Nebulae is a Greek word for cloud.

Nebulaes are gigantic clouds of interstellar gas and dust in the otherwise mostly empty space between the stars, that may be giving birth to new stars or being produced by a duying star. It may be lit by a nearby starlight, or caused to produced its own light by radiation from nearby stars, or it may be dark. When lit up by the light of nearby stars they form some of the most spectacular sights in our night sky.

Diffuse Nebulae are ones of irregular shape in which stars may be born. The most famous are the Great Nebulae of in Orion and the Lagoon Nebulae.

Emission Nebulae act like neon signs, glowing as ultraviolet light from nearby stars shines on the hydrogen within the nebula, exciting the atoms to give off a reddish light.

Reflection Nebulae glow by virtue of the reflection of light from nearby stars from the dust particles within the clouds. Since blue light is reflected more efficiently, they often appear bluish. Their light is simply light reflected off the nebulae from nearby stars. Some large Nebulaes have both emission and reflection areas.

Planetary Nebulae are named because many of them appear roughly like the outer planets, Uranus and Neptune in telescopes. Roundish, small green or blue and of rather high surface brightness. Such Nebulae are created when material is ejected from a very hot dying "central star". The result of a red giant star which has reached its dying phases, and blown off its outer layers to become a shell of gas surrounding, and most often illuminated by, the dying star. They often appear green, the color given off by hot, energized oxygen.

 

Dark Nebulae are are clouds of gas and dust that neither reflect not emit light. They may be seen by the absence of stars where they lie, or by their dark silhouette against a more distant bright Nebulae. One of the latters is the famous Horsehead Nebulae in Orion.