Planetary nebula
Also called: PN.
A glowing shell of gas thrown off by a dying Sun-like star — nothing to do with planets.
When a star like the Sun runs out of fuel, it swells into a red giant and puffs its outer layers into space. The hot, exposed core (a future white dwarf) makes the shell glow, mostly in the green-blue light of oxygen. The glow lasts only some tens of thousands of years — a blink in a star's life.
William Herschel named them in the 1780s because small ones looked like the disk of a planet in his telescope. They range from tiny bright disks a few arcseconds across to huge faint ones like the Helix.
At the eyepiece
Small planetaries need high power to look like more than a star; an OIII nebula filter makes them pop. Try “blinking”: flick the filter in and out — the nebula stays, the stars dim.
In AstroPilot
AstroPilot suggests high power for small planetaries and low power for large, faint ones like the Helix.