The interstellar medium (or ISM) is the name astronomers give to the tenuous gas and dust that pervade interstellar space. Whilst the ISM refers to the matter (interstellar matter, also abbreviated by ISM) that exists between the stars within a galaxy, the energy, in the form of electromagnetic radiation, that occupies the same volume is called the interstellar radiation field (or ISRF). A giant Hubble mosaic of the Crab Nebula, a supernova remnant. ... This article or section does not cite its references or sources. ... This article is about the astronomical object. ... NGC 4414, a typical spiral galaxy in the constellation Coma Berenices, is about 56,000 light-years in diameter and approximately 60 million light-years distant. ... Electromagnetic radiation can be imagined as a self-propagating transverse oscillating wave of electric and magnetic fields. ...

The ISM consists of an extremely dilute (by terrestrial standards) plasma, gas and dust, consisting of a mixture of ions, atoms, molecules, larger dust grains, electromagnetic radiation, cosmic rays, and magnetic fields. The matter consists of about 99% gas and 1% dust by mass. It fills interstellar space. This mixture is usually extremely tenuous, with typical densities ranging from a few hundred to a few hundred million particles per cubic meter. As a result of primordial nucleosynthesis, the gas is roughly 90% hydrogen and 10% helium by number of nucleii, with additional elements ("metals" in astronomical parlance) present in trace amounts. For other uses, see Plasma. ... A gas is one of the four major phases of matter (after solid and liquid, and followed by plasma, that subsequently appear as a solid material is subjected to increasingly higher temperatures. ... Porous chondrite interplanetary dust particle. ... An ion is an atom or group of atoms that normally are electrically neutral and achieve their status as an ion by loss or addition of one or more electrons. ... For other uses, see Atom (disambiguation). ... In chemistry, a molecule is an aggregate of two or more atoms in a definite arrangement held together by chemical bonds [1] [2] [3] [4] [5]. Chemical substances are not infinitely divisible into smaller fractions of the same substance: a molecule is generally considered the smallest particle of a pure... Porous chondrite interplanetary dust particle. ... Electromagnetic radiation can be imagined as a self-propagating transverse oscillating wave of electric and magnetic fields. ... Cosmic rays can loosely be defined as energetic particles originating outside of the Earth. ... To meet Wikipedias quality standards, this article may require cleanup. ... This page is a candidate for speedy deletion. ... General Name, Symbol, Number hydrogen, H, 1 Chemical series nonmetals Group, Period, Block 1, 1, s Appearance colorless Atomic mass 1. ... General Name, Symbol, Number helium, He, 2 Chemical series noble gases Group, Period, Block 18, 1, s Appearance colorless Atomic mass 4. ... Nucleus usually refers to the center of something, but can mean: In science: Atomic nucleus, the collection of protons and neutrons in the center of an atom that carries the bulk of the atoms mass and positive charge Cell nucleus, the membrane-bound subcellular organelle found in eukaryotes, visible... In astronomy, the metallicity of an object is the proportion of its matter made up of chemical elements other than hydrogen and helium. ...

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. In turn, 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. NGC 6543, the Cats Eye Nebula A planetary nebula is an astronomical object consisting of a glowing shell of gas and plasma formed by certain types of stars at the end of their lives. ... The plasma in the solar wind meeting the heliopause For the British comic, see Solar Wind (comic). ... Multiwavelength X-ray image of the remnant of Keplers Supernova, SN 1604. ...

The history of knowledge of interstellar space

The nature of the interstellar medium has received the attention of astronomers and scientists over the centuries. However, they first had to acknowledge the basic concept of "interstellar" space. The term appears to have been first used in print by Francis Bacon in 1626 where he wrote: "The Interstellar Skie.. hath .. so much Affinity with the Starre, that there is a Rotation of that, as well as of the Starre." (Sylva §354–5). Later, natural philosopher Robert Boyle surmised: "The inter-stellar part of heaven, which several of the modern Epicureans would have to be empty." (1674 Excell. Theol. ii. iv. 178) HIStory: Past, Present and Future – Book I is a two-disc album by Michael Jackson released in 1995 by the Epic Records division of Sony BMG. The first disc (HIStory Begins) is a fifteen-track greatest hits (later released as Greatest Hits - HIStory Volume I), while the second disc (HIStory... Francis Bacon, 1st Viscount St Alban, KC (22 January 1561 – 9 April 1626) was an English philosopher, statesman and essayist but is best known for leading the scientific revolution with his new observation and experimentation theory which is the way science has been conducted ever since. ... Robert Boyle Robert Boyle (January 25, 1627 – December 30, 1691) was an Irish natural philosopher, chemist, physicist, inventor and early gentleman scientist, noted for his work in physics and chemistry. ... Epicureanism is a system of philosophy based upon the teachings of Epicurus (c. ...

Before modern electromagnetic theory early physicists postulated that an invisible luminiferous aether existed as a medium to carry lightwaves. It was assumed that this aether extended into interstellar space, as R. H. Patterson wrote in 1862, "This efflux occasions a thrill, or vibratory motion, in the ether which fills the interstellar spaces" (Ess. Hist. & Art 10). The luminiferous aether: it was hypothesised that the Earth moves through a medium of aether that carries light In the late 19th century luminiferous aether (light-bearing aether) was the term used to describe a medium for the propagation of light. ... Look up aether, ether in Wiktionary, the free dictionary. ...

The advent of deep photographic imaging allowed Barnard to produce the first images of dark nebula silhouetted against the background star field of the Galaxy while the first actual detection of cold diffuse matter in interstellar space was made by Hartmann in 1904 through the use of absorption line spectroscopy. In this 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 demonstrated that absorption from the "K" line of calcium to appear "extraordinarily weak, but almost perfectly sharp" and also reported the "quite surprising result that the calcium line at 3934 Angstroms 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 was of course a discovery which would open up a new branch of astronomy, launching the study of the interstellar medium (ISM). Arguably the most famous dark nebula, the Horsehead Nebula. ... A materials absorption spectrum shows the fraction of incident electromagnetic radiation absorbed by the material over a range of frequencies. ... Delta Orionis (δ Ori) is one of the three stars of the belt of the constellation Orion. ... An angstrom, angström, or ångström (Å) is a non-SI unit of length equal to 10−10 metres, 0. ... A spectroscopic binary star is a binary star which cannot be resolved as a visual binary, even with telescopes of the highest existing resolving power. ...

Following the identification of interstellar Calcium absorption by Hartmann, interstellar Sodium was detected by Heger in 1919 through the observation of stationary absorption from the atoms "D" lines at 5890 and 5896 Angstroms towards Delta Orionis and Beta Sco. Beta Scorpii (β Sco / β Scorpii) is a star system in the constellation Scorpius. ...

Subsequent observations of the "H" and "K" lines of Calcium by Beals 1936 revealed double and asymmetric profiles in the spectra of Epsilon and Zeta Orionis. These were the first steps in the study of the very complex interstellar sightline towards 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 velocities. Because each cloud has a different velocity (either towards or away from the observer/Earth) the absorption lines occurring within each cloud is either red-shifted or blue-shifted (respectively) from the lines rest wavelength through the Doppler Effect. These observations highlight that matter is not distributed homogeneously and were the first evidence for the presence of multiple discrete clouds within the ISM. Epsilon Orionis (ε Ori / ε Orionis) is a large blue star in the constellation of Orion. ... Zeta Orionis is a star in the constellation Orion. ... Orion (IPA: ), a constellation often referred to as The Hunter, is a prominent constellation, perhaps the best-known and most conspicuous in the sky. ... Radial velocity is the velocity of an object in the direction of the line of sight. ... Redshift of spectral lines in the optical spectrum of a supercluster of distant galaxies (right), as compared with that of the Sun (left). ... Blue shift is the opposite of redshift, the latter being much more noted due to its importance to modern astronomy. ... A source of waves moving to the left. ...

The growing evidence for interstellar material led William Henry Pickering to comment in 1912 that "While the interstellar absorbing medium may be simply the ether, yet the character of its selective absorption, as indicated by Kapteyn, is characteristic of a gas, and free gaseous molecules are certainly there, since they are probably constantly being expelled by the Sun and stars..." William Henry Pickering (February 15, 1858 – January 17, 1938) was an American astronomer, brother of Edward Charles Pickering. ... Jacobus Cornelius Kapteyn, (January 19, 1851 – June 18, 1922) was a Dutch astronomer, best known for his extensive studies of the Milky Way and as the first discoverer of evidence for galactic rotation. ...

The same year Victor Hess's discovery of cosmic rays, highly energetic charged particles that rain down on 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 ions 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 [sic] systems or nebulae, but in "empty" space.' (See "Polar Magnetic Phenomena and Terrella Experiments", in The Norwegian Aurora Polaris Expedition 1902-1903 (publ. 1913, p.720). Victor Francis Hess (June 24, 1883 – December 17, 1964) was an Austrian-American physicist. ... Cosmic rays can loosely be defined as energetic particles originating outside of the Earth. ... Kristian Birkeland Kristian Birkeland (December 13, 1867 - June 15, 1917) was born in Christiania (Oslo today) and wrote his first scientific paper at the age of 18. ...

In 1930, Samuel L. Thorndike notes 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. If the millions of other stars are also ejecting ions, as is undoubtedly true, no absolute vacuum can exist within the galaxy".

Interstellar matter

The three-phase model

In 1969 Field, Goldsmith, & Habing put forward the a static two phase equilibrum model to explain the observed properties of the ISM. Their ISM consisted of a cold dense phase (T<300K), comprised of clouds of neutral and molecular hydrogen, and a warm intercloud phase (T~1,000K), comprised of rarefied neutral and ionized gas. McKee and Ostriker later added a dynamic third phase that represented the very hot (T = 1,000,000K) gas which had been expelled from supernovae and HII regions and constituted most of the volume of the ISM. Their 1977 paper formed the basis for further study over the past quarter-century. However, the relative proportions of the phases and their subdivisions are still a matter of considerable contention in scientific circles. 1969 (MCMLXIX) was a common year starting on Wednesday (the link is to a full 1969 calendar). ... Christopher Fulton McKee (born 1942) is an astrophysicist. ... Jeremiah (Jerry) Paul Ostriker (b. ... For the album by Ash, see 1977 (album). ...

The following table shows a breakdown of the properties and origin of the components of the three phases.

A molecular cloud is a type of interstellar cloud whose density and size permits the formation of molecules, most commonly molecular hydrogen (H2). ... NGC 604, a giant H II region in the Triangulum Galaxy. ... In astronomy, a corona is the luminous plasma atmosphere of the Sun or other celestial body, extending millions of kilometres into space, most easily seen during a total solar eclipse, but also observable in a coronagraph. ...

Structures

Features prominent in the study of the interstellar medium include molecular clouds, interstellar clouds, supernova remnants, planetary nebulae, and similar diffuse structures. A molecular cloud is a type of interstellar cloud whose density and size permits the formation of molecules, most commonly molecular hydrogen (H2). ... Interstellar cloud is the generic name given to an accumulation of gas, plasma and dust in our and other galaxies. ... The Crab Nebula is an expanding cloud of gas created by the 1054 supernova. ... NGC 6543, the Cats Eye Nebula A planetary nebula is an astronomical object consisting of a glowing shell of gas and plasma formed by certain types of stars at the end of their lives. ... Diffuse structures in the interstellar medium exist on all spatial scales, and the term clouds is misleading in a way. ...

Interstellar extinction

The medium is also responsible for extinction and reddening, the decreasing light intensity and dominant observable wavelengths of a star as the light travels through the medium. These effects are caused by scattering and absorption of photons and allows the ISM to be observed with the naked eye in a dark sky. The rifts that can be seen in the band of the Milky Way are caused by absorption of background starlight from the uniform disk of stars by molecular clouds within a few thousand light years. Extinction is a term used in astronomy to describe the absorption of light from astronomical objects by matter between them and the observer. ... In astronomy, interstellar reddening is a phenomenon associated with interstellar extinction where the spectrum of electromagnetic radiation from a radiation source changes characteristics from that which was emitted. ... Luminous intensity is a measure of the energy emitted by a light source in a particular direction. ... The wavelength is the distance between repeating units of a wave pattern. ...

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-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. General Name, Symbol, Number hydrogen, H, 1 Chemical series nonmetals Group, Period, Block 1, 1, s Appearance colorless Atomic mass 1. ... The Lyman series is the series of transitions and resulting emission lines of the hydrogen atom as an electron goes from n ≥ 2 to n = 1 (where n is the principal quantum number referring to the energy level of the electron). ... Earth (IPA: , often referred to as the Earth, Terra, the World or Planet Earth) is the third planet in the solar system in terms of distance from the Sun, and the fifth largest. ...

Interstellar radiation field

The interstellar radiation field (ISRF) is the sum total of all electromagnetic radiation travelling through interstellar space.

Heating of the interstellar medium

Heating by low-energy cosmic rays

The first mechanism proposed for heating the ISM was heating by low energy cosmic rays. Cosmic rays transfer energy to gas (through both ionization and excitation) and to free electrons through Coulomb interactions. Low energy cosmic rays (a few MeV) are more important because they are far more numerous than high-energy cosmic rays. Cosmic rays are an efficient heating source able to penetrate in the depths of molecular clouds. Cosmic rays can loosely be defined as energetic particles originating outside of the Earth. ...

Photoelectric heating in grains

The ultraviolet radiation emitted by hot stars can remove electrons 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: The solar corona as seen in deep ultraviolet light at 17. ... Kinetic energy is the energy by virtue of the motion of an object. ...

where r is the size of the dust particle, the grain area distribution is:

This indicates that the smallest dust grains dominate this method of heating.

Photoionization

When an electron is freed from an atom (typically from absorption of a UV photon) it carries kinetic energy away of the order: E_photon − E_ionization. This heating mechanism dominates in HII regions, but is negligible in the diffuse ISM due to the relative lack of neutral carbon atoms.

X-ray heating

X-rays remove electrons from atoms and ions, 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-rays 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. The Crab Nebula is an expanding cloud of gas created by the 1054 supernova. ...

Chemical heating

Molecular hydrogen (H₂) can be formed on the surface of dust grains when 2 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 H₂ molecule, as well as heating the dust grain. This kinetic energy, as well as the energy transfered from de-excitation of the hydrogen molecule through collisions heats the gas.

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.

Grain heating by thermal exchange is very important in supernova remnant where densities and temperatures are very high.

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: Image of two girls in mid-infrared (thermal) light (false color) Infrared (IR) radiation is electromagnetic radiation of a wavelength longer than that of visible light, but shorter than that of radio waves. ...

where T is the gas temperature, T_d the dust temperature, and T₂ the post-collision temperature of the gas atom/molecule. This coefficient was measure by Burke & Hollenbach (1983) as α = 0.35.

Other heating mechanisms

A variety of macroscopic heating mechanisms are present including:

• Gravitational collapse of a cloud
• Supernova explosions
• Stellar Winds
• Expansion of HII regions
• Magnetohydrodynamic waves created by supernova remnants

Cooling of the interstellar medium

Fine structure cooling

This process is dominant in most regions of the ISM, except regions of hot gas and regions deep in molecular clouds. This occurs most efficiently with abundant atoms 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 upper levels, which will eventually de-excite through photon emission, which will carry the energy out of the region.

Cooling by permitted lines

At higher temperature 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 Lyα photon upon de-excitation. In molecular clouds excitation of rotational lines of CO is important.

References

• Burke J.R. and Hollenbach D.J. 1983. ApJ, 265, 223.
• Dyson, J. (1997) Physics of the Interstellar Medium, London: Taylor & Francis
• Field, G. B., Goldsmith, D. W., & Habing, H. J. (2001). "Cosmic-Ray Heating of the Interstellar Gas". Astrophysical Journal 155: L149. (NASA Astronomy Abstract Service Entry)
• Ferriere, D. (2001). "The Interstellar Environment of our Galaxy.". Reviews of Modern Physics 73 (4): 1031-1066. (astro-ph preprint)
• Lequeux J. The Interstellar Medium. Springer 2005.
• Pickering, W. H. (1912). "Solar system, the motion of the, relatively to the intersteller absorbing medium". Monthly Notices of the Royal Astronomical Society 72: 740. (NASA Astronomy Abstract Service Entry)
• Thorndike, S. L. (1930). "Interstellar Matter". Publications of the Astronomical Society of the Pacific 42 (246): 99. (PASP online copy)
• Spitzer, L. (1978) Physical Processes in the Interstellar Medium New York: Wiley.
• Wisconsin H-Alpha Mapper Survey

See also

• Cosmic dust
• Diffuse interstellar band
• Interstellar masers
• Interstellar reddening
• Heliosphere
• Intergalactic space
• List of molecules in interstellar space
• Outer Solar System
• Outer space
• Solar system
• Stellar system
• Timeline of knowledge about the interstellar and intergalactic medium

Porous chondrite interplanetary dust particle. ... Relative strengths of observed diffuse interstellar bands Diffuse interstellar bands (DIBs) are absorption features seen in the spectra of astronomical objects in the Galaxy. ... An astrophysical maser is a naturally-occuring source of stimulated spectral line emission, typically in the microwave portion of the electromagnetic spectrum. ... In astronomy, interstellar reddening is a phenomenon associated with interstellar extinction where the spectrum of electromagnetic radiation from a radiation source changes characteristics from that which was emitted. ... The heliosphere is a bubble in space produced by the solar wind. ... Intergalactic space is the physical space between galaxies. ... This is a list of molecules that have been detected in the interstellar medium as of 2003. ... The outer solar system (as opposed to the outer planets) is that part of the Solar System which begins at roughly the orbit of Neptune and terminates at maximum orbit distance, approximately one Light Year from the sun in terms of orbital measurements. ... Layers of Atmosphere - not to scale (NOAA) Outer space, also simply called space, refers to the relatively empty regions of the universe outside the atmospheres of celestial bodies. ... This article is about the Solar System. ... A star system or stellar system is a group of stars (and possibly smaller bodies such as planets or asteroids) that orbit one another (a system with planetary bodies orbiting stars, are referred to as solar systems or planetary systems). ... Timeline of the interstellar medium and intergalactic medium 1848 - Lord Rosse studies M1 and names it the Crab Nebula 1864 - William Huggins studies the spectrum of the Orion Nebula and shows that it is a cloud of gas 1927 - Ira Bowen explains unidentified spectral lines from space as forbidden transition...

External links

• Freeview Video 'Chemistry of Interstellar Space' William Klemperer, Havard University. A Royal Institution Discourse by the Vega Science Trust.