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radial velocity
the velocity of an object toward or away from an observer
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radiant
1. the point in the sky from which the meteors belonging to a meteor shower appear to originate
2. vividly bright and shining
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radiation
electromagnetic waves (in astronomical usage)
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radiation pressure
a very small amount of pressure exerted on a surface by light or other electromagnetic radiation
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radio galaxy
a galaxy that emits an unusually large amount of radio waves
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radio telescope
a telescope designed to detect radio waves coming from space
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radio waves
the form of light with the longest wavelength and the least energy
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radiometer
a device that measures the total energy or power from an object in the form of radiation, especially infrared radiation
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red dwarf
a low-mass, main-sequence star much smaller, cooler, and less luminous than the sun
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red giant
a cool star near the end of its life cycle that has expanded to a size of a few dozen to a hundred times the diameter of the sun
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red supergiant
a cool, massive star near the end of its life that has expanded to a size from a hundred to a thousand times the diameter of the sun
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redshift
an increase in the wavelength of light coming from an object due to its motion away from Earth, the expansion of the universe, or a strong gravitational field
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reflection nebula
a cloud of gas and dust that is visible because the dust reflects a nearby star’s light
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reflector
a telescope that uses a curved mirror to gather light
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refractor
a telescope that uses a glass lens to gather light
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regolith
the unconsolidated residual or transported rock and soil that overlies solid bedrock on Earth, the moon, or another planet
the powdery soil of the moon produced by meteorite impacts
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relativity
the theories of physics developed by Albert Einstein that describe measurements made by two observers who are in relative motion
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resolution
the ability of a telescope or camera to pick out fine detail
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resolving power
the ability of a telescope or camera to pick out fine detail
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reticule
a grid or pattern of two or more fine wires set inthe focal plane of a telescope eyepiece and used in determining the position and/or size of a celestial object
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retrograde
objects that move or appear to move in the opposite direction of most solar system bodies; for example planets that appear to move east-to-west in the sky or objects that revolve or rotate clockwise as seen from north of the solar system
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reusable launch vehicle (RLV)
a single-stage-to-orbit spacecraft that may be reused on successive missions
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revolution
the orbital motion of one body around another body or a common center of mass
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ribonucleic acid
a nucleic acid that transmits genetic information
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rich clusters
large galaxy clusters with unusually high population densities
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rich-field telescope
a telescope designed to show a large field of view at low magnification
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right ascension
the angular distance of a celestial object east of the vernal equinox; the celestial sphere equivalent of longitude
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rotation
the spin of a galaxy, star, planet, moon, or asteroid about a central axis
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rotation period
the length of time it takes a body to complete one rotation
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