Editing Interstellar medium (section)

{{short description|Matter and radiation in the space between the star systems in a galaxy}}
[[File:WHAM survey.png|thumb|right|300px|The distribution of [[Plasma (physics)|ionized hydrogen]] (known by astronomers as H&nbsp;II from old spectroscopic terminology) in the parts of the Galactic interstellar medium visible from the Earth's northern hemisphere as observed with the Wisconsin Hα Mapper {{harvard citation|Haffner|Reynolds|Tufte|Madsen|2003}}.]]
The '''interstellar medium''' ('''ISM''') is the [[matter]] and radiation that exists in the [[outer space|space]] between the [[star system]]s in a [[galaxy]]. This matter includes [[gas]] in [[ion]]ic, [[atom]]ic, and [[molecular]] form, as well as [[cosmic dust|dust]] and [[cosmic ray]]s. It fills [[interstellar space]] and blends smoothly into the surrounding [[intergalactic medium]]. The [[energy]] that occupies the same volume, in the form of [[electromagnetic radiation]], is the '''interstellar radiation field'''. Although the [[density]] of atoms in the ISM is usually far below that in the best laboratory vacuums, the [[mean free path]] between collisions is short compared to typical interstellar lengths, so on these scales the ISM behaves as a gas (more precisely, as a [[Plasma (physics)|plasma]]: it is everywhere at least slightly [[ionized]]), responding to pressure forces, and not as a collection of non-interacting particles.

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]].<ref>{{Cite journal|last=Herbst|first=Eric|date=1995|title=Chemistry in The Interstellar Medium|journal=Annual Review of Physical Chemistry|volume=46|pages=27–54|bibcode=1995ARPC...46...27H|doi=10.1146/annurev.pc.46.100195.000331}}</ref> The thermal [[pressures]] of these phases are in rough equilibrium with one another. [[Magnetic field]]s and [[turbulent]] motions also provide pressure in the ISM, and are typically more important, [[dynamics (mechanics)|dynamically]], than the thermal pressure. In the interstellar medium, matter is primarily in molecular form and reaches number densities of 10<sup>12</sup> molecules per m<sup>3</sup> (1 trillion molecules per m<sup>3</sup>). In hot, diffuse regions, gas is highly ionized, and the density may be as low as 100 ions per m<sup>3</sup>. Compare this with a [[number density]] of roughly 10<sup>25</sup> molecules per m<sup>3</sup> for air at sea level, and 10<sup>16</sup> molecules per m<sup>3</sup> (10 quadrillion molecules per m<sup>3</sup>) for a laboratory high-vacuum chamber. Within our galaxy, by [[mass]], 99% of the ISM is gas in any form, and 1% is dust.<ref name="Boulanger">{{cite conference |author1=Boulanger, F. |author2=Cox, P. |author3=Jones, A. P. | title = Course 7: Dust in the Interstellar Medium | book-title = Infrared Space Astronomy, Today and Tomorrow | year = 2000 |editor1=F. Casoli |editor1-link= Fabienne Casoli |editor2=J. Lequeux |editor3=F. David | pages = 251 | bibcode=2000isat.conf..251B}}</ref> 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,<ref name="Ferriere2001">{{harv|Ferriere|2001}}</ref> known as "[[metallicity|metals]]" in [[astronomy|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 [[Circumstellar dust|enrichment]] (due to [[stellar nucleosynthesis]]) 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 cloud]]s and replenishes the ISM with matter and energy through [[planetary nebula]]e, [[stellar wind]]s, and [[supernova]]e. 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 estimated mission end date of 2025. Its twin ''[[Voyager 2]]'' entered the ISM on November 5, 2018.<ref name="NASA-2020">{{cite web |last=Nelson |first=Jon |title=Voyager - Interstellar Mission |url=https://voyager.jpl.nasa.gov/mission/interstellar-mission/ |archive-url=https://web.archive.org/web/20170825062644/https://voyager.jpl.nasa.gov/mission/interstellar-mission/ |url-status=dead |archive-date=2017-08-25 |date=2020 |work=[[NASA]] |access-date=November 29, 2020}}</ref>

[[File:Voyager.jpg|thumb|right|200px|''[[Voyager 1]]'' is the first artificial object to reach the interstellar medium.]]