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Key Terms

AstronomyKey Terms

baryon cycle
the cycling of mass in and out of the interstellar medium, including accretion of gas from intergalactic space, loss of gas back into intergalactic space, and conversion of interstellar gas into stars
cosmic rays
atomic nuclei (mostly protons) and electrons that are observed to strike Earth’s atmosphere with exceedingly high energies.
H II region
the region of ionized hydrogen in interstellar space
interstellar dust
tiny solid grains in interstellar space thought to consist of a core of rocklike material (silicates) or graphite surrounded by a mantle of ices; water, methane, and ammonia are probably the most abundant ices
interstellar extinction
the attenuation or absorption of light by dust in the interstellar medium
interstellar medium (ISM)
(or interstellar matter) the gas and dust between the stars in a galaxy
Local Bubble
(or Local Hot Bubble) a region of low-density, million degree gas in which the Sun and solar system are currently located
Local Fluff
a slightly denser cloud inside the Local Bubble, inside which the Sun also lies
molecular cloud
a large, dense, cold interstellar cloud; because of its size and density, this type of cloud can keep ultraviolet radiation from reaching its interior, where molecules are able to form
a cloud of interstellar gas or dust; the term is most often used for clouds that are seen to glow with visible light or infrared
reddening (interstellar)
the reddening of starlight passing through interstellar dust because dust scatters blue light more effectively than red
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