planetary nebula
/ NEB-yoo-luh /
Some of the most beautiful objects in the sky are glowing rings, butterflies, and bubbles of colored gas, like the famous Ring Nebula or the Cat's Eye. These are planetary nebulae — the exhaled outer layers of a dying low-mass star, lit up from within and drifting away into space. The name is a 200-year-old misnomer: through small early telescopes their round, soft disks looked vaguely like planets. They have nothing to do with planets at all.
A planetary nebula is the end of an AGB star's story. After shedding most of its envelope in powerful winds, the star exposes its hot, dead core. That core is so blisteringly hot — over 100,000 kelvin — that it floods the surrounding ejected gas with ultraviolet light, which makes the gas glow in the rich colors we see, much as electricity makes a neon sign shine. The glowing shell expands at tens of kilometers per second and is typically about a light-year across. The whole spectacle is fleeting by cosmic standards, lasting only about 10,000 to 20,000 years before the gas thins out and fades.
Planetary nebulae are how Sun-like stars die and give back. The expanding shell carries carbon, nitrogen, and other elements made and dredged up during the star's giant phases out into the interstellar medium, seeding the next generation of stars and planets. At the center, the bare core is left behind as a white dwarf, slowly cooling for eternity. This is the Sun's own future: in roughly 5 billion years it will puff off its outer layers as a planetary nebula and settle down as a white dwarf — a gentle death, utterly unlike a supernova.
The Ring Nebula in Lyra is a smoke-ring of glowing gas about a light-year wide, exhaled by a dying star whose tiny, fiercely hot core sits glowing white at the very center. In a few billion years the Sun will produce a similar, if humbler, ring of its own.
A planetary nebula is a Sun-like star's exhaled outer layers, lit by its exposed hot core.
Nothing to do with planets, and nothing to do with supernovae. A planetary nebula is the gentle death of a low-mass star — gas slowly puffed off, not violently blown apart. Only stars below about 8 solar masses end this way.