Interstellar Medium

In astronomy, the interstellar medium (or ISM) is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, 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 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 dilute 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. 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 1022 molecules per cm3 for liquid water. By mass, 99% of the ISM is gas in any form, and 1% is dust. Of the gas in the ISM, 89% of atoms are hydrogen and 9% are helium, with 2% of atoms being elements heavier than hydrogen or helium, which are called "metals" in astronomical parlance. The hydrogen and helium are a result of primordial nucleosynthesis, while the heavier elements in the ISM are 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, 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.

Read more about Interstellar MediumInterstellar Matter, Heating and Cooling, The History of Knowledge of Interstellar Space

Other articles related to "interstellar, interstellar medium":

Chandra Wickramasinghe
... Their joint work on the infrared spectra of interstellar grains led to developing the hypothesis of panspermia ... It proposes that cosmic dust in the interstellar medium and in comets is partly organic, and that life on Earth was 'seeded' from space rather than arising through abiogenesis ... of spectral features in comets and in the interstellar medium with degraded ...
Circular Polarization - Starlight
... partially linearly polarized by scattering from elongated interstellar dust grains whose long axes tend to be oriented perpendicular to the galactic magnetic field ... The explanation is that the interstellar medium is optically thin ... a starlight photon is scattered from a single interstellar grain multiple scattering (which produces circular polarization) is much less likely ...
Cosmic Dust - Stardust
... They differ greatly from the solids formed at low temperature within the interstellar medium ... which are expected to exist nowhere in the interstellar medium ... the isotopes could be diluted by mixing with the interstellar medium ...
Bow Shock Theory For Earth's Sun
... the Sun also has a bow shock as it travels through the interstellar medium ... This will occur if the interstellar medium is moving supersonically towards the Sun, since the sun's solar wind is moving supersonically away from the Sun ... The point where the interstellar medium becomes subsonic is the bow shock the point where the interstellar medium and solar wind pressures balance is at ...
Interstellar Medium - The History of Knowledge of Interstellar Space
... The nature of the interstellar medium has received the attention of astronomers and scientists over the centuries, and understanding of the ISM has developed ... However, they first had to acknowledge the basic concept of "interstellar" space ... term appears to have been first used in print by Bacon (1626, § 354–5) "The Interstellar Skie. ...

Famous quotes containing the word medium:

    Whenever, therefore, people are deceived and form opinions wide of the truth, it is clear that the error has slid into their minds through the medium of certain resemblances to that truth.
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