Stellar kinematics

From Wikipedia, the free encyclopedia

In astronomy, stellar kinematics is the observational study or measurement of the kinematics or motions of stars through space. The subject of stellar kinematics encompasses the measurement of stellar velocities in the Milky Way and its satellites as well as the measurement of the internal kinematics of more distant galaxies. Measurement of the kinematics of stars in different subcomponents of the Milky Way including the thin disk, the thick disk, the bulge, and the stellar halo provides important information about the formation and evolutionary history of our Galaxy. Kinematic measurements can also identify exotic phenomena such as hypervelocity stars escaping from the Milky Way, which are interpreted as the result of gravitational encounters of binary stars with the supermassive black hole at the Galactic Center. 

Stellar kinematics is related to but distinct from the subject of stellar dynamics, which involves the theoretical study or modeling of the motions of stars under the influence of gravity. Stellar-dynamical models of systems such as galaxies or star clusters are often compared with or tested against stellar-kinematic data to study their evolutionary history and mass distributions, and to detect the presence of dark matter or supermassive black holes through their gravitational influence on stellar orbits.

Space velocity

Relation between proper motion and velocity components of an object. At emission, the object was at distance d from the Sun, and moved at angular rate μ radian/s, that is, μ = v_t / d with v_t = the component of velocity transverse to line of sight from the Sun. (The diagram illustrates an angle μ swept out in unit time at tangential velocity v_t.)

The component of stellar motion toward or away from the Sun, known as radial velocity, can be measured from the spectrum shift caused by the Doppler effect. The transverse, or proper motion must be found by taking a series of positional determinations against more distant objects. Once the distance to a star is determined through astrometric means such as parallax, the space velocity can be computed. This is the star's actual motion relative to the Sun or the local standard of rest (LSR). The latter is typically taken as a position at the Sun's present location that is following a circular orbit around the Galactic Center at the mean velocity of those nearby stars with low velocity dispersion. The Sun's motion with respect to the LSR is called the "peculiar solar motion".

The components of space velocity in the Milky Way's Galactic coordinate system are usually designated U, V, and W, given in km/s, with U positive in the direction of the Galactic Center, V positive in the direction of galactic rotation, and W positive in the direction of the North Galactic Pole. The peculiar motion of the Sun with respect to the LSR is

(U, V, W) = (11.1, 12.24, 7.25) km/s,

with statistical uncertainty (+0.69−0.75, +0.47−0.47, +0.37−0.36) km/s and systematic uncertainty (1, 2, 0.5) km/s. (Note that V is 7 km/s larger than estimated in 1999 by Dehnen et al.)

Information Obtained from Stellar Kinematic Measurements

Stellar kinematics yields important astrophysical information about stars, and the galaxies in which they reside. Stellar kinematics data combined with astrophysical modeling produces important information about the galactic system as a whole. Measured stellar velocities in the innermost regions of galaxies including the Milky Way have provided evidence that many galaxies host supermassive black holes at their center. In farther out regions of galaxies such as within the galactic halo, velocity measurements of globular clusters orbiting in these halo regions of galaxies provides evidence for dark matter. Both of these cases derive from the key fact that stellar kinematics can be related to the overall potential in which the stars are bound. This means that if accurate stellar kinematics measurements are made for a star or group of stars orbiting in a certain region of a galaxy, the gravitational potential and mass distribution can be inferred given that the gravitational potential in which the star is bound produces its orbit and serves as the impetus for its stellar motion. Examples of using stellar kinematics measurements combined with modeling to construct an astrophysical system are:

• Rotation of the Milky Way's Disc From the proper motions and radial velocities of stars within the Milky way disc one can show that there is differential rotation. When combining these measurements of stars' proper motions and their radial velocities, along with careful modeling, it is possible to obtain a picture of the rotation of the Milky Way disc. The local character of galactic rotation in the solar neighborhood is encapsulated in the Oort constants.
• Structural Components of The Milky Way Using stellar kinematics, astronomers construct models which seek to explain the overall galactic structure in terms of distinct kinematic populations of stars. This is possible because these distinct populations are often located in specific regions of galaxies. For example, within the Milky Way, there are three primary components, each with its own distinct stellar kinematics: the disc, halo and bulge or bar. These kinematic groups are closely related to the stellar populations in the Milky Way, forming a strong correlation between the motion and chemical composition, thus indicating different formation mechanisms. For the Milky Way, the speed of disk stars is ${\displaystyle \mathrm {V} =220~\mathrm {km} ~\mathrm {s} ^{-1}}$ and an RMS velocity relative to this speed of ${\displaystyle \mathrm {V_{RMS}} =50~\mathrm {km} ~\mathrm {s} ^{-1}}$. For bulge population stars, the velocities are randomly oriented with a larger relative RMS velocity of ${\displaystyle \mathrm {V_{RMS}} =150~\mathrm {km} ~\mathrm {s} ^{-1}}$ and no net circular velocity. The Galactic stellar halo consists of stars with orbits that extend to the outer regions of the galaxy. Some of these stars will continually orbit far from the galactic center, while others are on trajectories which bring them to various distances from the galactic center. These stars have little to no average rotation. Many stars in this group belong to globular clusters which formed long ago and thus have a distinct formation history, which can be inferred from their kinematics and poor metallicities. The halo may be further subdivided into an inner and outer halo, with the inner halo having a net prograde motion with respect to the Milky Way and the outer a net retrograde motion.
• External Galaxies Spectroscopic observations of external galaxies make it possible to characterize the bulk motions of the stars they contain. While these stellar populations in external galaxies are generally not resolved to the level where one can track the motion of individual stars (except for the very nearest galaxies) measurements of the kinematics of the integrated stellar population along the line of sight provides information including the mean velocity and the velocity dispersion which can then be used to infer the distribution of mass within the galaxy. Measurement of the mean velocity as a function of position gives information on the galaxy's rotation, with distinct regions of the galaxy that are redshifted / blueshifted in relation to the galaxy's systemic velocity.
• Mass distributions Through measurement of the kinematics of tracer objects such as globular clusters and the orbits of nearby satellite dwarf galaxies, we can determine the mass distribution of the Milky Way or other galaxies. This is accomplished by combining kinematic measurements with dynamical modeling.

Recent advancements due to Gaia

In 2018 the Gaia data release 2 has yielded an unprecedented number of high quality stellar kinematic measurements as well as stellar parallax measurements which will greatly increase our understanding of the structure of the Milky Way. The Gaia data has also made it possible to determine the proper motions of many objects whose proper motions were previously unknown, including the absolute proper motions of 75 globular clusters orbiting at distances as far as 21 kpc. In addition, the absolute proper motions of nearby dwarf spheroidal galaxies have also been measured, providing multiple tracers of mass for the Milky Way. This increase in accurate measurement of absolute proper motion at such large distances is a major improvement over past surveys, such as those conducted with the Hubble Space Telescope.

Stellar kinematic types

Stars within galaxies may be classified based on their kinematics. For example, the stars in the Milky Way can be subdivided into two general populations, based on their metallicity, or proportion of elements with atomic numbers higher than helium. Among nearby stars, it has been found that population I stars with higher metallicity are generally located in the stellar disk while older population II stars are in random orbits with little net rotation. The latter have elliptical orbits that are inclined to the plane of the Milky Way. Comparison of the kinematics of nearby stars has also led to the identification of stellar associations. These are most likely groups of stars that share a common point of origin in giant molecular clouds.

There are many additional ways to classify stars based on their measured velocity components, and this provides detailed information about the nature of the star's formation time, its present location, and the general structure of the galaxy. As a star moves in a galaxy, the smoothed out gravitational potential of all the other stars and other mass within the galaxy plays a dominant role in determining the stellar motion. Stellar kinematics can provide insights into the location of where the star formed within the galaxy. Measurements of an individual star's kinematics can identify stars that are peculiar outliers such as a high-velocity star moving much faster than its nearby neighbors.

High-velocity stars

Depending on the definition, a high-velocity star is a star moving faster than 65 km/s to 100 km/s relative to the average motion of the stars in the Sun's neighborhood. The velocity is also sometimes defined as supersonic relative to the surrounding interstellar medium. The three types of high-velocity stars are: runaway stars, halo stars and hypervelocity stars. High-velocity stars were studied by Jan Oort, who used their kinematic data to predict that high-velocity stars have very little tangential velocity.

Runaway stars

Four runaway stars plowing through regions of dense interstellar gas and creating bright bow waves and trailing tails of glowing gas. The stars in these NASA Hubble Space Telescope images are among 14 young runaway stars spotted by the Advanced Camera for Surveys between October 2005 and July 2006

A runaway star is one that is moving through space with an abnormally high velocity relative to the surrounding interstellar medium. The proper motion of a runaway star often points exactly away from a stellar association, of which the star was formerly a member, before it was hurled out.

Mechanisms that may give rise to a runaway star include:

• Gravitational interactions between stars in a stellar system can result in large accelerations of one or more of the involved stars. In some cases, stars may even be ejected. This can occur in seemingly stable star systems of only three stars, as described in studies of the three-body problem in gravitational theory.
• A collision or close encounter between stellar systems, including galaxies, may result in the disruption of both systems, with some of the stars being accelerated to high velocities, or even ejected. A large-scale example is the gravitational interaction between the Milky Way Galaxy and the Large Magellanic Cloud.
• A supernova explosion in a multiple star system can accelerate both the supernova remnant and/or remaining stars to high velocities.

Multiple mechanisms may accelerate the same runaway star. For example, a massive star that was originally ejected due to gravitational interactions with its stellar neighbors may itself go supernova, producing a remnant with a velocity modulated by the supernova kick. If this supernova occurs in the very nearby vicinity of other stars, it is possible that it may produce more runaways in the process.

An example of a related set of runaway stars is the case of AE Aurigae, 53 Arietis and Mu Columbae, all of which are moving away from each other at velocities of over 100 km/s (for comparison, the Sun moves through the Milky Way at about 20 km/s faster than the local average). Tracing their motions back, their paths intersect near to the Orion Nebula about 2 million years ago. Barnard's Loop is believed to be the remnant of the supernova that launched the other stars.

Another example is the X-ray object Vela X-1, where photodigital techniques reveal the presence of a typical supersonic bow shock hyperbola.

Halo stars

Halo stars are very old stars that do not share the motion of the Sun or most other stars in the solar neighbourhood which are in similar circular orbits around the center of the Milky Way. Rather, they travel in elliptical orbits, which often take them well outside the plane of the Milky Way. Although their orbital velocities in the Milky Way may be no faster than the Sun's, their different paths result in the high relative velocities.
Typical examples are the halo stars passing through the disk of the Milky Way at steep angles. One of the nearest 45 stars, called Kapteyn's Star, is an example of the high-velocity stars that lie near the Sun. Its observed radial velocity is -245 km/s, and the components of its space velocity are U = 19 km/s, V = -288 km/s, and W = -52 km/s.

Hypervelocity stars

Hypervelocity stars (designated as HVS or HV in stellar catalogues) are stars with velocities that are substantially different from that expected for a star belonging to the normal distribution of stars in a galaxy. Such stars may have velocities so great that they exceed the escape velocity of the galaxy. Ordinary stars in the Milky Way have velocities on the order of 100 km/s, whereas hypervelocity stars (especially those near the center of the Milky Way, which is where most are thought to be produced), have velocities on the order of 1000 km/s. One of the fastest known stars discovered in our Galaxy is the O-class sub-dwarf US 708, moving away from the Milky Way with a total velocity of around 1200 km/s.

The existence of HVSs was first predicted by Jack G. Hills in 1988, and their existence confirmed by Warren Brown, Margaret Geller, Scott Kenyon, and Michael Kurtz in 2005. As of 2008, 10 unbound HVSs were known, one of which was believed to have originated from the Large Magellanic Cloud rather than the Milky Way. Further measurements placed its origin within the Milky Way. Due to uncertainty about the mass distribution within the Milky Way, determining whether a HVS is unbound is difficult; five additional known high-velocity stars may be unbound from the Milky Way and 16 HVSs are thought to be bound. The nearest currently known HVS (HVS2) is about 19 kpc from the Sun.

As of 1 September 2017, there have been roughly 20 observed hypervelocity stars. As most of these were observed in the Northern Hemisphere, the possibility remains that there are HVSs only observable from the Southern Hemisphere.

It is believed that about 1,000 HVSs exist in the Milky Way. Considering that there are around 100 billion stars in the Milky Way, this is a minuscule fraction (~0.000001%). Since the second data release of Gaia (DR2), most high-velocity late-type stars are found to have a high probability of being bound to the Milky Way.

Origin of hypervelocity stars

HVSs are believed to originate by close encounters of binary stars with the supermassive black hole in the center of the Milky Way. One of the two partners is captured by the black hole, whereas the other escapes with high velocity. Also, "captured" does not necessarily mean "swallowed", as the companion to the HVS may enter an orbit around the black hole. However, this can only happen if the binary stars are falling nearly directly toward the black hole from extremely far, otherwise the speed gain would not be sufficiently high.

Supernova-induced HVSs may also be possible, although they are presumably rare. In this scenario, a HVS is ejected from a close binary system as a result of the companion star undergoing a supernova explosion. Ejection velocities up to 770 km/s, as measured from the galactic rest frame, are possible for late-type B-stars. This mechanism can explain the origin of HVSs which are ejected from the galactic disk.

Known HVSs are main-sequence stars with masses a few times that of the Sun. HVSs with smaller masses are also expected and G/K-dwarf HVS candidates have been found.

HVSs that have come into the Milky Way came from the dwarf galaxy Large Magellanic Cloud. When the dwarf galaxy made its closest approach to the center of the Milky Way, it underwent intense gravitational tugs. These tugs boosted the energy of some of its stars so much that they broke free of the dwarf galaxy entirely and were thrown into space, due to the slingshot-like effect of the boost.

Some neutron stars are inferred to be traveling with similar speeds. This could be related to HVSs and the HVS ejection mechanism. Neutron stars are the remnants of supernova explosions, and their extreme speeds are very likely the result of an asymmetric supernova explosion or the loss of their near partner during the supernova explosions that forms them. The neutron star RX J0822-4300, which was measured to move at a record speed of over 1,500 km/s (0.5% of the speed of light) in 2007 by the Chandra X-ray Observatory, is thought to have been produced the first way.

Some kind of supernovas are expected to happen if a white dwarf collides with its nearby partner and consumes the outer matter of this partner. The white dwarf and its nearby partner have very high orbital velocities at this time. The huge mass lost of the white dwarf during the supernova causes the nearby partner to leave at its previous huge orbital speed of several hundred kilometers per second as a hypervelocity star. The supernova remnant of the exploding white dwarf leaves because of its own high orbital speed as a new fast-traveling neutron star. This seems to be the most likely origin of most HVSs and fast-traveling neutron stars.

Partial list of HVSs

As of 2014, twenty HVS were known.

• HVS 1 – (SDSS J090744.99+024506.8) (a.k.a. The Outcast Star) – the first hypervelocity star to be discovered
• HVS 2 – (SDSS J093320.86+441705.4 or US 708)
• HVS 3 – (HE 0437-5439) – possibly from the Large Magellanic Cloud
• HVS 4 – (SDSS J091301.00+305120.0)
• HVS 5 – (SDSS J091759.42+672238.7)
• HVS 6 – (SDSS J110557.45+093439.5)
• HVS 7 – (SDSS J113312.12+010824.9)
• HVS 8 – (SDSS J094214.04+200322.1)
• HVS 9 – (SDSS J102137.08-005234.8)
• HVS 10 – (SDSS J120337.85+180250.4)
• TYC 8840-1782-1

Kinematic groups

A set of stars with similar space motion and ages is known as a kinematic group. These are stars that could share a common origin, such as the evaporation of an open cluster, the remains of a star forming region, or collections of overlapping star formation bursts at differing time periods in adjacent regions. Most stars are born within molecular clouds known as stellar nurseries. The stars formed within such a cloud compose gravitationally bound open clusters containing dozens to thousands of members with similar ages and compositions. These clusters dissociate with time. Groups of young stars that escape a cluster, or are no longer bound to each other, form stellar associations. As these stars age and disperse, their association is no longer readily apparent and they become moving groups of stars.

Astronomers are able to determine if stars are members of a kinematic group because they share the same age, metallicity, and kinematics (radial velocity and proper motion). As the stars in a moving group formed in proximity and at nearly the same time from the same gas cloud, although later disrupted by tidal forces, they share similar characteristics.

Stellar associations

A stellar association is a very loose star cluster, whose stars share a common origin, but have become gravitationally unbound and are still moving together through space. Associations are primarily identified by their common movement vectors and ages. Identification by chemical composition is also used to factor in association memberships.

Stellar associations were first discovered by the Armenian astronomer Viktor Ambartsumian in 1947. The conventional name for an association uses the names or abbreviations of the constellation (or constellations) in which they are located; the association type, and, sometimes, a numerical identifier.

Types

Infrared ESO's VISTA view of a stellar nursery in Monoceros.

Viktor Ambartsumian first categorized stellar associations into two groups, OB and T, based on the properties of their stars. A third category, R, was later suggested by Sidney van den Bergh for associations that illuminate reflection nebulae. The OB, T, and R associations form a continuum of young stellar groupings. But it is currently uncertain whether they are an evolutionary sequence, or represent some other factor at work. Some groups also display properties of both OB and T associations, so the categorization is not always clear-cut.

OB associations

Carina OB1, a large OB association.

Young associations will contain 10 to 100 massive stars of spectral class O and B, and are known as OB associations. In addition, these associations also contain hundreds or thousands of low- and intermediate-mass stars. Association members are believed to form within the same small volume inside a giant molecular cloud. Once the surrounding dust and gas is blown away, the remaining stars become unbound and begin to drift apart. It is believed that the majority of all stars in the Milky Way were formed in OB associations. O-class stars are short-lived, and will expire as supernovae after roughly one million years. As a result, OB associations are generally only a few million years in age or less. The O-B stars in the association will have burned all their fuel within ten million years. (Compare this to the current age of the Sun at about five billion years.)

The Hipparcos satellite provided measurements that located a dozen OB associations within 650 parsecs of the Sun. The nearest OB association is the Scorpius–Centaurus Association, located about 400 light-years from the Sun.

OB associations have also been found in the Large Magellanic Cloud and the Andromeda Galaxy. These associations can be quite sparse, spanning 1,500 light-years in diameter.

T associations

Young stellar groups can contain a number of infant T Tauri stars that are still in the process of entering the main sequence. These sparse populations of up to a thousand T Tauri stars are known as T associations. The nearest example is the Taurus-Auriga T association (Tau-Aur T association), located at a distance of 140 parsecs from the Sun. Other examples of T associations include the R Corona Australis T association, the Lupus T association, the Chamaeleon T association and the Velorum T association. T associations are often found in the vicinity of the molecular cloud from which they formed. Some, but not all, include O-B class stars. Group members have the same age and origin, the same chemical composition, and the same amplitude and direction in their vector of velocity.

R associations

Associations of stars that illuminate reflection nebulae are called R associations, a name suggested by Sidney van den Bergh after he discovered that the stars in these nebulae had a non-uniform distribution. These young stellar groupings contain main sequence stars that are not sufficiently massive to disperse the interstellar clouds in which they formed. This allows the properties of the surrounding dark cloud to be examined by astronomers. Because R associations are more plentiful than OB associations, they can be used to trace out the structure of the galactic spiral arms. An example of an R association is Monoceros R2, located 830 ± 50 parsecs from the Sun.

Moving groups

If the remnants of a stellar association drift through the Milky Way as a somewhat coherent assemblage, then they are termed a moving group or kinematic group. Moving groups can be old, such as the HR 1614 moving group at two billion years, or young, such as the AB Dor Moving Group at only 120 million years.

Moving groups were studied intensely by Olin Eggen in the 1960s. A list of the nearest young moving groups has been compiled by López-Santiago et al. The closest is the Ursa Major Moving Group which includes all of the stars in the Plough/Big Dipper asterism except for α Ursae Majoris and η Ursae Majoris. This is sufficiently close that the Sun lies in its outer fringes, without being part of the group. Hence, although members are concentrated at declinations near 60° N, some outliers are as far away across the sky as Triangulum Australe at 70° S.

Stellar streams

A stellar stream is an association of stars orbiting a galaxy that was once a globular cluster or dwarf galaxy that has now been torn apart and stretched out along its orbit by tidal forces.

Known kinematic groups

Some nearby kinematic groups include:

• Local Association (Pleiades moving group)
• AB Doradus moving group
• Alpha Persei moving cluster
• Beta Pictoris moving group
• Castor moving group
• Corona Australis association
• Eta Chamaeleontis cluster
• Hercules stream
• Hyades Stream
• IC 2391 supercluster (Argus Association)
• Kapteyn group
• MBM 12 association
• Tucana-Horologium Association
• TW Hydrae association
• Ursa Major Moving Group
• Wolf 630 moving group
• Zeta Herculis moving group

Interstellar medium

From Wikipedia, the free encyclopedia

The distribution of 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 (Haffner et al. 2003).

In astronomy, the interstellar medium (ISM) is the matter and radiation that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as dust and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space. The energy that occupies the same volume, in the form of electromagnetic radiation, 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 followed by helium with trace amounts of carbon, oxygen, and nitrogen comparatively to hydrogen. The thermal pressures of these phases are in rough equilibrium with one another. Magnetic fields and turbulent motions also provide pressure in the ISM, and are typically more important dynamically than the thermal pressure is.

In all phases, the interstellar medium is extremely tenuous by terrestrial standards. In cool, dense regions of the ISM, matter is primarily in molecular form, and reaches number densities 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, 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 and 8.9% are helium, with 0.1% being atoms of elements heavier than hydrogen or helium, known as "metals" 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 in the process of stellar evolution.

The ISM plays a crucial role in astrophysics precisely because of its intermediate role between stellar and galactic scales. Stars form within the densest regions of the ISM, which ultimately contributes to molecular clouds and replenishes the ISM with matter and energy through planetary nebulae, stellar winds, and supernovae. This interplay between stars and the ISM helps determine the rate at which a galaxy depletes its gaseous content, and therefore its lifespan of active star formation.

Voyager 1 reached the ISM on August 25, 2012, making it the first artificial object from Earth to do so. Interstellar plasma and dust will be studied until the mission's end in 2025.

Voyager 1 is the first artificial object to reach the ISM

Interstellar matter

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

Table 1: Components of the interstellar medium

Component	Fractional volume	Scale height (pc)	Temperature (K)	Density (particles/cm3)	State of hydrogen	Primary observational techniques
Molecular clouds	< 1%	80	10–20	102–106	molecular	Radio and infrared molecular emission and absorption lines
Cold Neutral Medium (CNM)	1–5%	100–300	50–100	20–50	neutral atomic	H I 21 cm line absorption
Warm Neutral Medium (WNM)	10–20%	300–400	6000–10000	0.2–0.5	neutral atomic	H I 21 cm line emission
Warm Ionized Medium (WIM)	20–50%	1000	8000	0.2–0.5	ionized	Hα emission and pulsar dispersion
H II regions	< 1%	70	8000	102–104	ionized	Hα emission and pulsar dispersion
Coronal gas Hot Ionized Medium (HIM)	30–70%	1000–3000	106–107	10−4–10−2	ionized (metals also highly ionized)	X-ray emission; absorption lines of highly ionized metals, primarily in the ultraviolet

The three-phase model

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 K), consisting of clouds of neutral and molecular hydrogen, and a warm intercloud phase (T ~ 10⁴ K), consisting of rarefied neutral and ionized gas. McKee & Ostriker (1977) added a dynamic third phase that represented the very hot (T ~ 10⁶ K) gas which had been shock heated by supernovae 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 past three decades. However, the relative proportions of the phases and their subdivisions are still not well known.

The atomic hydrogen model

This model takes into account only atomic hydrogen : Temperature larger than 3000 K breaks molecules, lower than 50 000 K leaves atoms in their ground state. It is assumed that influence of other atoms (He ...) is negligible. Pressure is assumed very low, so that durations of free paths of atoms are larger than ~ 1 nanosecond duration of light pulses which make ordinary, temporally incoherent light .

In this collisionless gas, Einstein’s theory of coherent light-matter interactions applies, all gas-light interactions are spatially coherent. Suppose that a monochromatic light is pulsed, then scattered by molecules having a quadrupole (Raman) resonance frequency. If “length of light pulses is shorter than all involved time constants” (Lamb (1971)), an “impulsive stimulated Raman scattering (ISRS) ” (Yan, Gamble & Nelson (1985)) works: While light generated by incoherent Raman at a shifted frequency has a phase independent on phase of exciting light, thus generates a new spectral line, coherence between incident and scattered light allows their interference into a single frequency, thus shifts 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 first excited state. In this state, hyperfine periods are longer than 1 ns, so that an ISRS “may” redshift light frequency, populating high hyperfine levels. An other ISRS “may” transfer energy from hyperfine levels to thermal electromagnetic waves, so that redshift is permanent. Temperature of a light beam is defined from frequency and spectral radiance by Planck’s formula. As entropy must increase, “may” becomes “does”. However, where a previously absorbed line (first Lyman beta, ...) reaches Lyman alpha frequency, redshifting process stops and all hydrogen lines are strongly absorbed. But the stop is not perfect if there is energy at 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

Three-dimensional structure in Pillars of Creation.

 

The ISM is turbulent 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, stars interact physically with the ISM.

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

The Sun is currently traveling through the Local Interstellar Cloud, a denser region in the low-density Local Bubble.

Interaction with interplanetary medium

Play media

Short, narrated video about IBEX's interstellar matter observations.

The interstellar medium begins where the interplanetary medium of the Solar System ends. The solar wind slows to subsonic velocities at the termination shock, 90—100 astronomical units 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), 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 (Stone et al. 2005).

Interstellar extinction

The ISM is also responsible for extinction and reddening, the decreasing light intensity and shift in the dominant observable wavelengths of light from a star. These effects are caused by scattering and absorption of photons 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.
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.

Heating and cooling

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 A star (surface temperature of ~10,000 K) highly diluted. Therefore, bound levels within an atom or molecule in the ISM are rarely populated according to the Boltzmann formula (Spitzer 1978, § 2.4).

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 rays

The first mechanism proposed for heating the ISM was heating by low-energy cosmic rays. Cosmic rays are an efficient heating source able to penetrate in the depths of molecular clouds. 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.

Photoelectric heating by grains

The ultraviolet radiation emitted by hot stars can remove electrons from dust grains. The photon is absorbed by the dust grain, and some of its energy is used to overcome the potential energy barrier and remove the electron from the grain. This potential barrier is due to the binding energy of the electron (the work function) and the charge of the grain. The remainder of the photon's energy gives the ejected electron kinetic energy which heats the gas through collisions with other particles. A typical size distribution of dust grains is n(r) ∝ r^−3.5, where r is the radius of the dust particle. Assuming this, the projected grain surface area distribution is πr²n(r) ∝ r^−1.5. This indicates that the smallest dust grains dominate this method of heating^[7].

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 H II 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.

Chemical heating

Molecular hydrogen (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 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 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 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:

${\displaystyle \alpha ={\frac {T_{2}-T}{T_{d}-T}}}$

where T is the gas temperature, T_d the dust temperature, and T₂ the post-collision temperature of the gas atom or molecule. This coefficient was measured 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 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 atoms 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 will release a Ly-α 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.

Radiowave propagation

Atmospheric attenuation in dB/km as a function of frequency over the EHF band. Peaks in absorption at specific frequencies are a problem, due to atmosphere constituents such as water vapor (H₂O) and carbon dioxide (CO₂).

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 depends on compensating for the different propagation effects to uncover the desired signal.

History of knowledge of interstellar space

Herbig–Haro 110 object ejects gas through 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 Bacon (1626, § 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 Robert Boyle (1674) discussed "The inter-stellar part of heaven, which several of the modern Epicureans would have to be empty."

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 Patterson (1862) wrote, "this efflux occasions a thrill, or vibratory motion, in the ether which fills the interstellar spaces."

The advent of deep photographic imaging allowed Edward Barnard to produce the first images of dark nebulae 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 in 1904 through the use of 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.

In the series of investigations, Viktor Ambartsumian 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 was detected by 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.

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 are either Blue-shifted or 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.

This light-year-long knot of interstellar gas and dust resembles a caterpillar.

 

The growing evidence for interstellar material led 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 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."

The same year 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 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 nebulae, but in 'empty' space" (Birkeland 1913).

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 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."

In September 2012, NASA scientists reported that polycyclic aromatic hydrocarbons (PAHs), subjected to interstellar medium (ISM) conditions, are transformed, through hydrogenation, oxygenation and hydroxylation, to more complex organics – "a step along the path toward amino acids and nucleotides, the raw materials of proteins 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 grains, particularly the outer regions of cold, dense clouds or the upper molecular layers of protoplanetary disks."

In February 2014, NASA announced a greatly upgraded database for tracking polycyclic aromatic hydrocarbons (PAHs) in the universe. According to scientists, more than 20% of the carbon in the universe may be associated with PAHs, possible starting materials for the formation of life. PAHs seem to have been formed shortly after the Big Bang, are widespread throughout the universe, and are associated with new stars and exoplanets.