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Written by Rob Lambert
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A simple word that confounds even experienced amateur astronomers in how it is pronounced, nebulae is the plural form of nebula. Nebulae is pronounced "NEB-yu-lee". A nebula (Latin for "mist") is an interstellar cloud of dust, gas, and plasma. Originally nebula was a general name for any extended astronomical object, including galaxies beyond the Milky Way. (Some examples of the older usage still survive; for example, the Andromeda Galaxy is sometimes referred to as the Andromeda Nebula). There are several types of nebulae that we encounter in our search of the night sky.
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Reflection nebulae are clouds of space gas and dust that reflect light from a bright star. They are typically blue in astrophotography. A good example of a reflection nebula is found among the stars of M45, the Pleiades cluster, and is known as NGC 1435.
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Emission nebulae are space clouds that glow or emit light on their own. Atoms of hydrogen gas within an emission nebula are excited by powerful ultraviolet light emitted by nearby hot bright stars. The atoms absorb the UV energy and then re-emit that energy at longer wavelengths that are visible to the human eye. Typically, emission nebulae glow red or green. M42 in Orion is probably the most recognized emission nebula.
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Dark nebulae are non-illuminated clouds that are so dense they block the light from stars behind them. They are typically more difficult to locate and identify. The Horsehead nebula is probably the most famous of the dark nebulae. |
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Planetary nebulae are compact shells of gas and matter surrounding a dead star or an intermittently active star. The glowing gas and dust of planetary nebulae are cast off by medium-size stars like our Sun. Dying stars puff off their outer layers until their white hot cores are exposed, which then cause the cast-off gas and dust to glow. The Ring Nebula, M57, and the Dumbell Nebula, M27 are examples of planetary nebulae. |
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