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A nebula is a huge, diffuse cloud of gas and dust in intergalactic space. The gas in nebulae (the plural of nebula) is mostly hydrogen gas (H2).

A stellar nursery is a nebula ( a large cloud of hydrogen gas in space) in which star formation is occurring (stars are formed from gas). These nebulae are frequently illuminated by ultraviolet light which is emitted from the newborn stars. One example of a stellar nursery is the Eagle nebula pictured above.


Eskimo Nebula
The Eskimo Nebula (NGC 2392) in Gemini was first sighted by William Herschel in 1787. The picture above is from the Hubble Space Telescope

Hourglass Nebula
The Hourglass Nebula (NGC 6523) in M8
A planetary nebula is a nebula formed from by a shell of gas which was ejected from a certain kind of extremely hot star (a red giant or supergiant). As the giant star explodes, the core of the star is exposed. Planetary nebulae have nothing to do with planets.

An emission nebula is a nebula that glows (it emits light energy). The reddish light is produced when electrons and protons combine, forming hydrogen atoms. Emission nebulae are formed when energetic ultraviolet light from a very hot star excites a cloud of hydrogen gas; the UV radiation ionizes the hydrogen (it strips electrons from the hydrogen atoms). The free electrons combine with protons, forming hydrogen and red light. The Horsehead Nebula is in front of an emission nebula which illuminates the outline of the "horse head."

A reflection nebula is a nebula that glows as the dust in it reflects the light of nearby stars. These nebulae are frequently bluish in color because blue light is more efficiently reflected than red light. A reflection nebula surrounds the Pleiades Cluster.

A ring nebula is a huge cloud of dust and gas that is shaped like a ring. Pictured to the right is M-57, a ring nebula in Lyra which is 2,000 light-years from Earth (this...

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