A Planetary Nebula (PN or PNe) is ionized gas around an Asymptotic Giant Branch star. The term stems from the era of their first observation since given the observation methods of the time, they resembled planets.
They are presumed to live on the order of ten thousand years.
Vorontsov-Velyaminov Planetary Nebula Types:
MASH Catalog of Planetary Nebulae
Catalogue of Galactic Planetary Nebulae (PK)
Planetary Nebula Luminosity Function (PNLF)
Protoplanetary Nebula (PPN)
Strasbourg-ESO Catalog of Galactic Planetary Nebulae (PNG)