Interstellar medium
= Interstellar medium
{title2=ISM}
{wiki}
The interstellar medium is the <gas>, <interstellar dust>, <magnetic field> and <cosmic rays> between <stars> in a <galaxy>. It is a changing reservoir: <molecular clouds> supply <star formation>, while <stellar winds> and <supernova remnants> return material and stir or heat it. Different regions occupy different <temperature> and <ionization> states, so no single observation measures the whole medium. For example, the <neutral-hydrogen 21-centimeter line> probes neutral atomic <hydrogen>, whereas <X-rays> reveal much hotter <plasma>. Even neutral atomic <gas> can occupy cold and warm phases: the <heating and cooling balance in the interstellar medium> permits both over a restricted range of <pressure>.<The Neutral Atomic Phases of the ISM in the Galaxy by Mark G. Wolfire and others>[Abstract: cold and warm neutral phases, thermal balance and the pressure range for coexistence]{ref}