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"
Bright Stars and Dark Clouds Credit:
25/09/1996
Did you ever feel like a black cloud was following you around? Well don't feel bad - this even happened to the bright young stars of the open cluster NGC 6520. On the left are the cluster's bright blue stars. They formed only millions of years ago - much more recently than our ancient Sun which formed billions of years ago. On the right is an absorption nebula from which the stars might have formed. This nebula contains much opaque dust which blocks visible light from the many stars that would have been seen in the background. The study of open clusters is valuable for many reasons which include the understanding star formation and the calibration of the distance scale of our universe. Information: Thursday, September 26 - A Total Lunar Eclipse