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habitable zone (or ecosphere)
the zone around a star in which a planet can maintain liquid water on its surface
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halo
the outer region of a galaxy; contains globular clusters, a few stray stars, and dark matter
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heliacal rising
the period of time when an object, such as a star, is briefly seen in the eastern sky before dawn and is no longer hidden from the glare of the sun
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heliosphere
a vast region around the sun dominated by the solar wind
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helium
the second lightest element; consists of two protons, and usually two neutrons and two electrons; about 8 percent of the atoms in the universe are helium
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hydrazine
a colorless liquid which burns rapidly and is used as a common rocket and missile fuel
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hydrogen
the simplest and lightest element; usually consists of just a single proton and electron; about 90 percent of the atoms in the universe are hydrogen
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hypered film
film that has been treated, usually with gas, to enhance its response to low light levels
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