Planetary nebulae are clouds of glowing gas surrounding a “dying” star. Visually, many are relatively bright grey patches (by deep sky object standard, though very faint compared to an actual planet). Spectrographically, they typically have very strong doubly ionized oxygen (OIII) emissions compared to other nebulae. An OIII filter is a handy tool for the amateur astronomer, enhancing these nebulae and confirming their identity. Though not detectable visually, they also tend to be relatively strong in singly ionized Nitrogen (NII) and singly ionized Helium (denoted HeII, which I may avoid going forward due to the unfortunate confusion).
NGC 6818 aka the little gem is a small bright planetary nebula in sagittarius. Roughly planet sized and near the ecliptic (path of the planets through the sky) one can see why it gets the moniker “planetary”, but at roughly 6,000 light years away it’s 1/2 light year across. The blue/teal in this image is due to OIII emissions and red due to NII emissions.
Here’s the OIII
8" LX200R, SX Trius 694 0.4"/px
astrodon 3nm NII, 3nm OIII
NII 19x20 min OIII 24x5 min