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"
M17: The Omega Nebula
26/01/1999
The Omega Nebula contains glowing gas, dark dust, and some unusually massive stars. Also known as the M17 and the Swan Nebula, the Omega Nebula is about 5000 light-years away, 20 light-years across, and visible with binoculars in the constellation of Sagittarius. A recent epoch of star formation has created some very massive stars that haven't yet had time to self-destruct. Until then, these stars appear very bright and emit light so energetic it breaks up the surrounding gas and dust. The streaks across the bright stars are blemishes caused by the digital camera.