This interplay between stars and the ISM helps determine the rate at which a galaxy depletes its gaseous content, and therefore its lifespan of active star formation. Stars form within the densest regions of the ISM, which ultimately contributes to molecular clouds and replenishes the ISM with matter and energy through planetary nebulae, stellar winds, and supernovae. The ISM plays a crucial role in astrophysics precisely because of its intermediate role between stellar and galactic scales. The hydrogen and helium are primarily a result of primordial nucleosynthesis, while the heavier elements in the ISM are mostly a result of enrichment (due to stellar gravity and radiation pressure) in the process of stellar evolution. By mass this amounts to 70% hydrogen, 28% helium, and 1.5% heavier elements. Of the gas in the ISM, by number 91% of atoms are hydrogen and 8.9% are helium, with 0.1% being atoms of elements heavier than hydrogen or helium, known as " metals" in astronomical parlance. By mass, 99% of the ISM is gas in any form, and 1% is dust. Compare this with a number density of roughly 10 19 molecules per cm 3 for air at sea level, and 10 10 molecules per cm 3 (10 billion molecules per cm 3) for a laboratory high-vacuum chamber. In hot, diffuse regions of the ISM, matter is primarily ionized, and the density may be as low as 10 −4 ions per cm 3. In the interstellar medium, matter is primarily in molecular form, and reaches number densities of 10 6 molecules per cm 3 (1 million molecules per cm 3). Magnetic fields and turbulent motions also provide pressure in the ISM, and are typically more important, dynamically, than the thermal pressure is. The thermal pressures of these phases are in rough equilibrium with one another. The interstellar medium is composed, primarily, of hydrogen, followed by helium with trace amounts of carbon, oxygen, and nitrogen. 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 energy that occupies the same volume, in the form of electromagnetic radiation, is the interstellar radiation field. It fills interstellar space and blends smoothly into the surrounding intergalactic space. This matter includes gas in ionic, atomic, and molecular form, as well as dust and cosmic rays. In astronomy, the interstellar medium (ISM) is the matter and radiation that exist in the space between the star systems in a galaxy. The distribution of ionized hydrogen (known by astronomers as H II from old spectroscopic terminology) in the parts of the Galactic interstellar medium visible from the Earth's northern hemisphere as observed with the Wisconsin Hα Mapper ( Haffner et al.
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