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 Making of the Rotten Egg Nebula
03/09/2001
Fast expanding gas clouds mark the end for a central star in the Rotten Egg Nebula. The once-normal star has run out of nuclear fuel, causing the central regions to contract into a white dwarf. Some of the liberated energy causes the outer envelope of the star to expand. In this case, the result is a photogenic proto- planetary nebula. As the million-kilometer per hour gas rams into the surrounding interstellar gas, a supersonic shock front forms where ionized hydrogen and nitrogen glow blue. The complex shock front had been hypothesized previously but never so clearly imaged. Thick gas and dust hide the dying central star. The Rotten Egg Nebula, also known as the Calabash Nebula and OH231.8+4.2, will likely develop into a full bipolar planetary nebula over the next 1000 years. The nebula, pictured above, is about 1.4 light-years in extent and located about 5000 light-years away toward the constellation of Puppis.