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.
|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)
AND STAR CLUSTERS