The distribution of ionized hydrogen (known by astronomers as H II (aitch two) from old spectroscopic terminology) in the parts of the Galactic interstellar medium visible from the Earth's northern hemisphere (from the Wisconsin H-Alpha Mapper Survey)

In astronomy, the interstellar medium (or ISM) is the matter (interstellar matter, also abbreviated by ISM) and energy (interstellar radiation field, ISRF) content that exists between the stars within a galaxy. 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. In turn, 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 ISM consists of an extremely dilute (by terrestrial standards) plasma, gas and dust, consisting of a mixture of ions, atoms, molecules, larger dust grains, electromagnetic radiation, cosmic rays, and magnetic fields. The matter consists of about 99% gas and 1% dust by mass. It fills interstellar space. This mixture is usually extremely tenuous, with typical gas densities ranging from a few hundred to a few hundred million particles per cubic meter. As a result of primordial nucleosynthesis, the gas is roughly 90% hydrogen and 10% helium by number, with additional elements ("metals" in astronomical parlance) present in trace amounts.

Interstellar medium (ISM) phases
Molecular clouds < 1 % 20 - 50103 - 106 hydrogen molecules
Cold Neutral Medium (CNM) 1-5% 50 - 1001 - 103neutral hydrogen atoms
Warm Neutral Medium (WNM) 10-20% 1000 - 500010-1 - 10neutral hydrogen atoms
Warm Ionized Medium (WIM)20-50%103 - 1040.01 ionized hydrogen
H II regions~10% 104102 - 104 ionized hydrogen
Coronal gas
Hot Ionized Medium (HIM)
30-70% 106 - 10710-4 - 10-2highly ionized
(both hydrogen and trace metals)

The medium is also responsible for extinction and reddening, the decreasing light intensity and dominant observable wavelengths of a star as the light travels through the medium. 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.

The interstellar medium is usually divided into three phases, depending on the temperature of the gas: hot (millions of kelvins), warm (thousands of kelvins), and cold (tens of kelvins). These phases are the temperatures where heating and cooling can reach a stable equilibrium. This "three-phase" model of the ISM was initially developed by McKee and Ostriker in a 1977 paper, which has formed the basis for further study over the past quarter-century. The relative proportions of the phases are still a matter of considerable contention in scientific circles.

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


The term "interstellar" appears to be 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).

The nature of the interstellar medium has received the attention of astronomers and scientists over the centuries. Natural philosopher, Robert Boyle summised: "The inter-stellar part of heaven, which several of the modern Epicureans would have to be empty." (1674 Excell. Theol. ii. iv. 178)

In 1862, R. H. Patterson wrote: "This efflux occasions a thrill, or vibratory motion, in the ether which fills the interstellar spaces". (Ess. Hist. & Art 10).

The notion of an "aether" continues into the 20th century where its properties are considered. In 1912, William Henry Pickering wrote: "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..."

In 1913, 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".


  1. Physical Processes in the Interstellar Medium, L. Spitzer, 1978 (New York: Wiley)
  2. Physics of the Interstellar Medium, J. Dyson, 2nd Ed., 1997 (London: Taylor & Francis)
  3. Wisconsin H-Alpha Mapper Survey
  4. Pickering, W. H., "Solar system, the motion of the, relatively to the intersteller absorbing medium" (1912) Monthly Notices of the Royal Astronomical Society, Vol. 72, p.740
  5. Thorndike, S. L., "Interstellar Matter" (1930) Publications of the Astronomical Society of the Pacific, Vol. 42, No. 246, p.99

See also

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