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astronomy Astronomy () is a natural science that studies celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and evolution. Objects of interest include planets, moons, stars, nebulae, g ...
, the interstellar medium is the
matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atoms, which are made up of interacting subatomic part ...
and radiation that exist in the
space Space is the boundless three-dimensional extent in which objects and events have relative position and direction. In classical physics, physical space is often conceived in three linear dimensions, although modern physicists usually consi ...
between the
star system A star system or stellar system is a small number of stars that orbit each other, bound by gravitational attraction. A large group of stars bound by gravitation is generally called a '' star cluster'' or ''galaxy'', although, broadly speaking ...
s in a
galaxy A galaxy is a system of stars, stellar remnants, interstellar gas, dust, dark matter, bound together by gravity. The word is derived from the Greek ' (), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System ...
. This matter includes gas in ionic,
atom Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons. Every solid, liquid, gas, a ...
ic, and
molecular A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bio ...
form, as well as
dust Dust is made of fine particles of solid matter. On Earth, it generally consists of particles in the atmosphere that come from various sources such as soil lifted by wind (an aeolian process), volcanic eruptions, and pollution. Dust in ...
and
cosmic ray Cosmic rays are high-energy particles or clusters of particles (primarily represented by protons or atomic nuclei) that move through space at nearly the speed of light. They originate from the Sun, from outside of the Solar System in our own ...
s. It fills interstellar space and blends smoothly into the surrounding intergalactic space. The
energy In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of ...
that occupies the same volume, in the form of
electromagnetic radiation In physics, electromagnetic radiation (EMR) consists of waves of the electromagnetic (EM) field, which propagate through space and carry momentum and electromagnetic radiant energy. It includes radio waves, microwaves, infrared, (visib ...
, 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 interstellar medium is composed, primarily, of
hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-to ...
, followed by
helium Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic ta ...
with trace amounts of
carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon ma ...
,
oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements ...
, and
nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
. The thermal pressures of these phases are in rough equilibrium with one another.
Magnetic field A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...
s and
turbulent In fluid dynamics, turbulence or turbulent flow is fluid motion characterized by chaotic changes in pressure and flow velocity. It is in contrast to a laminar flow, which occurs when a fluid flows in parallel layers, with no disruption between ...
motions also provide pressure in the ISM, and are typically more important, dynamically, than the thermal pressure is. In the interstellar medium, matter is primarily in molecular form, and reaches number
densities Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematicall ...
of 106 molecules per cm3 (1 million 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 1019 molecules per cm3 for air at sea level, and 1010 molecules per cm3 (10 billion molecules per cm3) for a laboratory high-vacuum chamber. By
mass Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different ele ...
, 99% of the ISM is gas in any form, and 1% is dust. Of the gas in the ISM, by number 91% of atoms are
hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-to ...
and 8.9% are
helium Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic ta ...
, with 0.1% being atoms of elements heavier than hydrogen or helium, known as "
metals A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typical ...
" in 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
enrichment Enrichment may refer to: * Behavioral enrichment, the practice of providing animals under managed care with stimuli such as natural and artificial objects * Data enrichment, appending or enhancing data with relevant context from other sources, se ...
(due to stellar gravity and radiation pressure) in the process of
stellar evolution Stellar evolution is the process by which a star changes over the course of time. Depending on the mass of the star, its lifetime can range from a few million years for the most massive to trillions of years for the least massive, which is cons ...
. The ISM plays a crucial role in
astrophysics Astrophysics is a science that employs the methods and principles of physics and chemistry in the study of astronomical objects and phenomena. As one of the founders of the discipline said, Astrophysics "seeks to ascertain the nature of the h ...
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 clouds and replenishes the ISM with matter and energy through planetary nebulae,
stellar wind A stellar wind is a flow of gas ejected from the upper atmosphere of a star. It is distinguished from the bipolar outflows characteristic of young stars by being less collimated, although stellar winds are not generally spherically symmetric. ...
s, and
supernova A supernova is a powerful and luminous explosion of a star. It has the plural form supernovae or supernovas, and is abbreviated SN or SNe. This transient astronomical event occurs during the last evolutionary stages of a massive star or whe ...
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 ''Voyager 1'' is a space probe launched by NASA on September 5, 1977, as part of the Voyager program to study the outer Solar System and interstellar space beyond the Sun's heliosphere. Launched 16 days after its twin '' Voyager 2'', ''V ...
'' 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 ''Voyager 2'' is a space probe launched by NASA on August 20, 1977, to study the outer planets and interstellar space beyond the Sun's heliosphere. As a part of the Voyager program, it was launched 16 days before its twin, '' Voyager 1'', ...
'' entered the ISM on November 5, 2018.


Interstellar matter

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


The three-phase model

put forward the static two ''phase'' equilibrium model to explain the observed properties of the ISM. Their modeled ISM included of a cold dense phase (''T'' < 300  K), consisting of clouds of neutral and molecular hydrogen, and a warm intercloud phase (''T'' ~ 104  K), consisting of rarefied neutral and ionized gas. added a dynamic third phase that represented the very hot (''T'' ~ 106  K) gas that had been shock heated by
supernova A supernova is a powerful and luminous explosion of a star. It has the plural form supernovae or supernovas, and is abbreviated SN or SNe. This transient astronomical event occurs during the last evolutionary stages of a massive star or whe ...
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.


The atomic hydrogen model

This model takes into account only atomic hydrogen: A temperature higher than 3000 K breaks molecules, while that lower than 50000 K leaves atoms in their ground state. It is assumed that the influence of other atoms (He ...) is negligible. The pressure is assumed to be very low, so the durations of the free paths of atoms are longer than the ~ 1 nanosecond duration of the light pulses that constitute ordinary, temporally incoherent light. In this collisionless gas, Einstein's theory of coherent light-matter interactions applies: all the gas-light interactions are spatially coherent. Suppose that a monochromatic light is pulsed, then scattered by molecules with a quadrupole (Raman) resonance frequency. If the “length of light pulses is shorter than all involved time constants” (Lamb (1971)), an “impulsive stimulated Raman scattering (ISRS)” (Yan, Gamble & Nelson (1985)) applies: the light generated by incoherent Raman scattering at a shifted frequency has a phase independent of the phase of the exciting light, thus generating a new spectral line, and coherence between the incident and scattered light facilitates their interference into a single frequency, thus shifting the incident frequency. Assume that a star radiates a continuous light spectrum up to X-rays. Lyman frequencies are absorbed in this light and pump atoms mainly to the first excited state. In this state, the hyperfine periods are longer than 1 ns, so an ISRS “may” redshift the light frequency, populating high hyperfine levels. Another ISRS “may” transfer energy from hyperfine levels to thermal electromagnetic waves, so the redshift is permanent. The temperature of a light beam is defined by its frequency and spectral radiance with Planck's formula. As entropy must increase, “may” becomes “does”. However, where a previously absorbed line (first Lyman beta, ...) reaches the Lyman alpha frequency, the redshifting process stops, and all hydrogen lines are strongly absorbed. But this stop is not perfect if there is energy at the frequency shifted to Lyman beta frequency, which produces a slow redshift. Successive redshifts separated by Lyman absorptions generate many absorption lines, frequencies of which, deduced from absorption process, obey a law more dependable than Karlsson's formula. The previous process excites more and more atoms because a de-excitation obeys Einstein's law of coherent interactions: Variation dI of radiance I of a light beam along a path dx is dI=BIdx, where B is Einstein amplification coefficient which depends on medium. I is the modulus of Poynting vector of field, absorption occurs for an opposed vector, which corresponds to a change of sign of B. Factor I in this formula shows that intense rays are more amplified than weak ones (competition of modes). Emission of a flare requires a sufficient radiance I provided by random zero point field. After emission of a flare, weak B increases by pumping while I remains close to zero: De-excitation by a coherent emission involves stochastic parameters of zero point field, as observed close to quasars (and in polar auroras).


Structures

The ISM is
turbulent In fluid dynamics, turbulence or turbulent flow is fluid motion characterized by chaotic changes in pressure and flow velocity. It is in contrast to a laminar flow, which occurs when a fluid flows in parallel layers, with no disruption between ...
and therefore full of structure on all spatial scales. Stars are born deep inside large complexes of molecular clouds, typically a few parsecs in size. During their lives and deaths,
star A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...
s interact physically with the ISM.
Stellar wind A stellar wind is a flow of gas ejected from the upper atmosphere of a star. It is distinguished from the bipolar outflows characteristic of young stars by being less collimated, although stellar winds are not generally spherically symmetric. ...
s from young clusters of stars (often with giant or supergiant
HII region An H II region or HII region is a region of interstellar atomic hydrogen that is ionized. It is typically in a molecular cloud of partially ionized gas in which star formation has recently taken place, with a size ranging from one to hundreds ...
s surrounding them) and
shock wave In physics, a shock wave (also spelled shockwave), or shock, is a type of propagating disturbance that moves faster than the local speed of sound in the medium. Like an ordinary wave, a shock wave carries energy and can propagate through a me ...
s created by
supernova A supernova is a powerful and luminous explosion of a star. It has the plural form supernovae or supernovas, and is abbreviated SN or SNe. This transient astronomical event occurs during the last evolutionary stages of a massive star or whe ...
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 A stellar-wind bubble is a cavity light-years across filled with hot gas blown into the interstellar medium by the high-velocity (several thousand km/s) stellar wind from a single massive star of type O or B. Weaker stellar winds also blow bub ...
s and
superbubble A superbubble or supershell is a cavity which is hundreds of light years across and is populated with hot (106  K) gas atoms, less dense than the surrounding interstellar medium, blown against that medium and carved out by multiple superno ...
s of hot gas, seen by X-ray satellite telescopes or turbulent flows observed in
radio telescope A radio telescope is a specialized antenna and radio receiver used to detect radio waves from astronomical radio sources in the sky. Radio telescopes are the main observing instrument used in radio astronomy, which studies the radio frequency ...
maps. The Sun is currently traveling through the
Local Interstellar Cloud The Local Interstellar Cloud (LIC), also known as the Local Fluff, is an interstellar cloud roughly across, through which the Solar System is moving. This feature overlaps a region around the Sun referred to as the solar neighborhood. It is un ...
, a denser region in the low-density Local Bubble. In October 2020, astronomers reported a significant unexpected increase in density in the
space Space is the boundless three-dimensional extent in which objects and events have relative position and direction. In classical physics, physical space is often conceived in three linear dimensions, although modern physicists usually consi ...
beyond the
Solar System The Solar System Capitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar ...
as detected by the ''
Voyager 1 ''Voyager 1'' is a space probe launched by NASA on September 5, 1977, as part of the Voyager program to study the outer Solar System and interstellar space beyond the Sun's heliosphere. Launched 16 days after its twin '' Voyager 2'', ''V ...
'' and ''
Voyager 2 ''Voyager 2'' is a space probe launched by NASA on August 20, 1977, to study the outer planets and interstellar space beyond the Sun's heliosphere. As a part of the Voyager program, it was launched 16 days before its twin, '' Voyager 1'', ...
''
space probe A space probe is an artificial satellite that travels through space to collect scientific data. A space probe may orbit Earth; approach the Moon; travel through interplanetary space; flyby, orbit, or land or fly on other planetary bodies; o ...
s. According to the researchers, this implies that "the density gradient is a large-scale feature of the VLISM (very local interstellar medium) in the general direction of the heliospheric nose".


Interaction with interplanetary medium

The interstellar medium begins where the interplanetary medium of the
Solar System The Solar System Capitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar ...
ends. The solar wind slows to subsonic velocities at the
termination shock The heliosphere is the magnetosphere, astrosphere and outermost atmospheric layer of the Sun. It takes the shape of a vast, bubble-like region of space. In plasma physics terms, it is the cavity formed by the Sun in the surrounding interstell ...
, 90–100
astronomical unit The astronomical unit (symbol: au, or or AU) is a unit of length, roughly the distance from Earth to the Sun and approximately equal to or 8.3 light-minutes. The actual distance from Earth to the Sun varies by about 3% as Earth orbits ...
s from the Sun. In the region beyond the termination shock, called the heliosheath, interstellar matter interacts with the solar wind. ''
Voyager 1 ''Voyager 1'' is a space probe launched by NASA on September 5, 1977, as part of the Voyager program to study the outer Solar System and interstellar space beyond the Sun's heliosphere. Launched 16 days after its twin '' Voyager 2'', ''V ...
'', the farthest human-made object from the Earth (after 1998), crossed the termination shock December 16, 2004 and later entered interstellar space when it crossed the heliopause on August 25, 2012, providing the first direct probe of conditions in the ISM .


Interstellar extinction

The ISM is also responsible for
extinction Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the Endling, last individual of the species, although the Functional ext ...
and reddening, the decreasing light intensity and shift in the dominant observable
wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tr ...
s of light from a star. These effects are caused by scattering and absorption of
photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they alwa ...
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 The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. ...
– a uniform disk of stars – are caused by absorption of background starlight by molecular clouds within a few thousand light years from Earth. Far ultraviolet light is absorbed effectively by the neutral components of the ISM. For example, a typical absorption wavelength of atomic
hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-to ...
lies at about 121.5 nanometers, the
Lyman-alpha The Lyman-alpha line, typically denoted by Ly-α, is a spectral line of hydrogen (or, more generally, of any one-electron atom) in the Lyman series. It is emitted when the atomic electron transitions from an ''n'' = 2 orbital to the ...
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.


Heating and cooling

The ISM is usually far from
thermodynamic equilibrium Thermodynamic equilibrium is an axiomatic concept of thermodynamics. It is an internal state of a single thermodynamic system, or a relation between several thermodynamic systems connected by more or less permeable or impermeable walls. In the ...
. Collisions establish a
Maxwell–Boltzmann distribution In physics (in particular in statistical mechanics), the Maxwell–Boltzmann distribution, or Maxwell(ian) distribution, is a particular probability distribution named after James Clerk Maxwell and Ludwig Boltzmann. It was first defined and use ...
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 A star (surface temperature of ~10,000 K) highly diluted. Therefore, bound levels within an
atom Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons. Every solid, liquid, gas, a ...
or
molecule A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and b ...
in the ISM are rarely populated according to the Boltzmann formula . 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.


Heating mechanisms

; Heating by low-energy
cosmic ray Cosmic rays are high-energy particles or clusters of particles (primarily represented by protons or atomic nuclei) that move through space at nearly the speed of light. They originate from the Sun, from outside of the Solar System in our own ...
s: The first mechanism proposed for heating the ISM was heating by low-energy
cosmic rays Cosmic rays are high-energy particles or clusters of particles (primarily represented by protons or atomic nuclei) that move through space at nearly the speed of light. They originate from the Sun, from outside of the Solar System in our ...
.
Cosmic ray Cosmic rays are high-energy particles or clusters of particles (primarily represented by protons or atomic nuclei) that move through space at nearly the speed of light. They originate from the Sun, from outside of the Solar System in our own ...
s are an efficient heating source able to penetrate in the depths of molecular clouds.
Cosmic ray Cosmic rays are high-energy particles or clusters of particles (primarily represented by protons or atomic nuclei) that move through space at nearly the speed of light. They originate from the Sun, from outside of the Solar System in our own ...
s transfer energy to gas through both ionization and excitation and to free
electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have n ...
s through
Coulomb The coulomb (symbol: C) is the unit of electric charge in the International System of Units (SI). In the present version of the SI it is equal to the electric charge delivered by a 1 ampere constant current in 1 second and to elementary char ...
interactions. Low-energy
cosmic ray Cosmic rays are high-energy particles or clusters of particles (primarily represented by protons or atomic nuclei) that move through space at nearly the speed of light. They originate from the Sun, from outside of the Solar System in our own ...
s (a few MeV) are more important because they are far more numerous than high-energy
cosmic ray Cosmic rays are high-energy particles or clusters of particles (primarily represented by protons or atomic nuclei) that move through space at nearly the speed of light. They originate from the Sun, from outside of the Solar System in our own ...
s. ; Photoelectric heating by grains: The
ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30  PHz) to 400 nm (750  THz), shorter than that of visible light, but longer than X-rays. UV radiation ...
radiation emitted by hot
star A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...
s can remove
electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have n ...
s from dust grains. The
photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they alwa ...
is absorbed by the dust grain, and some of its energy is used to overcome the potential energy barrier and remove the
electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have n ...
from the grain. This potential barrier is due to the binding energy of the electron (the
work function In solid-state physics, the work function (sometimes spelt workfunction) is the minimum thermodynamic work (i.e., energy) needed to remove an electron from a solid to a point in the vacuum immediately outside the solid surface. Here "immediately ...
) and the charge of the grain. The remainder of the photon's energy gives the ejected
electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have n ...
kinetic energy In physics, the kinetic energy of an object is the energy that it possesses due to its motion. It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acce ...
which heats the gas through collisions with other particles. A typical size distribution of dust grains is ''n''(''r'') ∝ ''r'', where ''r'' is the radius of the dust particle. Assuming this, the projected grain surface area distribution is ''πr'n''(''r'') ∝ ''r''. This indicates that the smallest dust grains dominate this method of heating. ; Photoionization: When an
electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have n ...
is freed from an
atom Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons. Every solid, liquid, gas, a ...
(typically from absorption of a UV
photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they alwa ...
) it carries kinetic energy away of the order ''E'' − ''E''. This heating mechanism dominates in H II regions, but is negligible in the diffuse ISM due to the relative lack of neutral
carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon ma ...
atom Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons. Every solid, liquid, gas, a ...
s. ;
X-ray An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10  nanometers, corresponding to frequencies in the range 30&nb ...
heating:
X-ray An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10  nanometers, corresponding to frequencies in the range 30&nb ...
s remove
electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have n ...
s from
atom Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons. Every solid, liquid, gas, a ...
s 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-ray An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10  nanometers, corresponding to frequencies in the range 30&nb ...
s can penetrate and
x-ray An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10  nanometers, corresponding to frequencies in the range 30&nb ...
heating can be ignored. This is assuming the region is not near an
x-ray An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10  nanometers, corresponding to frequencies in the range 30&nb ...
source such as a supernova remnant. ; Chemical heating: Molecular
hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-to ...
(H2) can be formed on the surface of dust grains when two 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 transferred 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 less 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 remnants 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 Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of Light, visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from ...
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: \alpha = \frac where ''T'' is the gas temperature, ''Td'' the dust temperature, and ''T''2 the post-collision temperature of the gas atom or molecule. This coefficient was measured by as ''α'' = 0.35. ; Other heating mechanisms: A variety of macroscopic heating mechanisms are present including: :* Gravitational collapse of a cloud :*
Supernova A supernova is a powerful and luminous explosion of a star. It has the plural form supernovae or supernovas, and is abbreviated SN or SNe. This transient astronomical event occurs during the last evolutionary stages of a massive star or whe ...
explosions :*
Stellar wind A stellar wind is a flow of gas ejected from the upper atmosphere of a star. It is distinguished from the bipolar outflows characteristic of young stars by being less collimated, although stellar winds are not generally spherically symmetric. ...
s :* Expansion of H II regions :* Magnetohydrodynamic waves created by supernova remnants


Cooling mechanisms

; 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 Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons. Every solid, liquid, gas, a ...
s having fine structure levels close to the fundamental level such as: C II and O I in the neutral medium and O II, O III, N II, N III, Ne II and Ne III in H II 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. ; 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 Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-to ...
will release a Ly-α photon upon de-excitation. In molecular clouds, excitation of rotational lines of CO is important. Once a
molecule A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and b ...
is excited, it eventually returns to a lower energy state, emitting a photon which can leave the region, cooling the cloud.


Radiowave propagation

Radio waves from ≈10 kHz ( very low frequency) to ≈300 GHz ( extremely high frequency) propagate differently in interstellar space than on the Earth's surface. There are many sources of interference and signal distortion that do not exist on Earth. A great deal of
radio astronomy Radio astronomy is a subfield of astronomy that studies celestial objects at radio frequencies. The first detection of radio waves from an astronomical object was in 1933, when Karl Jansky at Bell Telephone Laboratories reported radiation comin ...
depends on compensating for the different propagation effects to uncover the desired signal.


Discoveries

In 1864, William Huggins used spectroscopy to determine that a nebula is made of gas. Huggins had a private observatory with an 8-inch telescope, with a lens by Alvin Clark; but it was equipped for spectroscopy which enabled breakthrough observations. In 1904, one of the discoveries made using the Potsdam Great Refractor telescope was of calcium in the interstellar medium. The astronomer Johannes Frank Hartmann determined from spectrograph observations of the binary star Mintaka in Orion, that there was the element
calcium Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar ...
in the intervening space. Interstellar gas was further confirmed by Slipher in 1909, and then by 1912 interstellar dust was confirmed by Slipher. In this way the overall nature of the interstellar medium was confirmed in a series of discoveries and postulizations of its nature. In September 2020, evidence was presented of solid-state water in the interstellar medium, and particularly, of water ice mixed with silicate grains in cosmic dust grains.


History of knowledge of interstellar space

The nature of the interstellar medium has received the attention of astronomers and scientists over the centuries and 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 : "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 discussed "The inter-stellar part of heaven, which several of the modern
Epicureans Epicureanism is a system of philosophy founded around 307 BC based upon the teachings of the ancient Greek philosopher Epicurus. Epicureanism was originally a challenge to Platonism. Later its main opponent became Stoicism. Few writings by ...
would have to be empty." Before modern electromagnetic theory, early
physicist A physicist is a scientist who specializes in the field of physics, which encompasses the interactions of matter and energy at all length and time scales in the physical universe. Physicists generally are interested in the root or ultimate cau ...
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 wrote, "this efflux occasions a thrill, or vibratory motion, in the
ether In organic chemistry, ethers are a class of compounds that contain an ether group—an oxygen atom connected to two alkyl or aryl groups. They have the general formula , where R and R′ represent the alkyl or aryl groups. Ethers can again ...
which fills the interstellar spaces." The advent of deep photographic imaging allowed Edward Barnard to produce the first images of
dark nebula A dark nebula or absorption nebula is a type of interstellar cloud, particularly molecular clouds, that is so dense that it obscures the visible wavelengths of light from objects behind it, such as background stars and emission or reflection neb ...
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 Hartmann Johannes Hartmann (Amberg, 14 January 1568 – Kassel, 7 December 1631) was a German chemist A chemist (from Greek ''chēm(ía)'' alchemy; replacing ''chymist'' from Medieval Latin ''alchemist'') is a scientist trained in the study of ...
in 1904 through the use of absorption line spectroscopy. In his historic study of the spectrum and orbit of
Delta Orionis Mintaka , designation Delta Orionis (δ Orionis, abbreviated Delta Ori, δ Ori) and 34 Orionis (34 Ori), is a multiple star system some 1,200 light-years from the Sun in the constellation of Orion. Together with Alnitak (Zeta Ori ...
, 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 Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar ...
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 A binary star is a system of two stars that are gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved using a telescope as separate stars, in w ...
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. In the series of investigations,
Viktor Ambartsumian Viktor Amazaspovich Ambartsumian (russian: Виктор Амазаспович Амбарцумян; hy, Վիկտոր Համազասպի Համբարձումյան, ''Viktor Hamazaspi Hambardzumyan''; 12 August 1996) was a Soviet Armenian ast ...
introduced the now commonly accepted notion that interstellar matter occurs in the form of clouds. Following Hartmann's identification of interstellar calcium absorption, interstellar
sodium Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable ...
was detected by 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. Subsequent observations of the "H" and "K" lines of calcium by revealed double and asymmetric profiles in the spectra of
Epsilon Epsilon (, ; uppercase , lowercase or lunate ; el, έψιλον) is the fifth letter of the Greek alphabet, corresponding phonetically to a mid front unrounded vowel or . In the system of Greek numerals it also has the value five. It was d ...
and
Zeta Orionis Alnitak is a triple star system in the constellation of Orion. It has the designations ζ Orionis, which is Latinised to Zeta Orionis and abbreviated Zeta Ori or ζ Ori, and 50 Orionis, abbreviated 50 Ori. The system is located at a ...
. 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 are either blue-shifted or red-shifted (respectively) from the lines' rest wavelength, through the
Doppler Effect The Doppler effect or Doppler shift (or simply Doppler, when in context) is the change in frequency of a wave in relation to an observer who is moving relative to the wave source. It is named after the Austrian physicist Christian Doppler, who ...
. These observations confirming that matter is not distributed homogeneously were the first evidence of multiple discrete clouds within the ISM. The growing evidence for interstellar material led to comment 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
molecule A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and b ...
s are certainly there, since they are probably constantly being expelled by the Sun and
star A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...
s." The same year
Victor Hess Victor Franz Hess (; 24 June 188317 December 1964) was an Austrian-American physicist, and Nobel laureate in physics, who discovered cosmic rays. Biography He was born to Vinzenz Hess and Serafine Edle von Grossbauer-Waldstätt, in Waldstein ...
's discovery of
cosmic rays Cosmic rays are high-energy particles or clusters of particles (primarily represented by protons or atomic nuclei) that move through space at nearly the speed of light. They originate from the Sun, from outside of the Solar System in our ...
, 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 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 systems or
nebula A nebula ('cloud' or 'fog' in Latin; pl. nebulae, nebulæ or nebulas) is a distinct luminescent part of interstellar medium, which can consist of ionized, neutral or molecular hydrogen and also cosmic dust. Nebulae are often star-forming regio ...
e, but in 'empty' space" . 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 emitted by the Sun. If the millions of other
star A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...
s are also ejecting ions, as is undoubtedly true, no absolute vacuum can exist within the galaxy." In September 2012, NASA scientists reported that polycyclic aromatic hydrocarbons (PAHs), subjected to ''interstellar medium (ISM)'' conditions, are transformed, through
hydrogenation Hydrogenation is a chemical reaction between molecular hydrogen (H2) and another compound or element, usually in the presence of a catalyst such as nickel, palladium or platinum. The process is commonly employed to reduce or saturate organic ...
, oxygenation and
hydroxylation In chemistry, hydroxylation can refer to: *(i) most commonly, hydroxylation describes a chemical process that introduces a hydroxyl group () into an organic compound. *(ii) the ''degree of hydroxylation'' refers to the number of OH groups in a ...
, to more complex organics – "a step along the path toward
amino acids Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha am ...
and nucleotides, the raw materials of
proteins Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...
and DNA, respectively". Further, as a result of these transformations, the PAHs lose their spectroscopic signature which could be one of the reasons "for the lack of PAH detection in
interstellar ice Interstellar ice consists of grains of volatiles in the ice phase that form in the interstellar medium. Ice and dust grains form the primary material out of which the Solar System was formed. Grains of ice are found in the dense regions of molecular ...
grains, particularly the outer regions of cold, dense clouds or the upper molecular layers of
protoplanetary disks A protoplanetary disk is a rotating circumstellar disc of dense gas and dust surrounding a young newly formed star, a T Tauri star, or Herbig Ae/Be star. The protoplanetary disk may also be considered an accretion disk for the star itself, be ...
." In February 2014,
NASA The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research. NASA was established in 1958, succeedin ...
announced a greatly upgraded database for tracking polycyclic aromatic hydrocarbons (PAHs) in the universe. According to scientists, more than 20% of the
carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon ma ...
in the universe may be associated with PAHs, possible starting materials for the
formation Formation may refer to: Linguistics * Back-formation, the process of creating a new lexeme by removing or affixes * Word formation, the creation of a new word by adding affixes Mathematics and science * Cave formation or speleothem, a secondar ...
of
life Life is a quality that distinguishes matter that has biological processes, such as signaling and self-sustaining processes, from that which does not, and is defined by the capacity for growth, reaction to stimuli, metabolism, energy ...
. PAHs seem to have been formed shortly after the
Big Bang The Big Bang event is a physical theory that describes how the universe expanded from an initial state of high density and temperature. Various cosmological models of the Big Bang explain the evolution of the observable universe from the ...
, are widespread throughout the universe, and are associated with new stars and exoplanets. In April 2019, scientists, working with the
Hubble Space Telescope The Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the first space telescope, but it is one of the largest and most vers ...
, reported the confirmed detection of the large and complex ionized molecules of buckminsterfullerene (C60) (also known as "buckyballs") in the interstellar medium spaces between the
star A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...
s.


See also

* Astrophysical maser * Diffuse interstellar band *
Fossil stellar magnetic field Fossil stellar magnetic fields or ''fossil fields'' are proposed as possible interstellar magnetic fields that became locked into certain stars. Vincent Duez, Stéphane Mathis, Sylvaine Turck-Chièze. ''Effect of a fossil magnetic field on the stru ...
* Heliosphere * List of interstellar and circumstellar molecules * List of plasma physics articles *
Photodissociation region In astrophysics, photodissociation regions (or photon-dominated regions, PDRs) are predominantly neutral regions of the interstellar medium in which far ultraviolet photons strongly influence the gas chemistry and act as the most important source ...


References


Citations


Sources

* * * * * * * * * Th
Wisconsin Hα Mapper
is funded by the
National Science Foundation The National Science Foundation (NSF) is an independent agency of the United States government that supports fundamental research and education in all the non-medical fields of science and engineering. Its medical counterpart is the National ...
. * * * * * * * * * *


External links


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


{{DEFAULTSORT:Interstellar Medium Astrochemistry Medium, interstellar Articles containing video clips Concepts in astronomy