The Iris Nebula (NGC 7023) in the constellation Cepheus. It lies 1,300 light-years away, and the region show is about 6 light-years across. Dusty nebular material surrounds a massive, hot, young star. Image credit: Jean-Charles Cuillandre (CFHT).
A gaseous nebula is an H II region, a supernova remnant, or a planetary nebula. H II regions have an emission-line optical spectrum, and a thermal continuous spectrum declining in intensity as the wavelength increases (from maximum in the ultraviolet) through infrared and radio. Supernova remnants have an emission-line optical spectrum and a nonthermal radio spectrum. Temperatures of planetary nebulae are much higher than those of H II regions.