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dark adaptation
the process by which the human eye becomes well adjusted to seeing dim objects in the dark.
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dark energy
a type of "negative gravity" that seems to play a role in the acceeleration of universal expansion
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dark matter
matter that exerts gravitational force but does not emit any detectable light or radiation; dark matter comprises most of the mass of the universe but its exact nature remains unknown
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dark nebula
a cloud of dust grains that is thick enough to obscure the light from background stars
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declination
the angular distance of a celestial object above or below the celestial equator; the celestial sphere equivalent of latitude
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deep-sky objects
objects located beyond the solar system; consist of stars, nebulae, star clusters, and galaxies.
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degree
1. a unit of angular size equal to 1/360 the circumference of the celestial sphere; the sun and full moon both appear about half a degree wide
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density
the amount of mass per unit volume of an object or region of space
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deuterium
an isotope of hydrogen; its nucleus, consisting of one proton and one neutron, has double the mass of the nucleus of ordinary hydrogen
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diffraction
the spreading out of light as it passes the edge of an obstacle
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dust
tiny particles floating in space
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dwarf galaxy
a small galaxy containing a few million stars; the most common type of galaxy in the universe
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dwarf star
a main-sequence or smaller star
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