Content of the region between the stars, including vast, diffuse clouds of gases and minute solid particles. Such tenuous matter in the Milky Way Galaxy accounts for about 5percnt of its total mass. By no means a complete vacuum, the interstellar medium contains mainly hydrogen gas, with a smaller amount of helium and sizable quantities of dust particles of uncertain composition. Primary cosmic rays also travel through interstellar space, and magnetic fields extend across much of it. Most interstellar matter occurs in cloudlike concentrations, which can condense to form stars. Stars, in turn, continually lose mass through stellar winds (see solar wind). Supernovas and planetary nebulae also feed mass back to the interstellar medium, where it mixes with matter that has not yet formed stars (see Populations I and II).
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Substance comparatively opaque to X-rays, which appears lighter on X-ray film and allows a body structure that does not normally contrast with its background to be seen clearly on the film. Common contrast media include barium sulfate and iodized organic compounds. They are given by the route that introduces them into the structure to be examined—swallowed or as an enema for the digestive tract, inhaled for the respiratory tract, or injected for blood vessels and for organs and tissues they supply. Serious reactions to contrast media are not infrequent. Seealso diagnostic imaging.
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