Editing Interstellar medium (section)

==Interstellar matter==<!--Molecular cloud links here, as of 2008 July 24, primarily for components of the ISM table-->

Table 1 shows a breakdown of the properties of the components of the ISM of the Milky Way.

{|class="wikitable"
|+ '''Table 1: Components of the interstellar medium'''<ref name="Ferriere2001"/>
|- align=center bgcolor=#eeeeee
!Component||Fractional <br /> volume||[[Scale height]]<br />([[parsec|pc]])||Temperature<br />([[Kelvin|K]])||Density<br />(particles/cm<sup>3</sup>)||State of [[hydrogen]] || Primary observational techniques
|- align=center
|[[Molecular cloud]]s|| < 1% || 80 || 10–20 || 10<sup>2</sup>–10<sup>6</sup> || molecular || [[Radio astronomy|Radio]] and [[Infrared astronomy|infrared]] molecular emission and absorption lines
|- align=center
|Cold neutral medium (CNM) || 1–5% || 100–300 || 50–100 || 20–50 || neutral atomic || [[Hydrogen line|H&nbsp;I 21&nbsp;cm line]] absorption
|- align=center
|Warm neutral medium (WNM) ||10–20% || 300–400 ||6000–10000 || 0.2–0.5 || neutral atomic|| [[Hydrogen line|H&nbsp;I 21&nbsp;cm line]] emission
|- align=center
|Warm ionized medium (WIM)||20–50%|| 1000 || 8000 || 0.2–0.5 || ionized || [[Hα]] emission and [[Dispersion (optics)#Pulsar emissions|pulsar dispersion]]
|- align=center
|[[H II region|H&nbsp;II regions]] || < 1% || 70 || 8000 || 10<sup>2</sup>–10<sup>4</sup> || ionized || [[Hα]] emission, [[Dispersion (optics)#Pulsar emissions|pulsar dispersion]], and [[radio recombination line]]s
|- align=center
|[[Galactic corona|Coronal gas]]<br />Hot ionized medium (HIM)||30–70% || 1000–3000 || 10<sup>6</sup>–10<sup>7</sup> || 10<sup>−4</sup>–10<sup>−2</sup> || ionized<br />(metals also highly ionized) || [[X-ray astronomy|X-ray]] emission; absorption lines of highly ionized metals, primarily in the [[Ultraviolet astronomy|ultraviolet]]
|}

===The three-phase model===

{{harvtxt|Field|Goldsmith|Habing|1969}} put forward the static two ''phase'' equilibrium model to explain the observed properties of the ISM. Their modeled ISM included a cold dense phase (''T''&nbsp;<&nbsp;300&nbsp;[[Kelvin|K]]), consisting of clouds of neutral and molecular hydrogen, and a warm intercloud phase (''T''&nbsp;~&nbsp;10<sup>4</sup>&nbsp;K), consisting of rarefied neutral and ionized gas. {{harvtxt|McKee|Ostriker|1977}} added a dynamic third phase that represented the very hot (''T''&nbsp;~&nbsp;10<sup>6</sup>&nbsp;K) gas that had been shock heated by [[supernova]]e and constituted most of the volume of the ISM.
These phases are the temperatures where heating and cooling can reach a stable equilibrium. Their paper formed the basis for further study over the subsequent three decades. However, the relative proportions of the phases and their subdivisions are still not well understood.<ref name=Ferriere2001 />

The basic physics behind these phases can be understood through the behaviour of hydrogen, since this is by far the largest constituent of the ISM. The different phases are roughly in pressure balance over most of the Galactic disk, since regions of excess pressure will expand and cool, and likewise under-pressure regions will be compressed and heated. Therefore, since [[Ideal gas law|''P = n k T'']], hot regions (high ''T'') generally have low particle number density ''n''. Coronal gas has low enough density that collisions between particles are rare and so little radiation is produced, hence there is little loss of energy and the temperature can stay high for periods of hundreds of millions of years. In contrast, once the temperature falls to O(10<sup>5</sup> K) with correspondingly higher density, protons and electrons can recombine to form hydrogen atoms, emitting photons which take energy out of the gas, leading to runaway cooling. Left to itself this would produce the warm neutral medium. However, [[OB stars]] are so hot that some of their photons have energy greater than the [[Lyman limit]], ''E'' > 13.6 [[electron volt|eV]], enough to ionize hydrogen. Such photons will be absorbed by, and ionize, any neutral hydrogen atom they encounter, setting up a dynamic equilibrium between ionization and recombination such that gas close enough to OB stars is almost entirely ionized, with temperature around 8000 K (unless already in the coronal phase), until the distance where all the ionizing photons are used up. This ''ionization front'' marks the boundary between the Warm ionized and Warm neutral medium.

OB stars, and also cooler ones, produce many more photons with energies below the Lyman limit, which pass through the ionized region almost unabsorbed. Some of these have high enough energy (> 11.3 eV) to ionize carbon atoms, creating a C II ("ionized carbon") region outside the (hydrogen) ionization front. In dense regions this may also be limited in size by the availability of photons, but often such photons can penetrate throughout the neutral phase and only get absorbed in the outer layers of molecular clouds. Photons with ''E'' > 4 eV or so can break up molecules such as H<sub>2</sub> and CO, creating a [[photodissociation region]] (PDR) which is more or less equivalent to the Warm neutral medium. These processes contribute to the heating of the WNM. The distinction between Warm and Cold neutral medium is again due to a range of temperature/density in which runaway cooling occurs.

The densest molecular clouds have significantly higher pressure than the interstellar average, since they are bound together by their own gravity.  When stars form in such clouds, especially OB stars, they convert the surrounding gas into the warm ionized phase, a temperature increase of several hundred. Initially the gas is still at molecular cloud densities, and so at vastly higher pressure than the ISM average: this is a classical H II region. The large overpressure causes the ionized gas to expand away from the remaining molecular gas (a [[Champagne flow model|Champagne flow]]), and the flow will continue until either the molecular cloud is fully evaporated or the OB stars reach the end of their lives, after a few millions years. At this point the OB stars explode as [[supernova]]s, creating blast waves in the warm gas that increase temperatures to the coronal phase ([[supernova remnants]], SNR). These too expand and cool over several million years until they return to average ISM pressure.

=== The ISM in different kinds of galaxy ===
[[File:Three-dim-pillars-creation.jpg|thumb|Three-dimensional structure in [[Pillars of Creation]].<ref>{{cite web|title=The Pillars of Creation Revealed in 3D|url=http://www.eso.org/public/news/eso1518/|access-date=14 June 2015 |publisher=European Southern Observatory |date=30 April 2015}}</ref>]]
Most discussion of the ISM concerns [[spiral galaxies]] like the [[Milky Way]], in which nearly all the mass in the ISM is confined to a relatively thin [[Galactic disc|disk]], typically with [[scale height]] about 100 [[parsec]]s (300 [[light year]]s), which can be compared to a  typical disk diameter of 30,000 parsecs. Gas and stars in the disk orbit the galactic centre with typical orbital speeds of 200&nbsp;km/s. This is much faster than the random motions of atoms in the ISM, but since the orbital motion of the gas is coherent, the average motion does not directly affect structure in the ISM.  The vertical scale height of the ISM is set in roughly the same way as the Earth's atmosphere, as a balance between the local gravitation field (dominated by the stars in the disk) and the pressure. Further from the disk plane, the ISM is mainly in the low-density warm and coronal phases, which extend at least several thousand parsecs away from the disk plane. This [[galactic halo]] or 'corona' also contains significant magnetic field and cosmic ray energy density.

The rotation of galaxy disks influences ISM structures in several ways. Since the [[angular velocity]] declines with increasing distance from the centre, any ISM feature, such as giant molecular clouds or magnetic field lines, that extend across a  range of radius are sheared by differential rotation, and so tend to become stretched out in the tangential direction; this tendency is opposed by interstellar turbulence (see below) which tends to randomize the structures.  [[Density wave theory|Spiral arms]] are due to perturbations in the disk orbits - essentially ripples in the disk, that cause orbits to alternately converge and diverge, compressing and then expanding the local ISM. The visible spiral arms are the regions of maximum density, and the compression often triggers star formation in molecular clouds, leading to an abundance of H&nbsp;II regions along the arms. [[Coriolis force]] also influences large ISM features.

Irregular galaxies such as the [[Magellanic Clouds]] have similar interstellar mediums to spirals, but less organized. In [[Elliptical galaxy|elliptical galaxies]] the ISM is almost entirely in the coronal phase, since there is no coherent disk motion to support cold gas far from the center: instead, the scale height of the ISM must be comperable to the radius of the galaxy. This is consistent with the observation that there is little sign of current star formation in ellipticals. Some elliptical galaxies do show evidence for a small disk component, with ISM similar to spirals, buried close to their centers. The ISM of [[Lenticular galaxy|lenticular galaxies]], as with their other properties, appear intermediate between spirals and ellipticals.

Very close to the center of most galaxies (within a few hundred light years at most), the ISM is profoundly modified by the central [[supermassive black hole]]: see [[Galactic Center]] for the Milky Way, and [[Active galactic nucleus]] for extreme examples in other galaxies. The rest of this article will focus on the ISM in the disk plane of spirals, far from the galactic center.

===Structures===
[[File:The Local Interstellar Cloud and neighboring G-cloud complex.svg|thumb|200px|Map showing the [[Sun]] located near the edge of the Local Interstellar Cloud and [[Alpha Centauri]] about 4 [[light-year]]s away in the neighboring [[G-Cloud]] complex]]
[[File:Interstellar medium annotated.jpg|thumb|upright=2|Interstellar medium and [[astrosphere]] meeting]]

Astronomers describe the ISM as [[turbulence|turbulent]], meaning that the gas has quasi-random motions coherent over a large range of spatial scales. Unlike normal turbulence, in which the [[fluid]] motions are highly [[Subsonic speed|subsonic]], the bulk motions of the ISM are usually larger than the [[sound speed]]. Supersonic collisions between gas clouds cause [[shock waves]] which compress and heat the gas, increasing the sounds speed so that the flow is locally subsonic; thus supersonic turbulence has been described as 'a box of shocklets', and is inevitably associated with complex density and temperature structure. In the ISM this is further complicated by the magnetic field, which provides wave modes such as [[Alfvén wave]]s which are often faster than pure sound waves: if turbulent speeds are supersonic but below the Alfvén wave speed, the behaviour is more like subsonic turbulence.

[[Star formation|Stars are born]] deep inside large complexes of [[molecular clouds]], typically a few parsecs in size. During their lives and deaths, [[star]]s interact physically with the ISM.

Stellar winds from young clusters of stars (often with giant or supergiant [[HII region]]s surrounding them) and [[shock wave]]s created by supernovae inject enormous amounts of energy into their surroundings, which leads to hypersonic turbulence. The resultant structures – of varying sizes – can be observed, such as [[stellar wind bubble]]s and [[superbubble]]s of hot gas, seen by X-ray satellite telescopes or turbulent flows observed in [[radio telescope]] maps.

Stars and planets, once formed, are unaffected by pressure forces in the ISM, and so do not take part in the turbulent motions, although stars formed in molecular clouds in a galactic disk share their general orbital motion around the galaxy center. Thus stars are usually in motion relative to their surrounding ISM. The [[Sun]] is currently traveling through the [[Local Interstellar Cloud]], an irregular clump of the warm neutral medium a few parsecs  across, within the low-density [[Local Bubble]], a 100-parsec radius region of coronal gas.

In October 2020, astronomers reported a significant unexpected increase in density in the [[outer space|space]] beyond the [[Solar System]] as detected by the ''Voyager 1'' and ''[[Voyager 2]]'' [[space probe]]s. According to the researchers, this implies that "the density gradient is a large-scale feature of the [[#Structures|VLISM]] (very local interstellar medium) in the general direction of the [[Heliosphere#Outer structure|heliospheric nose]]".<ref name="SA-20201019">{{cite news |last=Starr |first=Michelle |title=Voyager Spacecraft Detect an Increase in The Density of Space Outside The Solar System |url=https://www.sciencealert.com/for-some-reason-the-density-of-space-is-higher-just-outside-the-solar-system |date=19 October 2020 |work=[[ScienceAlert]] |access-date=19 October 2020 }}</ref><ref name="AJL-20200825">{{cite journal |last1=Kurth |first1=W.S. |last2=Gurnett |first2=D.A. |title=Observations of a Radial Density Gradient in the Very Local Interstellar Medium by Voyager 2 |date=25 August 2020 |journal=[[The Astrophysical Journal Letters]] |volume=900 |number=1 |pages=L1 |doi=10.3847/2041-8213/abae58 |bibcode=2020ApJ...900L...1K |s2cid=225312823 |doi-access=free }}</ref>

===Interaction with interplanetary medium===
The interstellar medium begins where the [[interplanetary medium]] of the [[Solar System]] ends. The [[solar wind]] slows to [[Speed of sound|subsonic]] velocities at the [[termination shock]], 90–100 [[astronomical unit]]s from the Sun. In the region beyond the termination shock, called the [[heliosheath]], interstellar matter interacts with the solar wind. ''Voyager 1'', the farthest human-made object from the Earth (after 1998<ref>{{cite web |url=http://voyager.jpl.nasa.gov/mission/fastfacts.html |title=Voyager: Fast Facts |publisher=Jet Propulsion Laboratory}}</ref>), crossed the termination shock December 16, 2004 and later entered interstellar space when it crossed the [[Heliopause (astronomy)|heliopause]] on August 25, 2012, providing the first direct probe of conditions in the ISM {{harvard citation|Stone|Cummings|McDonald|Heikkila|2005}}.

===Interstellar extinction===
[[File:Short, narrated video about IBEX's interstellar matter observations.ogv|thumb|left|350px|Short, narrated video about [[Interstellar Boundary Explorer|IBEX's]] interstellar matter observations.]]
[[Cosmic dust|Dust grains]] in the ISM are responsible for [[Extinction (astronomy)|extinction]] and [[interstellar reddening|reddening]], the decreasing [[Radiance|light intensity]] and shift in the dominant observable [[wavelength]]s of light from a star. These effects are caused by scattering and absorption of [[photon]]s and allow the ISM to be observed with the naked eye in a dark sky. The apparent rifts that can be seen in the band of the [[Milky Way]] – a uniform disk of stars – are caused by absorption of background starlight by dust in molecular clouds within a few thousand light years from Earth. This effect decreases rapidly with increasing wavelength ("reddening" is caused by greater absorption of blue than red light), and becomes almost negligible at mid-[[infrared]] wavelengths (>&nbsp;5&nbsp;μm).

Extinction provides one of the best ways of mapping the three-dimensional structure of the ISM, especially since the advent of accurate distances to millions of stars from the [[Gaia (spacecraft)|''Gaia'' mission]]. The total amount of dust in front of each star is determined from its reddening, and the dust is then located along the line of sight by comparing the dust [[Area density|column density]] in front of stars projected close together on the sky, but at different distances. By 2022 it was possible to generate a map of ISM structures within 3&nbsp;kpc (10,000 light years) of the Sun.<ref>{{Cite journal |last1=Vergely |first1=J. L. |last2=Lallement |first2=R. |last3=Cox |first3=N. L. J. |date=August 2022 |title=Three-dimensional extinction maps: Inverting inter-calibrated extinction catalogues |url=https://www.aanda.org/10.1051/0004-6361/202243319 |journal=Astronomy & Astrophysics |volume=664 |pages=A174 |doi=10.1051/0004-6361/202243319 |s2cid=248863272 |issn=0004-6361|arxiv=2205.09087 |bibcode=2022A&A...664A.174V }}</ref>

[[Far ultraviolet|Far ultraviolet light]] is absorbed effectively by the neutral hydrogen gas in the ISM. Specifically, atomic [[hydrogen]] absorbs very strongly at about 121.5 nanometers, the [[Lyman series|Lyman-alpha]] transition, and also at the other Lyman series lines. Therefore, it is nearly impossible to see light emitted at those wavelengths 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. All photons with wavelength <&nbsp;91.6&nbsp;nm, the Lyman limit, can ionize hydrogen and are also very strongly absorbed. The absorption gradually decreases with increasing photon energy, and the ISM begins to become transparent again in [[soft X-ray]]s, with wavelengths shorter than about 1&nbsp;nm.