interstellar medium

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interstellar medium

(ISM) The matter contained in the region between the stars of the Galaxy, constituting about 10% of the galactic mass. It is largely confined to a thin layer in the galactic plane and tends to be concentrated in the spiral arms. Several different constituents have been observed in the ISM, including clouds of ionized hydrogen – H II regions – and smaller relatively cool (100 kelvin) clouds of neutral hydrogen – H I regions – surrounded by more tenuous regions of hot gas (1000–10 000 K). The presence of some very much hotter regions has been inferred from a diffuse background of soft X-ray emission and by ultraviolet absorption lines. There are also very cool (10 K) dense clouds of molecular hydrogen – molecular clouds – in which a number of other molecules and radicals have been observed.

There are several components of the ISM. In the three phase model of the ISM these are referred to as the cold neutral medium (H I gas at about 70 K and a density of 30 cm–3 filling 3–4% of the total volume), the warm neutral medium (H I, 6000 K, 0.3 cm–3, 20% of the volume), and the hot ionized medium (H II, 106 K, 10–3 cm–3, 70% of the volume). The three components are in approximate pressure equilibrium, and gas may cycle back and forth between the three phases.

In addition to the gas, cosmic dust is to be found throughout the medium. The dust comprises small solid grains that are between about 0.01 to 0.1 μm in size. The grains are thought to be composed of carbon, silicates, or iron material, with mantles of water and ammonia ice and possibly solid carbon dioxide, when they occur in dense molecular clouds. The total mass of interstellar dust is thought to be about 1% of the gas mass. The dust causes dimming and reddening of starlight (see interstellar extinction) and also interstellar polarization of starlight. The interstellar grains can undergo a complex life cycle, passing into and out of molecular clouds: the ISM is constantly churned up by shock waves from expanding supernova remnants, by stellar winds and bipolar outflows, and by other forces. While in a molecular cloud the grains may be incorporated in a newly forming star, and may later be ejected back into the ISM in a stellar wind or supernova explosion.

The medium is permeated by a flux of cosmic rays that spiral along the field lines of the galactic magnetic field of a few microgauss and cause the synchrotron emission that is the galactic radio background radiation. Radio maps show that this radiation is largely confined to the galactic plane but has several spurs, in particular the north galactic spur, radiating away from it. The north galactic spur is the most prominent segment of a huge fragmentary ring of gas detected at radio and X-ray wavelengths. This giant gas shell is probably a nearby old supernova remnant.

Interstellar Medium


the tenuous material—interstellar gas and minute dust particles—filling the space between the stars in our and other galaxies. The interstellar medium also includes cosmic rays, interstellar magnetic fields, and quanta of electromagnetic radiation of various wavelengths. Near the sun (and other stars), the interstellar medium merges into the interplanetary medium; the space between the galaxies is filled by the intergalactic medium. V. la. Struve was the first to deduce (in 1847) the existence of an interstellar medium that absorbs star-light, but actual proof came only in the 1930’s (the American astronomer R. Trumpler and the Soviet astronomer B. A. Vorontsov-Vel’iaminov).

Interstellar gas consists of neutral and ionized atoms and molecules. The main mass of the gas is made up of atoms of hydrogen and helium (90 percent and 10 percent, respectively, by the number of atoms) with small admixtures of oxygen, carbon, neon, and nitrogen (about 0.01 percent of each). Of the molecules, the most abundantly represented is H2, which is concentrated in clouds. In addition, there are small quantities of CH, OH, H2O, NH3, CH2O, and other organic and inorganic molecules. The interstellar gas is almost equally mixed with interstellar dust, which consists of particles measuring from 10-4 to 3 X 10-6 cm in diameter. The smaller particles consist of Fe and SiOa, and the larger ones have partially graphite cores, possibly with an admixture of iron and coated with condensed gases, such as CH4, NH3, and H2O. Gas and dust are almost entirely absent in elliptical galaxies, but in spiral galaxies of types Sa, Sb, and Sc they constitute about 1 percent, 3 percent, and 10 percent, respectively, of the galactic mass, and in irregular galaxies about 16 percent. The interstellar gas and dust are strongly concentrated in the galactic plane, forming a disk whose thickness averages several hundred parsecs and increases toward the periphery, sometimes to several kiloparsecs. The concentration of gas in the disks averages one atom or several atoms per cubic centimeter (density about 10-24 g/cm3); outside the disk and at its edges, the density of the gas is significantly less. In spiral galaxies, much of the gas and dust is concentrated in the spiral arms: the density of gas between the arms is three to ten times less than within the arms. In the arms, about 80-90 percent of the gas is concentrated in interstellar clouds, which often coalesce to form gas-dust complexes, located mainly on the inside (concave) side of the spiral arms. The parameters of interstellar clouds are extremely varied.

In our galaxy the diameters of the interstellar clouds are usually 5-40 parsecs; the concentration of atoms in them ranges from 2 to 100 atoms per cubic centimeter, and the temperature from 20° to 100°K. Clouds occupy about 10 percent of the volume of the galactic disk. The gas and dust of the interstellar medium along with the stars move around the center of the galactic disk in near-circular orbits with mean velocities of 100-200 km/sec. Individual clouds of interstellar gas have proper (peculiar) velocities averaging 10 km/sec and sometimes reaching 50-100 km/sec. In the galactic halo, gas of unknown origin has been observed falling onto the Galactic plane with velocities of tens or hundreds (up to 200) km/sec. The concentration of atoms between the clouds is 0.02-0.2 per cubic centimeter, and the temperature is 7°-10,000°K.

Hydrogen, helium, and other elements whose ionization potential is larger than that of hydrogen are ionized very weakly within the clouds, but between clouds the hydrogen is ionized to several tens of a percent. The remaining elements are singly ionized by the light of the stars. Such clouds and the medium between them are called H I (neutral hydrogen) regions and occupy the main part of the galactic disk. Around hot O-type stars the hydrogen is strongly (up to 99 percent) ionized by ultraviolet radiation. Such regions are termed H II (ionized hydrogen) regions, or Stromgren spheres. The temperature of the H II regions reaches 6000°-8000°K; the size of these regions varies, depending on the temperature of the star and on the density of the gas, from fractions of a parsec to several tens of parsecs; in exceptional cases it reaches several hundred parsecs. In addition to the ionized interstellar clouds, significantly denser diffuse nebulas, in which the concentration reaches tens or hundreds of atoms per cubic centimeter, are usually observed around hot stars. Possibly these nebulae are remnants of the dense complex out of which these hot stars were formed. The H II regions gradually expand under the influence of the hot gas. If in the way of such a cloud there happens to be a condensation from an H I region, then the boundary of the H II region bends around the condensation, eroding it from all sides. In this manner are formed the dark (against the bright background of the H II regions), cold, dense H I regions in the shape of filaments (“elephant trunks”) or spherical clumps (globules). Bright emission lines of hydrogen and forbidden lines of oxygen, nitrogen, sulfur, and some other elements are observed in the spectra of H II regions, as are weak continuous spectra. At radio wavelengths these regions radiate a continuous spectrum, as well as lines of hydrogen and helium arising from quantum transitions between very high energy levels. In the H I regions the gas does not radiate at optical wavelengths. These regions are studied by means of absorption lines in the light from stars situated behind the regions. Much information is provided by resonance absorption lines of atoms and ions, which are located in the ultraviolet region of the spectrum and are observable from space probes. Information about the neutral hydrogen in our galaxy and in other galaxies, such as the hydrogen’s distribution and motion, is obtained by observing the line of neutral hydrogen at the radio wavelength of 21 cm. However, only a small fraction of the thermal energy of the gas in the H I regions is radiated at this wavelength. Most of the energy is radiated by H I regions in the far-infrared spectral lines of O atoms and C, Si, Fe, and other ions.

The mean density of the dust in the Milky Way Galaxy’s disk is 10-26 g/cm3 (0.01 of the gas density). This dust absorbs starlight, absorbing more strongly in the blue than in the red. Therefore, because of the dust, the light observed from distant stars appears dimmer and redder than it actually is. The presence of dust prevents the observation of stars lying in the galactic plane at distances of more than 3 kiloparsecs from the earth. The dense clouds of gas and of dust that absorbs the light appear dark against the bright background of the Milky Way. Gas-dust clouds stand out even more sharply when projected against a bright nebula. Near very bright stars (primarily class B) the dust is illuminated sufficiently to be photographed from the earth; these bright clouds are called reflection nebulae. The layer of gas and dust in other galaxies, observed edge on, is seen as a dark band. Interstellar dust particles are nonspherical in shape and are generally oriented in a definite way with respect to the Milky Way Galaxy’s magnetic field; this causes the polarization of starlight.

The masses of large gas-dust complexes can reach tens or hundreds of thousands of solar masses. In the central regions the temperature is very low (sometimes only 5°-6°K) with concentrations of atoms up to several hundred per cubic centimeter and more, and the density of the dust is greater than 1/100 of the density of the gas. The latter circumstance is associated with the fact that at low temperatures and high densities molecule formation occurs, including the formation of polyatomic molecules, followed by the adherence of these molecules to the dust particles. Stars may be formed in such places. In connection with this, it is highly significant that in the central portions of these complexes, compact objects are observed (of the order of 1015 cm and smaller in size), out of which possibly stars (protostars) and planets form. They radiate intensively at radio wavelengths of the molecules OH, H2O, and others, the radiation sometimes resembling that from lasers. Much less common in the interstellar medium are the particles making up cosmic rays and having enormous energies—from 106 to 1020 eV, but their total energy in a cubic centimeter is about 1 eV, that is, it exceeds the energy of thermal motion of the interstellar gas. High-energy cosmic rays interact weakly with the gas and dust, occasionally causing nuclear reactions. Less energetic particles (106-107 eV) are capable of heating and ionizing the interstellar gas and are among the principal sources of the heating of H I regions. The intensity of the interstellar magnetic field is low (105 times weaker than the earth’s magnetic field), but the field’s energy is approximately equal to that of the cosmic rays. Therefore, the pressure of the cosmic rays and the pressure of the magnetic field play an essential role in the dynamics of the interstellar medium. Electromagnetic quanta in the interstellar medium have frequencies ranging from radio wavelengths to hard gamma radiation. The greatest influence on the interstellar gas and dust is exerted by optical radiation, ultraviolet radiation, and soft X-radiation (with energy quanta of less than 1 keV). The last, which partly comes from intergalactic space and partly originates in X-ray sources within the Milky Way Galaxy, causes (together with cosmic rays) the heating and partial ionization of the H I regions. Optical and ultraviolet quanta in interstellar space are the result of radiation from stars in our galaxy.

In galaxies there is a constant exchange of matter between the stars and the interstellar medium. The interstellar medium serves as material for the formation of stars, and the stars, in turn, eject part of their matter into the interstellar medium, simultaneously imparting kinetic energy to the gas. This occurs even during the quiet stages of stellar evolution, as well as at the end of the evolution, when the star throws off an envelope, forming a planetary nebula, or explodes as a supernova. A constant circulation of matter takes place, during which the quantity of gas in the interstellar medium is gradually depleted. In particular, the latter circumstance explains why there is no gas in elliptical galaxies while at the same time there is a great deal of it in irregular galaxies—here it has been depleted least of all. Insofar as during the process of stellar evolution, and especially during supernova outbursts, nuclear reactions alter the chemical composition of the gas, the composition of the interstellar medium, and consequently of the stars formed from it, also changes. In addition, an exchange of gas occurs between galactic cores and the interstellar medium.


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