Nebula
A nebula is a giant cloud of gas (mostly hydrogen and helium) and cosmic dust situated between stars in the interstellar medium. Nebulae serve as sites of stellar birth and death—including emission nebulae that glow from ionized gas, reflection nebulae that scatter starlight, and dark nebulae that obscure background stars.
Source: science.nasa.gov
APODs including "Nebula"
IC 59 and IC 63 in Cassiopeia
03/11/2011
These bright rims and flowing shapes suggest to some melting ice cream on a cosmic scale. Looking toward the constellation Cassiopeia, the colorful (zoomable) skyscape features the swept back, comet-shaped clouds IC 59 (left) and IC 63. About 600 light-years distant, the clouds aren't actually melting, but they are slowly dissipating under the influence of ionizing ultraviolet radiation from hot,luminous star gamma Cas. Gamma Cas is physically located only 3 to 4 light-years from the nebulae, just off the upper right edge of the frame. In fact, slightly closer to gamma Cas, IC 63 is dominated by red H-alpha light emitted as the ionized hydrogen atoms recombine with electrons. Farther from the star, IC 59 shows proportionally less H-alpha emission but more of the characteristic blue tint of dust reflected star light. The field of view spans about 1 degree or 10 light-years at the estimated distance of gamma Cas and friends.