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"
A Chamaeleon Sky
23/03/1999
A photogenic group of nebulae can be found in Chamaeleon, a constellation visible predominantly in skies south of the Earth's equator. Towards Chamaeleon, dark molecular clouds and bright planetary nebula NGC 3195 can be found. Visible near the center of the above photograph is a reflection nebula surrounding a young bright star. On the lower right, a dark molecular cloud blocks the light from stars behind it. It takes light hundreds of years to reach us from these objects.