A Nebula is an interstellar cloud of dust, hydrogen, helium and other ionized gases.
- It is an Astrographic Feature.
Interstellar clouds are regions of gas, dust, and plasma that have a higher average density than the general interstellar medium. They are typically composed of about 70% hydrogen, the bulk of the remainder being helium with traces of other elements. Interstellar clouds can be classified into 3 general types based on the primary form of hydrogen found within:
- Neutral Clouds (H I regions - formed of atomic hydrogen)
- Ionized Clouds (H II regions - formed of ionized hydrogen [i.e. "Plasma"])
- Molecular Clouds (Molecular Hydrogen - formed of H2)
Under normal circumstances interstellar clouds exist in a state of hydrostatic equilibrium, in which the natural self-gravitation that would otherwise cause the cloud to collapse is balanced by the outward pressure within the cloud due to motion of the atoms and molecules that compose it. Star formation is normally caused within molecular clouds by a triggering event which locally disturbs this equilibrium, causing runaway local collapsing regions within the cloud. Such causes can be the collision of one cloud with another, or the shock-wave within the interstellar medium caused by a nearby supernova explosion.
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History & Background (Dossier)
Nebulae are typically measured in the following units:
- Astrographic hexes
References & Contributors (Sources)
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