Nebula: Derived from the Latin word meaning cloud, this is precisely what these objects are. Originally this term was applied to any object that was larger and fuzier than a star. Observation records from a centruy ago referred to galaxies as nebula. Today with improved optics and technology, the term applies to specific objects that are vast clouds of gas and dust that drift among the stars.

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Many of these are dark and invisible and only reveal their presence in that they obscure star fields that lie more distant. The rifts (dark lanes) in galaxies including our own Milky Way galaxy are examples of this type of "Dark Nebula". Other nebula produce spectacular displays of color as hot, active stars cause the gas and dust to flouresce (Emission Nebula) or reflect (Reflection Nebula).

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Nebula are the Universe's stellar nuseries, where new stars are being born. Some nebula (Super Nova remnants and Planetary Nebula) are the remaining evidence of stars that reached the end of their life with violent explosions that expel much of their mass light years into space. These clouds frequently continue to expand at rates of 1,000 - 1,500 miles per second.

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So nebula can mark both the beginning of a stars life and the end. In fact they are the "stuff" of both in a cycle of star death and rebirth. As mentioned above, the clouds can be formed from the remnanats of an exploded star. They can remain in relative equalibrium for long periods of time until a major disturbance (nearby super nova blast), triggers a series of events causing compression of the dust and gas into pockets of greater density. The increased density causes an increase in gravetational pull and a process of accretion begins.

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The colorful light emitted by nebulae are due to specific emission lines caused by the ionization (stripping away of electrons in the valance) of the atoms due to a nearby star's intense ultraviolet radiation. Visible light is emitted as the electrons are recaptured. Hydrogen is the most abundant of the elements in nebula and so these regions are referred to as HII regions. They emit visible light strongly in the red (656 nm) spectrum. Many also cantain oxygen, which emit light in the 496 to 501 nm range, which is seen as green. Some nebula are seen because they scatter (reflect) starlight. These are generally seen as blue due to this refraction. Although nebula look very tangible and somtimes almost dense in appearance, they are usually of a much more tenuous nature typically comprising only a few kilograms of material dispersed over a volume of a thousand million cubic kilometers. On earth we would consider this type of density to be a very sterile vacuum environment.