A nebula is simply a large cloud of interstellar gas, dust and plasma. Often seen in the dark night sky as beautiful bright bursts or color, nebulas are created by both the formation and destruction of stars.
Nebulas are generally classified according to the degree of luminosity. The most common types of nebulas include planetary nebulas and diffuse nebulas. Diffuse nebulas consist of reflecting nebulas, emission nebulas, and dark nebulas. Ruminants from celestial supernova explosions are also often classified as nebulas.
Planetary nebulas, which are actually composed of material left over from the giant red stage of star formation, most commonly resemble planets. There are several Planetary Nebulas that exist in our own Milky Way Galaxy.
H II regions, which are a common type of emission nebulae, are actually the birth place of many stars. This occurs when diffused molecular clouds begin to collapse under their own gravity, most often due to the influence of a supernova explosion nearby. After the cloud collapses and fragments, hundreds of new stars can be formed.
Prior to the invention of research tools like the telescope, the term nebula was used to describe any celestial object with a diffused appearance, as a result many objects now known to be star clusters or galaxies were originally classified as nebulas.
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