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"
The Red Rectangle
02/11/1995
The unusual geometry of this stellar nebula creates somewhat of a mystery. At the nebula's center is a young binary star system that probably created the nebula, but how? This type of nebula shows a "bipolar flow" which carries a significant amount of mass away from the central stars. It has been speculated that the central stars create a pair of jets that precess like a spinning top. These jets might throw gas into a thick disk which we see here edge on - so that it appears to us as a rectangle. The nebula emission is also unusual in that some of the infrared light it emits might be associated with unusual carbon-containing molecules.