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interstellar medium
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File:WHAM survey.png|thumb|medium|right|300px|The distribution of ionized hydrogenionized hydrogenIn 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.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.JOURNAL, Herbst, Eric, 1995, Chemistry in The Interstellar Medium, Annual Review of Physical Chemistry, 46, 27–54, 1995ARPC...46...27H, 10.1146/annurev.pc.46.100195.000331, 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.In all phases, the interstellar medium is extremely tenuous by terrestrial standards. In cool, dense regions of the ISM, matter is primarily in molecular form, and reaches number densities of 106 molecules per cm3 (1 million molecules per cm3). In hot, diffuse regions of the ISM, matter is primarily ionized, and the density may be as low as 10−4 ions per cm3. Compare this with a number density of roughly 1019 molecules per cm3 for air at sea level, and 1010 molecules per cm3 (10 billion molecules per cm3) for a laboratory high-vacuum chamber. By mass, 99% of the ISM is gas in any form, and 1% is dust.CONFERENCE, Boulanger, F., Cox, P., Jones, A. P., Course 7: Dust in the Interstellar Medium, Infrared Space Astronomy, Today and Tomorrow, 2000, F. Casoli, J. Lequeux, F. David, 251, 2000isat.conf..251B, 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,Ferriere (2001) known as "metals" in astronomical parlance. By mass this amounts to 70% hydrogen, 28% helium, and 1.5% heavier elements. The hydrogen and helium are primarily a result of primordial nucleosynthesis, while the heavier elements in the ISM are mostly a result of enrichment in the process of stellar evolution.The ISM plays a crucial role in astrophysics precisely because of its intermediate role between stellar and galactic scales. 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. 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.Voyager 1 reached the ISM on August 25, 2012, making it the first artificial object from Earth to do so. Interstellar plasma and dust will be studied until the mission's end in 2025. Its twin, Voyager 2 entered the ISM in November 2018.File:Voyager.jpg|thumb|right|200px|Voyager 1Voyager 1

Interstellar matter

Table 1 shows a breakdown of the properties of the components of the ISM of the Milky Way.{|class="wikitable"|+ Table 1: Components of the interstellar medium align=center bgcolor=#eeeeee!Component||Fractional volume||Scale height(pc)||Temperature(K)||Density(particles/cm3)||State of hydrogen || Primary observational techniques
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Molecular clouds>Radio astronomy>Radio and infrared molecular emission and absorption lines
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Hydrogen line>H I 21 cm line absorption
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Hydrogen line>H I 21 cm line emission
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Hα emission and Dispersion (optics)#Dispersion in pulsar timing>pulsar dispersion
align=center< 1%
H II region>H II regions 70 8000 102–104 ionized Hα emission and pulsar dispersion
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Galactic corona>Coronal gasHot Ionized Medium (HIM)30–70% 1000–3000 106–107 10−4–10−2 ionized(metals also highly ionized) X-ray astronomy emission; absorption lines of highly ionized metals, primarily in the Ultraviolet astronomy>ultraviolet

The three-phase model

{{harvtxt|Field|Goldsmith|Habing|1969}} put forward the static two phase equilibrium model to explain the observed properties of the ISM. Their modeled ISM consisted of a cold dense phase (T 

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