nebula: A cloud of gas or dust in a
galaxy which can be detected optically.
Opaque clouds of dust that obscure
light from distant objects are called
dark nebulae, while those that reflect
light are called reflection nebulae.
Two types of gaseous nebulae are: 
 emission nebulae: These nebulae
glow from the radiation of hot stars
inside them. Because the stars are
usually quite young, emission
nebulae are often thought to be
stellar nurseries. Many emission
nebulae are characterized by a reddish
color caused by the presence of
ionized hydrogen gas.