Nebulae
The enigmatic nebulae range from compact planetary forms to disperse gaseous clouds and reflection nebulae. Colors reveal the chemistry of the gases, with red the main emission of ionized H-alpha (656 nm), blues mainly doubly-ionized O (~500 nm), and for reflection nebula blue from light scattering.
Astrophotography by Rex A Parker PhD from New Jersey USA (home astronomy observatory); or from Cerro Tololo Chile (Star Shadows Remote Observatory, CTIO, PROMPT2/UNC-Chapel Hill); or Mt Lemmon Sky Cntr, AZ USA (thanks to A Block).