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igneous rock  Any rock produced by cooling from a molten state. 
inclination (of an orbit)  The angle between the orbital plane of a revolving body and some fundamental plane-usually the plane of the celestial equator or of the ecliptic. 
inertia  The property of matter that requires a force to act on it to change its state of motion; the tendency of objects to continue doing what they are doing in the absence of outside forces.
inertial system  A system of coordinates that is not itself accelerated, but that either is at rest or is moving with constant velocity.
inflationary universe  A theory of cosmology in which the universe is assumed to have undergone a phase of very rapid expansion during the first 10^-30 S. After this period of rapid expansion, the Big Bang and inflationary rdodels are identical. 
infrared cirrus  Patches of interstellar dust, which emit infrared radiation and look like cirrus clouds on the images of the sky produced by the Infrared Astronomy Satellite. 
infrared radiation  Electromagnetic radiation of wavelength longer than the longest (red) wavelengths that can be perceived by the eye, but shorter than radio wavelengths. 
interstellar dust  Tiny solid grains in interstellar space, thought to consist of a core of rock-like 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. 
inverse-square law (for light)  The amount of energy (light) flowing through a given area in a given time (flux) decreases in proportion to the square of the distance from the source of energy or light. 
ion An atom that has become electrically charged by the addition or loss of one or more electrons. 
ionization The process by which an atom gains or loses electrons.
ionosphere The upper region of the Earth's atmosphere in which many of the atoms are ionized.
ion tail (of comet)  See plasma.
irregular galaxy  A galaxy without rotational symmetry; neither a spiral nor an elliptical galaxy. 
isotropic The same in all directions. 
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