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 Beautiful Trifid
26/05/2005
The beautiful Trifid Nebula (aka M20), a photogenic study in cosmic contrasts, lies about 5,000 light-years away toward the nebula rich constellation Sagittarius. A star forming region in the plane of our galaxy, the Trifid fittingly illustrates three basic types of astronomical nebulae; red emission nebulae dominated by light from hydrogen atoms, blue reflection nebulae produced by dust reflecting starlight, and dark absorption nebulae where dense dust clouds appear in silhouette. The bright red emission nebula, roughly separated into three parts by obscuring dust lanes, lends the Trifid its popular name. In this gorgeous wide view, the red emission region is also surrounded by the telltale blue haze of reflection nebulae. Light-year long pillars and jets sculpted by newborn stars - visible here below the center of the emission nebula - appear in Hubble Space Telescope close-up images of the region.