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The interstellar medium (or ISM) is the name astronomers give to the gas and dust that pervade interstellar space. While the interstellar medium refers to the matter that exists between the stars within a galaxy, the energy, in the form of electromagnetic radiation, that occupies the same volume is called the interstellar radiation field.

The interstellar medium consists of an extremely dilute (by terrestrial standards) mixture of ions, atoms, molecules, larger dust grains, cosmic rays, and magnetic fields. The matter consists of about 99% gas and 1% dust by mass. It fills interstellar space, and blends smoothly into the surrounding intergalactic medium. The ISM is usually extremely tenuous, with densities ranging from a few thousand to a few hundred million particles per cubic meter, and an average value in the Milky Way Galaxy of a million particles per cubic meter. As a result of primordial nucleosynthesis, the gas is roughly 90% hydrogen and 10% helium by number of nuclei, with additional heavier elements ("metals" in astronomical parlance) present in trace amounts.

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, molecular clouds, and replenish 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.

The history of knowledge of interstellar space

The nature of the interstellar medium has received the attention of astronomers and scientists over the centuries. However, they first had to acknowledge the basic concept of "interstellar" space. The term appears to have been first used in print by Francis Bacon in 1626 where he wrote: "The Interstellar Skie.. hath .. so much Affinity with the Starre, that there is a Rotation of that, as well as of the Starre." (Sylva §354–5). Later, natural philosopher Robert Boyle surmised: "The inter-stellar part of heaven, which several of the modern Epicureans would have to be empty." (1674 Excell. Theol. ii. iv. 178)

Before modern electromagnetic theory early physicists postulated that an invisible luminiferous aether existed as a medium to carry lightwaves. It was assumed that this aether extended into interstellar space, as R. H. Patterson wrote in 1862, "This efflux occasions a thrill, or vibratory motion, in the ether which fills the interstellar spaces" (Ess. Hist. & Art 10).

The advent of deep photographic imaging allowed Edward Barnard to produce the first images of dark nebulae silhouetted against the background star field of the galaxy while the first actual detection of cold diffuse matter in interstellar space was made by Johannes Hartmann in 1904 through the use of absorption line spectroscopy. In this historic study of the spectrum and orbit of Delta Orionis, Hartmann observed the light coming from this star and realized that some of this light was being absorbed before it reached the Earth. Hartmann reported that absorption from the "K" line of calcium appeared "extraordinarily weak, but almost perfectly sharp" and also reported the "quite surprising result that the calcium line at 393.4 nanometres does not share in the periodic displacements of the lines caused by the orbital motion of the spectroscopic binary star". The stationary nature of the line led Hartmann to conclude that the gas responsible for the absorption was not present in the atmosphere of Delta Orionis, but was instead located within an isolated cloud of matter residing somewhere along the line-of-sight to this star. This discovery launched the study of the ISM.

Following the identification of interstellar calcium absorption by Hartmann, interstellar sodium was detected by Heger in 1919 through the observation of stationary absorption from the atom's "D" lines at 589.0 and 589.6 nanometres towards Delta Orionis and Beta Sco.

Subsequent observations of the "H" and "K" lines of calcium by Beals (1936) revealed double and asymmetric profiles in the spectra of Epsilon and Zeta Orionis. These were the first steps in the study of the very complex interstellar sightline towards Orion. Asymmetric absorption line profiles are the result of the superposition of multiple absorption lines, each corresponding to the same atomic transition (for example the "K" line of calcium), but occurring in interstellar clouds with different radial velocities. Because each cloud has a different velocity (either towards or away from the observer/Earth) the absorption lines occurring within each cloud is either Blue-shifted or Red-shifted (respectively) from the lines rest wavelength through the Doppler Effect. These observations highlight that matter is not distributed homogeneously and were the first evidence for the presence of multiple discrete clouds within the ISM.

The growing evidence for interstellar material led William Henry Pickering to comment in 1912 that "While the interstellar absorbing medium may be simply the ether, yet the character of its selective absorption, as indicated by Kapteyn, is characteristic of a gas, and free gaseous molecules are certainly there, since they are probably constantly being expelled by the Sun and stars..."

The same year Victor Hess's discovery of cosmic rays, highly energetic charged particles that rain down on the Earth from space, led others to speculate whether they also pervaded interstellar space. The following year the Norwegian explorer and physicist Kristian Birkeland wrote: 'It seems to be a natural consequence of our points of view to assume that the whole of space is filled with electrons and flying electric ions of all kinds. We have assumed that each stellar system in evolutions throws off electric corpuscles into space. It does not seem unreasonable therefore to think that the greater part of the material masses in the universe is found, not in the solar [sic] systems or nebulae, but in "empty" space.' (See "Polar Magnetic Phenomena and Terrella Experiments", in The Norwegian Aurora Polaris Expedition 1902–1903 (publ. 1913, p.720).

In 1930, Samuel L. Thorndike notes that ".. it could scarcely have been believed that the enormous gaps between the stars are completely void. Terrestrial aurorae are not improbably excited by charged particles from the Sun emitted by the Sun. If the millions of other stars are also ejecting ions, as is undoubtedly true, no absolute vacuum can exist within the galaxy".

Interstellar matter

The three-phase model

In 1969 Field, Goldsmith, & Habing put forward the static two phase equilibrium model to explain the observed properties of the ISM. Their ISM consisted of a cold dense phase (T < 300 K), consisting of clouds of neutral and molecular hydrogen, and a warm intercloud phase (T ~ 1,000 K), consisting of rarefied neutral and ionized gas. McKee and Ostriker in 1977 added a dynamic third phase that represented the very hot (T = 1,000,000 K) gas which had been shock heated by supernovae, and constituted most of the volume of the ISM. Their paper formed the basis for further study over the past quarter-century. However, the relative proportions of the phases and their subdivisions are still a matter of considerable contention among astronomers.

The following table shows a breakdown of the properties and origin of the components of the three phases.

**Interstellar medium (ISM) phases**
Component	Fractional Volume	Temperature (K)	Density (atoms/cm³)	State
Molecular clouds	< 1%	20 - 50	10³ - 10⁶	hydrogen molecules
Cold Neutral Medium (CNM)	1-5%	50 - 100	1 - 10³	neutral hydrogen atoms
Warm Neutral Medium (WNM)	10-20%	1000 - 5000	10^-1 - 10	neutral hydrogen atoms
Warm Ionized Medium (WIM)	20-50%	10³ - 10⁴	10^-2	ionized hydrogen
H II regions	~10%	10⁴	10² - 10⁴	ionized hydrogen
Coronal gas Hot Ionized Medium (HIM)	30-70%	10⁶ - 10⁷	10^-4 - 10^-2	highly ionized (both hydrogen and trace metals)

Structures

The ISM is turbulent, and so full of structure on all spatial scales. The term interstellar "clouds" is misleading.

Stars are born deep inside large complexes of molecular clouds, typically a few parsecs in size. During their life and even till the very end of their lifecycle, stars interact with the ISM physically.

Particle winds from young clusters of stars (often with giant or supergiant HII regions surrounding them) and shock waves created by supernovae inject enormous energy into their surroundings, which leads to hypersonic turbulence. Resultant structures, of sizes varying from small to large, can be observed using widely different telescopes, such as bubbles of hot gas seen by X-ray satellite telescopes or turbulent eddies observed in radio telescope maps.

Features prominent in the study of the interstellar medium include molecular clouds, interstellar clouds, supernova remnants, planetary nebulae, and other diffuse structures.

Interstellar extinction

The ISM is also responsible for extinction and reddening, the decreasing light intensity and shift in the dominant observable wavelengths of light from a star. These effects are caused by scattering and absorption of photons and allows the ISM to be observed with the naked eye in a dark sky. The rifts that can be seen in the band of the Milky Way are caused by absorption of background starlight from the uniform disk of stars by molecular clouds within a few thousand light years.

Far ultraviolet light is absorbed effectively by the neutral components of the ISM. For example, a typical absorption wavelength of atomic hydrogen lies at about 121.5 nanometers, the Lyman-alpha transition. Therefore, it is nearly impossible to see light emitted at that wavelength from a star farther than a few hundred light years from Earth, because most of it is absorbed during the trip to Earth by intervening neutral hydrogen.

Heating of the interstellar medium

Heating by low-energy cosmic rays

The first mechanism proposed for heating the ISM was heating by low energy cosmic rays. Cosmic rays transfer energy to gas (through both ionization and excitation) and to free electrons through Coulomb interactions. Low energy cosmic rays (a few MeV) are more important because they are far more numerous than high-energy cosmic rays. Cosmic rays are an efficient heating source able to penetrate in the depths of molecular clouds.

Photoelectric heating in grains

The ultraviolet radiation emitted by hot stars can remove electrons from dust grains. The photon hits the dust grain and some of its energy is used in overcoming the potential energy barrier (due to the possible positive charge of the grain) to remove the electron from the grain. The remainder of the photon's energy heats the grain and gives the ejected electron kinetic energy. Since the size distribution of dust grains is:

$n(r)\propto r^{-3.5}$

where r is the size of the dust particle, the grain area distribution is:

$r^{2}n\propto r^{-1.5}$

This indicates that the smallest dust grains dominate this method of heating.

Photoionization

When an electron is freed from an atom (typically from absorption of a UV photon) it carries kinetic energy away of the order: $E_{photon}-E_{ionization}$ . This heating mechanism dominates in HII regions, but is negligible in the diffuse ISM due to the relative lack of neutral carbon atoms.

X-ray heating

X-rays remove electrons from atoms and ions, and those photoelectrons can provoke secondary ionizations. As the intensity is often low, this heating is only efficient in warm, less dense atomic medium (as the column density is small). For example in molecular clouds only hard x-rays can penetrate and x-ray heating can be ignored. This is assuming the region is not near an x-ray source such as a supernova remnant.

Chemical heating

Molecular hydrogen ( $H_{2}$ ) can be formed on the surface of dust grains when 2 H atoms (which can travel over the grain) meet. This process yields 4.48 eV of energy distributed over the rotational and vibrational modes, kinetic energy of the $H_{2}$ molecule, as well as heating the dust grain. This kinetic energy, as well as the energy transferred from de-excitation of the hydrogen molecule through collisions heats the gas.

Grain-gas heating

Collisions at high densities between gas atoms and molecules with dust grains can transfer thermal energy. This is not important in HII regions because UV radiation is more important. It is also not important in diffuse ionized medium due to the low density. In the neutral diffuse medium grains are always colder, but do not effectively cool the gas due to the low densities.

Grain heating by thermal exchange is very important in supernova remnant where densities and temperatures are very high.

Gas heating via grain-gas collisions is dominant deep in giant molecular clouds (especially at high densities). Far infrared radiation penetrates deeply due to the low optical depth. Dust grains are heated via this radiation and can transfer thermal energy during collisions with the gas. A measure of efficiency in the heating is given by the accommodation coefficient:

$\alpha ={\frac {T_{2}-T}{T_{d}-T}}$

where T is the gas temperature, $T_{d}$ the dust temperature, and $T_{2}$ the post-collision temperature of the gas atom/molecule. This coefficient was measure by Burke & Hollenbach (1983) as $\alpha =0.35$ .

Other heating mechanisms

A variety of macroscopic heating mechanisms are present including:

Gravitational collapse of a cloud
Supernova explosions
Stellar winds
Expansion of HII regions
Magnetohydrodynamic waves created by supernova remnants

Cooling of the interstellar medium

Fine structure cooling

This process is dominant in most regions of the ISM, except regions of hot gas and regions deep in molecular clouds. This occurs most efficiently with abundant atoms having fine structure levels close to the fundamental level such as: CII and OI in the neutral medium and OII, OIII, NII, NIII, NeII and NeIII in HII regions. Collisions will excite these atoms to upper levels, which will eventually de-excite through photon emission, which will carry the energy out of the region.

Cooling by permitted lines

At higher temperature more levels than fine structure levels can be populated via collisions. For example collisional excitation of the n=2 level of hydrogen will release a Ly $\alpha$ photon upon de-excitation. In molecular clouds excitation of rotational lines of CO is important.

References

Burke J.R. and Hollenbach D.J. (1983). "The gas-grain interaction in the interstellar medium - Thermal accommodation and trapping". Astrophysical Journal. 265: 223. (NASA Astronomy Abstract Service Entry)
Dyson, J. (1997) Physics of the Interstellar Medium, London: Taylor & Francis
Field, G. B., Goldsmith, D. W., & Habing, H. J. (2001). "Cosmic-Ray Heating of the Interstellar Gas". Astrophysical Journal. 155: L149.{{cite journal}}: CS1 maint: multiple names: authors list (link) (NASA Astronomy Abstract Service Entry)
Ferriere, K. (2001). "The Interstellar Environment of our Galaxy". Reviews of Modern Physics. 73 (4): 1031–1066. (astro-ph preprint)
Lequeux J. The Interstellar Medium. Springer 2005.
McKee, C. F. and Ostriker, J. P. (1977). "A theory of the interstellar medium - Three components regulated by supernova explosions in an inhomogeneous substrate". Astrophysical Journal. 218: 148.{{cite journal}}: CS1 maint: multiple names: authors list (link) (NASA Astronomy Abstract Service Entry)
Pickering, W. H. (1912). "The Motion of the Solar System relatively to the Interstellar Absorbing Medium". Monthly Notices of the Royal Astronomical Society. 72: 740. (NASA Astronomy Abstract Service Entry)
Thorndike, S. L. (1930). "Interstellar Matter". Publications of the Astronomical Society of the Pacific. 42 (246): 99. (PASP online copy)
Spitzer, L. (1978) Physical Processes in the Interstellar Medium New York: Wiley.
Wisconsin H-Alpha Mapper Survey

External links

Freeview Video 'Chemistry of Interstellar Space' William Klemperer, Havard University. A Royal Institution Discourse by the Vega Science Trust.

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 The following table shows a breakdown of the properties and origin of the components of the three phases.
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