The word "nebula," in Latin, means "cloud." Early astronomers believed that nebulae were made up of many dim stars. They tried to separate what they could observe into different stars. This approach worked some of the time, but often they could not distinguish bewtween them. Late in the nineteenth century, the spectroscope (a device for analyzing light) was used to prove that nebulae were not star groups at all.
Nebulae are actually large clouds of gas and dust. There are four types of nebulae: emission, reflective, dark, and planetary. Emission and reflection are sometimes labeled as diffuse nebulae. Emission nebulae are those that are very colorful. They are also known as "bright" nebulae. They lie near stars with a surface temperature of about 13,900 degress Celsius or greater. The immense amount of energy being radiated by these stars excites the atoms of the nebulae, which absorb the UV light. The atoms, in order to reach a lower energy state, must emit this energy in their own form of radiation. We see this as the many wonderful colors that appear. Most emission nebulae contain a lot of red because of the large amounts of hydrogen (the most abundant element) present. They also contain helium, carbon, nitrogen, oxygen, and sulfur. The Great Nebula, also known as M42, is an emission nebula. It is in the constellation Orion.
When these clouds of particles are near a star with a surface temperature less than 13,900° C, the light from the star is reflected by the particles and there exists what is known as a reflective type of nebula. Most nebula are reflective.
Another type, dark nebulae, occur when the particles of the nebula block out light behind it. This creates a dark patch against the sky, where there are apparently no stars to be seen. Two very famous dark nebulae are the Horsehead and the Coalsack, both named for their shapes. The term globules refers to smaller, denser, dark nebulae.
The last type is the planetary nebulae, which appear to be planets but are really a result of a dying star. Also, many supernovae create nebulae. The Crab Nebula is one of these.
Nebula are not very dense. The typical density is only a hundred or less atoms per cubic centimeter. However, these are visible because the atoms, in their excited state, have so much room to move and give off light. Also, many nebulae are several light years across. Atomic collisions are quite rare in nebulae. It is when stars begin to form from the gases that the atoms heat up tremendously and collide more often as their volume is compressed. A nebula exists throughout the Milky Way that is only about one atom per cubic centimeter. It is observed as a dim haze and obscures the center of the galaxy from our sight.
A Gallery of Planetary Nebulae - images of nebulae
Types of Nebulae - pretty, illustrative images of nebulae types
Nebulae - explains different types, black and white images included.