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 Close-Up of the Lagoon's Hourglass
21/08/1996
In the central part of the Lagoon Nebula lies the above pictured Hourglass Nebula. In this region of recent star formation, obscuring dark lanes of dust permeate the red-glowing hydrogen gas. Blocking some of the gas cloud from our view, they chance to create a glowing shape that appears from our vantage point like an hourglass. In the upper right of this picture from the Hubble Space Telescope is a bright young blue star from the open cluster NGC 6530 - visible below center in yesterday's APOD. A recent study of the Lagoon Nebula (M8), has shown that this emission nebula houses large magnetic fields and unusually large dust particles.