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.
Other articles related to "interstellar, interstellar medium":
... abundances, chemical composition, and temperatures of stars and interstellar clouds ... Interstellar formaldehyde was the first polyatomic organic molecule detected in the interstellar medium ... chemical species is radio astronomy, which has resulted in the detection of over a hundred interstellar species, including radicals and ions, and organic (i.e ...
... Hot Neptunes / Super-Earths Transiting planets Rogue / Interstellar planets Hypothetical planet types Chthonian planets Ocean planets Trojan planets ...
Famous quotes containing the words ism and/or medium:
“Grandpa: Penny, why dont you write a play about ism-mania?
Grandpa: Yeah, sure. You know, communism, fascism, voodooism. Everybodys got an ism these days.
Penny: It sounds like an itch or something.
Grandpa: Its just as catching. When things go a little bad these days, you go out and get yourself an ism and youre in business.”
—Robert Riskin (18971955)
“As a medium of exchange,... worrying regulates intimacy, and it is often an appropriate response to ordinary demands that begin to feel excessive. But from a modernized Freudian view, worryingas a reflex response to demandnever puts the self or the objects of its interest into question, and that is precisely its function in psychic life. It domesticates self-doubt.”
—Adam Phillips, British child psychoanalyst. Worrying and Its Discontents, in On Kissing, Tickling, and Being Bored, p. 58, Harvard University Press (1993)