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| {{Other uses|Interstellar (disambiguation)}}
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| [[File:WHAM survey.png|thumb|medium|right|300px|The distribution of [[Plasma (physics)|ionized hydrogen]] (known by astronomers as H 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|}}.]]
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| <!--[[File:Cold and Dark Dust in Space.jpg|thumb|Some of the coldest and darkest dust in space shines brightly in this [[Infrared|infra-red]] image from the [[Herschel Space Observatory|Herschel Observatory]].]]-->
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| In [[astronomy]], the '''interstellar medium''' (or '''ISM''') is the [[matter]] 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, [[cosmic dust|dust]], and [[cosmic ray]]s. It fills interstellar space and blends smoothly into the surrounding [[Outer space#Intergalactic|intergalactic space]]. The [[energy]] that occupies the same volume, in the form of [[electromagnetic radiation]], is the '''interstellar radiation field'''.
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| 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 field]]s and [[turbulent]] motions also provide pressure in the ISM, and are typically more important [[dynamics (mechanics)|dynamically]] than the thermal pressure is. | | The name of the author is Eric Rister. What he loves doing is playing chess but he can't make it his professional. Since I was 18 I've been working like a customer service representative. I've always loved living in West Virginia and I've everything which need on this website. If you want to ascertain more check out my website: http://mittel-gegen-haarausfalll.de<br><br>my web-site :: [http://mittel-gegen-haarausfalll.de wie wachsen die haare schneller] |
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| 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 [[density|densities]] of 10<sup>6</sup> molecules per cm<sup>3</sup>. In hot, diffuse regions of the ISM, matter is primarily ionized, and the density may be as low as 10<sup>−4</sup> ions per cm<sup>3</sup>. Compare this with a number density of roughly 10<sup>22</sup> molecules per cm<sup>3</sup> for liquid water. By [[mass]], 99% of the ISM is gas in any form, and 1% is dust.<ref name=Boulanger>{{cite conference | author = Boulanger, F.; Cox, P.; and Jones, A. P. | title = Course 7: Dust in the Interstellar Medium | booktitle = Infrared Space Astronomy, Today and Tomorrow | year = 2000 | editor = F. Casoli, J. Lequeux, & F. David | pages = 251 | bibcode=2000isat.conf..251B}}</ref> 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 "[[metallicity|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]].
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| 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 cloud]]s, and replenish the ISM with matter and energy through [[planetary nebula]]e, [[solar wind|stellar winds]], 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.
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| On September 12 2013, [[NASA]] officially announced that [[Voyager 1]] had reached the ISM on August 25, 2012, making it the first manmade object to do so. Interstellar plasma and dust will be studied until the mission's end in 2025.
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| [[File:Voyager.jpg|thumb|right|200px|[[Voyager 1]] is the first manmade object to reach the ISM]]
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| ==Interstellar matter==<!--Molecular cloud links here, as of 2008 July 24, primarily for components of the ISM table-->
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| Table 1 shows a breakdown of the properties of the components of the ISM of the Milky Way.
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| {|class="wikitable"
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| |+ '''Table 1: Components of the interstellar medium'''<ref name=Ferriere2001>[[#Ferriere2001|Ferriere (2001)]]</ref>
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| |- align=center bgcolor=#eeeeee
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| !Component||Fractional <br /> Volume||Scale Height<br />([[parsec|pc]])||Temperature<br />([[Kelvin|K]])||Density<br />([[atom]]s/cm³)||State of [[hydrogen]] || Primary observational techniques
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| |- align=center
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| |[[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
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| |- align=center
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| |Cold Neutral Medium (CNM) || 1—5% || 100—300 || 50—100 || 20—50 || neutral atomic || [[Hydrogen line|H I 21 cm line]] absorption
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| |- align=center
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| |Warm Neutral Medium (WNM) ||10—20% || 300—400 ||6000—10000 || 0.2—0.5 || neutral atomic|| [[Hydrogen line|H I 21 cm line]] emission
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| |- align=center
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| |Warm Ionized Medium (WIM)||20—50%|| 1000 || 8000 || 0.2—0.5 || ionized || [[Hα]] emission and [[Dispersion (optics)#Dispersion in pulsar timing|pulsar dispersion]]
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| |- align=center
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| |[[H II region|H II regions]] || < 1% || 70 || 8000 || 10<sup>2</sup>—10<sup>4</sup> || ionized || [[Hα]] emission and [[Dispersion (optics)#Dispersion in pulsar timing|pulsar dispersion]]
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| |- align=center
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| |[[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]]
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| |}
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| ===The three-phase model===
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| {{harvtxt|Field|Goldsmith|Habing|1969}} put forward the static two ''phase'' equilibrium model to explain the observed properties of the ISM. Their modeled ISM consisted of a cold dense phase (T < 300 [[Kelvin|K]]), consisting of clouds of neutral and molecular hydrogen, and a warm intercloud phase (T ~ 10<sup>4</sup> [[Kelvin|K]]), consisting of rarefied neutral and ionized gas. {{harvtxt|McKee|Ostriker|1977}} added a dynamic third phase that represented the very hot (T ~ 10<sup>6</sup> [[Kelvin|K]]) gas which had been shock heated by [[supernova]]e and constituted most of the volume of the ISM.
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| These phases are the temperatures where heating and cooling can reach a stable equilibrium. Their paper formed the basis for further study over the past three decades. However, the relative proportions of the phases and their subdivisions are still not well known.<ref name=Ferriere2001 />
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| ===Structures===
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| The ISM is [[turbulence|turbulent]] and therefore full of structure on all spatial scales.
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| [[Star formation|Stars are born]] deep inside large complexes of [[molecular clouds]], typically a few [[parsec]]s in size. During their lives and deaths, [[star]]s interact physically with the ISM.
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| [[Stellar wind]]s from young clusters of stars (often with giant or supergiant [[HII region]]s surrounding them) and [[shock wave]]s created by [[supernova]]e 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.
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| The [[Sun]] is currently traveling through the [[Local Interstellar Cloud]], a denser region in the low-density [[Local Bubble]].
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| ===Interaction with interplanetary medium===
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| [[File:Short, narrated video about IBEX's interstellar matter observations.ogv|thumb|350px|Short, narrated video about [[Interstellar Boundary Explorer|IBEX's]] interstellar matter observations.]]
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| 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>[http://voyager.jpl.nasa.gov/mission/fastfacts.html Voyager: Fast Facts]</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}}.
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| ===Interstellar extinction===
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| The ISM is also 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 molecular clouds within a few thousand light years from Earth.
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| [[Far ultraviolet|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 series|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.
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| == Heating and cooling ==
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| The ISM is usually far from [[thermodynamic equilibrium]]. Collisions establish a [[Maxwell-Boltzmann distribution]] of velocities, and the 'temperature' normally used to describe interstellar gas is the 'kinetic temperature', which describes the temperature at which the particles would have the observed Maxwell-Boltzmann velocity distribution in thermodynamic equilibrium. However, the interstellar radiation field is typically much weaker than a medium in thermodynamic equilibrium; it is most often roughly that of an [[Stellar_classification#Class_A|A star]] (surface temperature of ~10,000 [[Kelvin|K]]) highly diluted. Therefore, [[energy level|bound levels]] within an [[atom]] or [[molecule]] in the ISM are rarely populated according to the Boltzmann formula {{harv|Spitzer|1978|loc=§ 2.4}}.
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| Depending on the temperature, density, and ionization state of a portion of the ISM, different heating and cooling mechanisms determine the temperature of the [[gas]].
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| === Heating mechanisms ===
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| ; Heating by low-energy [[cosmic ray]]s : The first mechanism proposed for heating the ISM was heating by low-energy [[cosmic rays]]. [[Cosmic ray]]s are an efficient heating source able to penetrate in the depths of molecular clouds. [[Cosmic ray]]s transfer energy to [[gas]] through both ionization and excitation and to free [[electron]]s through [[Coulomb]] interactions. Low-energy [[cosmic ray]]s (a few [[MeV]]) are more important because they are far more numerous than high-energy [[cosmic ray]]s.
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| ; Photoelectric heating in grains : The [[ultraviolet]] radiation emitted by hot [[star]]s can remove [[electron]]s from dust grains. The [[photon]] hits the dust grain, and some of its energy is used in overcoming the potential energy barrier (due to the possible positive charge of the grain) to remove the [[electron]] from the grain. The remainder of the photon's energy heats the grain and gives the ejected [[electron]] [[kinetic energy]]. Since the size distribution of dust grains is <math>n(r) \propto r^{-3.5}</math>, where r is the size of the dust particle, the grain area distribution is <math>r^2 n \propto r^{-1.5}</math>. This indicates that the smallest dust grains dominate this method of heating.
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| ; Photoionization : When an [[electron]] is freed from an [[atom]] (typically from absorption of a UV [[photon]]) it carries kinetic energy away of the order: <math>E_{photon} - E_{ionization}</math>. This heating mechanism dominates in HII regions, but is negligible in the diffuse ISM due to the relative lack of neutral [[carbon]] [[atom]]s.
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| ; [[X-ray]] heating : [[X-ray]]s remove [[electron]]s from [[atom]]s and [[ion]]s, and those photoelectrons can provoke secondary ionizations. As the intensity is often low, this heating is only efficient in warm, less dense atomic medium (as the column density is small). For example in molecular clouds only hard [[x-ray]]s can penetrate and [[x-ray]] heating can be ignored. This is assuming the region is not near an [[x-ray]] source such as a [[supernova remnant]].
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| ; Chemical heating : Molecular [[hydrogen]] (<math>H_2</math>) can be formed on the surface of dust grains when two [[Hydrogen|H]] atoms (which can travel over the grain) meet. This process yields 4.48 eV of energy distributed over the rotational and vibrational modes, kinetic energy of the <math>H_2</math> molecule, as well as heating the dust grain. This kinetic energy, as well as the energy transferred from de-excitation of the hydrogen molecule through collisions, heats the gas.
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| ; Grain-gas heating : Collisions at high densities between gas atoms and molecules with dust grains can transfer thermal energy. This is not important in HII regions because UV radiation is more important. It is also not important in diffuse ionized medium due to the low density. In the neutral diffuse medium grains are always colder, but do not effectively cool the gas due to the low densities.
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| Grain heating by thermal exchange is very important in supernova remnants where densities and temperatures are very high.
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| Gas heating via grain-gas collisions is dominant deep in giant molecular clouds (especially at high densities). Far [[infrared]] radiation penetrates deeply due to the low optical depth. Dust grains are heated via this radiation and can transfer thermal energy during collisions with the gas. A measure of efficiency in the heating is given by the accommodation coefficient:
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| :<math>\alpha = \frac{T_2 - T}{T_d - T}</math>
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| where <math>T</math> is the gas temperature, <math>T_d</math> the dust temperature, and <math>T_2</math> the post-collision temperature of the gas atom/molecule. This coefficient was measured by {{harvard citation|Burke|Hollenbach|1983}} as <math>\alpha = 0.35</math>.
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| ; Other heating mechanisms : A variety of macroscopic heating mechanisms are present including:
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| :* [[Gravitational collapse]] of a cloud
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| :* [[Supernova]] explosions
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| :* [[Stellar wind]]s
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| :* Expansion of [[H II region|H II regions]]
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| :* [[Magnetohydrodynamic]] waves created by supernova remnants
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| === Cooling mechanisms ===
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| ; Fine structure cooling : The process of fine structure cooling is dominant in most regions of the Interstellar Medium, except regions of hot [[gas]] and regions deep in molecular clouds. It occurs most efficiently with abundant [[atom]]s having fine structure levels close to the fundamental level such as: CII and OI in the neutral medium and OII, OIII, NII, NIII, NeII and NeIII in HII regions. Collisions will excite these atoms to higher levels, and they will eventually de-excite through photon emission, which will carry the energy out of the region.
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| ; Cooling by permitted lines : At lower temperatures, more levels than fine structure levels can be populated via collisions. For example, collisional excitation of the n=2 level of [[hydrogen]] will release a Ly<math>\alpha</math> photon upon de-excitation. In molecular clouds, excitation of rotational lines of CO is important. Once a [[molecule]] is excited, it eventually returns to a lower energy state, emitting a photon which can leave the region, cooling the cloud.
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| ==The history of knowledge of interstellar space==
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| [[File:Herbig-Haro object HH 110.jpeg|thumb|[[Herbig–Haro 110]] object ejects gas through interstellar space.<ref>{{cite news|title=A geyser of hot gas flowing from a star|url=http://www.spacetelescope.org/news/heic1210/|accessdate=3 July 2012|newspaper=ESA/Hubble Press Release}}</ref> ]]
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| The nature of the interstellar medium has received the attention of astronomers and scientists over the centuries, and [[Timeline of knowledge about the interstellar and intergalactic medium|understanding of the ISM has developed]]. However, they first had to acknowledge the basic concept of "interstellar" space. The term appears to have been first used in print by {{harvtxt|Bacon|1626|loc=§ 354–5}}: "The Interstellar Skie.. hath .. so much Affinity with the Starre, that there is a Rotation of that, as well as of the Starre." Later, [[natural philosopher]] {{harvard citations|first=Robert | last=Boyle | year=1674 | txt=yes | authorlink=Robert Boyle}} discussed "The inter-stellar part of heaven, which several of the modern [[Epicureanism|Epicureans]] would have to be empty."
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| Before modern [[electromagnetic theory]], early [[physicist]]s postulated that an invisible [[luminiferous aether]] existed as a medium to carry lightwaves. It was assumed that this aether extended into interstellar space, as {{harvtxt|Patterson|1862}} wrote, "this efflux occasions a thrill, or vibratory motion, in the [[Aether (classical element)|ether]] which fills the interstellar spaces."
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| The advent of deep photographic imaging allowed [[Edward Emerson Barnard|Edward Barnard]] to produce the first images of [[dark nebula]]e silhouetted against the background star field of the galaxy, while the first actual detection of cold diffuse matter in interstellar space was made by [[Johannes Franz Hartmann|Johannes Hartmann]] in 1904<ref>{{Citation|authorlink=Isaac Asimov|first=Isaac|last=Asimov|title=[[Asimov's Biographical Encyclopedia of Science and Technology]]|edition=2nd}}</ref> through the use of [[absorption spectrum|absorption line spectroscopy]]. In his historic study of the spectrum and orbit of [[Delta Orionis]], Hartmann observed the [[light]] coming from this star and realized that some of this light was being absorbed before it reached the Earth. Hartmann reported that absorption from the "K" line of [[calcium]] appeared "extraordinarily weak, but almost perfectly sharp" and also reported the "quite surprising result that the calcium line at 393.4 nanometres does not share in the periodic displacements of the lines caused by the orbital motion of the [[spectroscopic binary]] star". The stationary nature of the line led Hartmann to conclude that the gas responsible for the absorption was not present in the atmosphere of Delta Orionis, but was instead located within an isolated cloud of matter residing somewhere along the line-of-sight to this star. This discovery launched the study of the Interstellar Medium.
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| In the series of investigations, [[Viktor Ambartsumian]] introduced the now commonly accepted notion that interstellar matter occurs in the form of clouds.<ref>{{Citation | title= To Victor Ambartsumian on his 80th birthday | author=[[Subrahmanyan Chandrasekhar|S. Chandrasekhar]] | journal= Journal of Astrophysics and Astronomy | volume=18 | pages=3 | doi=10.1007/BF01005852|bibcode = 1988Ap.....29..408C | year= 1989 }}</ref>
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| Following Hartmann's identification of interstellar calcium absorption, interstellar [[sodium]] was detected by {{harvtxt|Heger|1919}} through the observation of stationary absorption from the atom's "D" lines at 589.0 and 589.6 nanometres towards Delta Orionis and [[Beta Scorpii]].
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| Subsequent observations of the "H" and "K" lines of calcium by {{harvtxt|Beals|1936}} revealed double and asymmetric profiles in the spectra of [[Epsilon Orionis|Epsilon]] and [[Zeta Orionis]]. These were the first steps in the study of the very complex interstellar sightline towards [[Orion (constellation)|Orion]]. Asymmetric absorption line profiles are the result of the superposition of multiple absorption lines, each corresponding to the same atomic transition (for example the "K" line of calcium), but occurring in interstellar clouds with different [[Radial velocity|radial velocities]]. Because each cloud has a different velocity (either towards or away from the observer/Earth) the absorption lines occurring within each cloud are either [[Blueshift|Blue-shifted]] or [[Red shift|Red-shifted]] (respectively) from the lines' rest wavelength, through the [[Doppler Effect]]. These observations confirming that matter is not distributed homogeneously were the first evidence of multiple discrete clouds within the ISM.
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| [[File:Hubble sees a cosmic caterpillar.jpg|thumb|This light-year-long knot of interstellar gas and dust resembles a [[caterpillar]].<ref>{{cite web|title=Hubble sees a cosmic caterpillar|url=http://www.spacetelescope.org/images/opo1335a/|work=Image Archive|publisher=ESA/Hubble|accessdate=9 September 2013}}</ref> ]]
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| The growing evidence for interstellar material led {{harvtxt|Pickering|1912}} to comment that "While the interstellar absorbing medium may be simply the ether, yet the character of its selective absorption, as indicated by [[Jacobus Kapteyn|Kapteyn]], is characteristic of a gas, and free gaseous [[molecule]]s are certainly there, since they are probably constantly being expelled by the [[Sun]] and [[star]]s."
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| The same year [[Victor Francis Hess|Victor Hess]]'s discovery of [[cosmic rays]], highly energetic charged particles that rain onto the [[Earth]] from space, led others to speculate whether they also pervaded interstellar space. The following year the 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 [[ion]]s 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 systems or [[nebula]]e, but in 'empty' space" {{harv|Birkeland|1913}}.
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| {{harvtxt|Thorndike|1930}} noted 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]].<!--transcription: one of these phrases may be duplicative--> If the millions of other [[star]]s are also ejecting [[ion]]s, as is undoubtedly true, no absolute vacuum can exist within the galaxy."
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| In September 2012, [[NASA|NASA scientists]] reported that [[polycyclic aromatic hydrocarbons|polycyclic aromatic hydrocarbons (PAHs)]], subjected to ''interstellar medium (ISM)'' conditions, are transformed, through [[hydrogenation]], [[Oxygenate|oxygenation]] and [[hydroxylation]], to more complex [[Organic compound|organics]] - "a step along the path toward [[amino acids]] and [[nucleotides]], the raw materials of [[proteins]] and [[DNA]], respectively".<ref name="Space-20120920">{{cite web |author=Staff |title=NASA Cooks Up Icy Organics to Mimic Life's Origins|url=http://www.space.com/17681-life-building-blocks-nasa-organic-molecules.html|date=September 20, 2012 |publisher=[[Space.com]] |accessdate=September 22, 2012 }}</ref><ref name="AJL-20120901">{{Citation |last1=Gudipati |first1=Murthy S. |last2=Yang |first2=Rui|title=In-Situ Probing Of Radiation-Induced Processing Of Organics In Astrophysical Ice Analogs—Novel Laser Desorption Laser Ionization Time-Of-Flight Mass Spectroscopic Studies|url=http://iopscience.iop.org/2041-8205/756/1/L24 |date=September 1, 2012 |journal=[[The Astrophysical Journal Letters]] |volume=756 |doi=10.1088/2041-8205/756/1/L24|accessdate=September 22, 2012 |bibcode = 2012ApJ...756L..24G |issue=1 |pages=L24 }}</ref> Further, as a result of these transformations, the PAHs lose their [[Spectroscopy|spectroscopic signature]] which could be one of the reasons "for the lack of PAH detection in [[interstellar ice]] [[Cosmic_dust#Dust_grain_formation|grains]], particularly the outer regions of cold, dense clouds or the upper molecular layers of [[protoplanetary disks]]."<ref name="Space-20120920" /><ref name="AJL-20120901" />
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| ==See also==
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| {{Portal|Astronomy|Solar System|Space}}
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| <div style="column-count:2;-moz-column-count:2;-webkit-column-count:2">
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| * [[Diffuse interstellar band]]
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| * [[Fossil stellar magnetic field]]
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| * [[Heliosphere]]
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| * [[Interstellar masers]]
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| * [[List of molecules in interstellar space]]
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| * [[Photodissociation region]]
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| * [[List of plasma (physics) articles]]
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| </div>
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| == Notes ==
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| {{Reflist}}
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| ==References==
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| * <cite id=Bacon1626>{{citation | last=Bacon | first=Francis | authorlink=Francis Bacon | year=1626 | title=Sylva | edition=354–5}}</cite>
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| * <cite id=Beals1936>{{citation | last=Beals | first=C. S. | year=1936 | journal=Monthly Notices of the Royal Astronomical Society | title=On the interpretation of interstellar lines | volume=96 | pages=661 | bibcode=1936MNRAS..96..661B }}</cite>
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| * <cite id=Birkeland1913>{{citation | last=Birkeland | first=Kristian | authorlink=Kristian Birkeland | year=1913 | title=The Norwegian Aurora Polaris Expedition, 1902-03 (section 2) | publisher=New York: Christiania (now Oslo), H. Aschelhoug & Co. | pages=720 | contribution=Polar Magnetic Phenomena and Terrella Experiments |url=http://www.archive.org/details/norwegianaurorap01chririch}} out-of-print, full text online</cite>
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| * <cite id=Boyle1674>{{citation | last=Boyle | first=Robert | authorlink=Robert Boyle | year=1674 | journal=Excell. Theol. | volume = ii. iv. | pages=178 }}</cite>
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| * <cite id=Burke1983>{{citation | last1=Burke | first1=J. R. | last2=Hollenbach | first2=D.J. | title= The gas-grain interaction in the interstellar medium - Thermal accommodation and trapping | journal=Astrophysical Journal | year=1983 | volume=265 | pages=223 | bibcode=1983ApJ...265..223B | doi=10.1086/160667}}</cite>
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| * <cite id=Dyson1997>{{citation | last=Dyson | first=J. | year=1997 | title=Physics of the Interstellar Medium | publisher=London: Taylor & Francis}}</cite>
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| * <cite id=Field1969>{{citation | last1=Field | first1=G. B. | last2=Goldsmith | first2=D. W. | last3=Habing | first3=H. J. | title=Cosmic-Ray Heating of the Interstellar Gas | journal=Astrophysical Journal | year=1969 | volume=155 | pages= L149 | bibcode=1969ApJ...155L.149F | doi=10.1086/180324}}</cite>
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| * <cite id=Ferriere2001>{{citation | last=Ferriere | first=K. | title= The Interstellar Environment of our Galaxy | journal=Reviews of Modern Physics | year=2001| volume=73 | issue=4 | pages= 1031–1066 | doi=10.1103/RevModPhys.73.1031 | id= | arxiv=astro-ph/0106359 | bibcode=2001RvMP...73.1031F}}</cite>
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| * <cite id=Haffner2003>{{citation|last=Haffner | last2=Reynolds | last3=Tufte | last4=Madsen | first=L. M. | first2=R. J. | first3=S. L. | journal=Astrophysical Journal Supplement | year=2003 | volume=145|issue=2 | pages=405 | title= The Wisconsin Hα Mapper Northern Sky Survey | doi=10.1086/378850|first4=G. J.|last5=Jaehnig|first5=K. P.|last6=Percival|first6=J. W. | bibcode=2003ApJS..149..405H|arxiv = astro-ph/0309117 }}.</cite> The [http://www.astro.wisc.edu/wham/ Wisconsin Hα Mapper] is funded by the [[National Science Foundation]].
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| *<cite id=Heger1919>{{citation | last=Heger | first=Mary Lea | journal=Publications of the Astronomical Society of the Pacific | volume=31 | pages=304 | year=1919 | bibcode=1919PASP...31..304H | title=Stationary Sodium Lines in Spectroscopic Binaries | doi=10.1086/122890 | issue=184}}</cite>
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| * <cite id=Lequeux2005>Lequeux, J. ''The Interstellar Medium''. Springer 2005.</cite>
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| * <cite id=McKee1977>{{citation | last1=McKee | first1=C. F. |last2=Ostriker |first2=J. P. | author1-link=Christopher McKee | author2-link=Jeremiah P. Ostriker | title=A theory of the interstellar medium - Three components regulated by supernova explosions in an inhomogeneous substrate | journal=Astrophysical Journal | year=1977 | volume=218 | pages=148 | bibcode=1977ApJ...218..148M | doi=10.1086/155667}}</cite>
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| * <cite id=Patterson1896>{{citation | last=Patterson | first=Robert Hogarth | title= Colour in nature and art | journal= Essays in History and Art| volume=10 | year=1862}} Reprinted from ''Blackwood's Magazine''.</cite>
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| * <cite id=Pickering1912>{{citation | last1=Pickering | first1=W. H. | authorlink=William Henry Pickering | title=The Motion of the Solar System relatively to the Interstellar Absorbing Medium | journal=Monthly Notices of the Royal Astronomical Society | year=1912| volume=72 | pages= 740 | bibcode=1912MNRAS..72..740P}}</cite>
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| * <cite id=Spitzer1978>{{citation | author=Spitzer, L. | authorlink=Lyman Spitzer | year=1978 | title=Physical Processes in the Interstellar Medium | publisher=Wiley | isbn=0-471-29335-0}}</cite>
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| * <cite id=Stone2005>{{citation | last1=Stone | first1=E. C. | last2=Cummings | first2=A. C. | last3=McDonald | first3=F. B. | last4=Heikkila | first4=B. C. | last5=Lal | first5=N. | last6=Webber | first6=W. R. | journal=Science | title=Voyager 1 Explores the Termination Shock Region and the Heliosheath Beyond | year=2005 | volume=309 | pages=2017–20 | doi= 10.1126/science.1117684 | pmid=16179468 | issue=5743|bibcode = 2005Sci...309.2017S }}</cite>
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| * <cite id=Thorndike1930>{{citation | last=Thorndike | first=S. L. | title=Interstellar Matter | journal=Publications of the Astronomical Society of the Pacific | year=1930| volume=42 | issue=246 | pages= 99 | url=http://articles.adsabs.harvard.edu//full/seri/PASP./0042//0000099.000.html | doi=10.1086/124007 | bibcode=1930PASP...42...99T}}</cite>
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| ==External links==
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| * [http://www.vega.org.uk/video/programme/64 Freeview Video 'Chemistry of Interstellar Space' William Klemperer, Harvard University. A Royal Institution Discourse by the Vega Science Trust.]
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| * [http://www-ssg.sr.unh.edu/ism/intro.html The interstellar medium: an online tutorial]
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| {{DEFAULTSORT:Interstellar Medium}}
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| [[Category:Interstellar media| ]]
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| [[Category:Outer space|Medium, interstellar]]
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| [[Category:Astrochemistry]]
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