Planetary Nebula란 무엇인가?
A planetary nebula is the glowing shell of gas a Sun-like star exhales at the end of its life, set alight by the ultraviolet radiation of the naked core it has just exposed. It has nothing whatsoever to do with planets — and it lasts only about twenty thousand years, an eyeblink in stellar terms.
The name is an eighteenth-century mistake that stuck. William Herschel, sweeping the sky in the 1780s, saw small round greenish disks that looked like the planet he had just discovered, and called them planetary. In fact each one is the outer envelope of a star of up to about eight solar masses, puffed off during its unstable asymptotic-giant-branch phase and now expanding at a few tens of kilometres per second.
What makes them glow is the core. Once the envelope drifts away, the exposed remnant is a 100,000-kelvin ember pouring out ultraviolet light. That radiation ionises the surrounding gas, which then re-emits at very specific wavelengths — the red of hydrogen-alpha, the vivid green-teal of doubly ionised oxygen that so confused early observers that they briefly attributed it to a new element, nebulium.
They are also short-lived and structurally strange. The Ring Nebula in Lyra, the Cat's Eye in Draco with its nested shells, the Helix with its thousands of cometary knots — none of them are the simple spheres the physics naively predicts. Binary companions, magnetic fields and earlier bouts of mass loss sculpt them into bipolar lobes, jets and rings. In roughly five billion years the Sun will make one of its own, and the white dwarf at the centre of this model is what will remain when it fades.