’t centred in its own creation — it sits noticeably off to one side. The molecular cloud around it is denser on that side, so the shell has been pushing harder and faster into the thinner gas on the other, giving the whole structure a lopsided, almost breathing quality rather than a clean circle.

That same cloud is doing double duty: it confines the bubble, and it glows. SAO 20575 is pouring out ultraviolet radiation, and where it hits the surrounding gas it lights up in the red and blue hues that make this one of the more striking emission nebulae in the northern sky.

It won’t last. A star this massive burns through its fuel fast, and the current thinking is SAO 20575 has somewhere between ten and twenty million years left before it goes supernova — an eyeblink, astronomically speaking. When it does, the explosion will tear through the very bubble the star spent its life inflating. What we’re looking at tonight is a structure actively building toward its own destruction, just on a timescale too slow for any of us to watch it happen.

Worth the 2h 11m. Full details and the 3D distance view are on the gallery card.