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"
Hot Stars in the Trifid Nebula
21/12/1995
In the center of the glowing red gas on the Trifid Nebula lies an open cluster of young hot stars. The energetic light from these stars strikes hydrogen atoms in the surrounding nebula causing them to lose their electrons. When an electron finds its way back to a hydrogen proton, it emits light at very specific colors - one of which is the red color of the nebula seen here. The red glow is thus indicative of an emission nebula. The dramatic dark sheets are made of interstellar dust grains, tiny needle shaped pellets which are thought to be created and expelled in the atmospheres of cooler stars.