image The representation of an object produced when light from the object is reflected or refracted by a mirror or a lens.
inertia The tendency of an object to continue in motion at the same speed and in the same direction, unless acted upon by a force.
inflation Short period of extremely rapid cosmic expansion early in the history of the universe. During inflation, the universe swelled in size by a factor of 10 to the 50th power.
infrared telescope Telescopes designed to detect infrared radiation.
intensity A basic property of electromagnetic radiation that specifies the amount or strength of the radiation flowing in a specific direction.
interference The ability of two or more waves to interact in such a way that they either reinforce or cancel each other.
interferometer Collection of two or more telescopes working together as a team, observing the same object at the same time and at the same wavelength. The effective diameter of an interferometer is equal to the distance between its outermost dishes.
interferometry Technique in widespread use to dramatically improve the resolution of telescopes, especially radio telescopes. Several radio telescopes observe the object simultaneously, and a computer analyzes how the signals interfere with each other.
intergalactic matter Matter in the space between galaxies.
intergalactic space The space between galaxies.
interplanetary matter Matter in the space between planets.
interplanetary space The space between planets.
interstellar dust Dust particles in the space between the stars. Sizes vary over a wide range but are typically about one micrometer, comparable to the wavelength of visible light
interstellar matter (or medium) Interstellar gas and dust.
interstellar space The space between stars.
inverse-square law The law that a field follows if its strength decreases with the square of the distance. Fields that follow the inverse square law rapidly decrease in strength as the distance increases, but never quite reach zero.
ion An atom with one or more electrons removed (or added), giving it a positive (or negative) charge.
ionization The process by which ions are produced, typically by collisions of electrons, ions, or photons.
irregular galaxy A strangely shaped galaxy, often rich in interstellar matter, but apparently not a member of any of the major classes of spiral or elliptical galaxies.
isotopes Nuclei containing the same number of protons but different numbers of neutrons. Most elements can exist in several isotopic forms. A common example of an isotope is deuterium, which differs from normal hydrogen by the presence of an extra neutron in the nucleus.
isotropy Assumed property of the universe such that the universe looks the same way in every direction.