<Tr> <Td> <Ul> <Li> </Li> <Li> </Li> <Li> </Li> </Ul> </Td> </Tr> <Ul> <Li> </Li> <Li> </Li> <Li> </Li> </Ul> <P> In astronomy, the interstellar medium (ISM) is the matter and radiation that exists in the space between the star systems in a galaxy . This matter includes gas in ionic, atomic, and molecular form, as well as dust and cosmic rays . It fills interstellar space and blends smoothly into the surrounding intergalactic space . The energy that occupies the same volume, in the form of electromagnetic radiation, is the interstellar radiation field . </P> <P> The interstellar medium is composed of multiple phases, distinguished by whether matter is ionic, atomic, or molecular, and the temperature and density of the matter . The interstellar medium is composed primarily of hydrogen followed by helium with trace amounts of carbon, oxygen, and nitrogen comparatively to hydrogen . The thermal pressures of these phases are in rough equilibrium with one another . Magnetic fields and turbulent motions also provide pressure in the ISM, and are typically more important dynamically than the thermal pressure is . </P>

What do we mean by the interstellar medium
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