In simple terms, a cloud of interstellar gas and dust. Though they can exist in many
forms, nebulae tend to be associated with the beginnings and endings of stars.
Nebulae form the raw material for the formation of stars, but are also created by material
'cast off' by stars in their final stages of life.
A typical Reflection Nebula - clouds of interstellar material are illuminated by the
two brightstars in the foreground.
