# 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](https://astropilot.space/glossary/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.

**Examples:** [M57](https://astropilot.space/object/M57.md), [M27](https://astropilot.space/object/M27.md), [NGC7293](https://astropilot.space/object/NGC7293.md), [NGC6543](https://astropilot.space/object/NGC6543.md)

**Related:** [Nebula](https://astropilot.space/glossary/nebula.md), [Nebula filter](https://astropilot.space/glossary/nebula-filter.md), [Supernova remnant](https://astropilot.space/glossary/supernova-remnant.md), [Angular size](https://astropilot.space/glossary/angular-size.md)

From the AstroPilot astronomy glossary: https://astropilot.space/glossary/planetary-nebula