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"
Dumbbell Nebula Close-Up from Hubble
11/02/2003
What causes unusual knots of gas and dust in planetary nebulas? Seen previously in the Ring Nebula, the Helix Nebula, and NGC 2392, the knots' existence was not predicted previously and still not well understood. Pictured above is a newly released image of the Dumbbell Nebula by the Hubble Space Telescope showing details of its gaseous knots. Also visible are many bright young stars and dark sheets of interstellar dust. The Dumbbell Nebula, also known as M27, is a planetary nebula thought similar to what our Sun will become when it runs out of core fuel for nuclear fusion. Recent study of similar cometary knots indicates that they include concentrations of relatively cold molecular gas and that they change significantly as the planetary nebula ages. APOD Update: APOD now also available from Japan in Japanese.