Interstellar matter is gas and dust existing between stars in a galaxy. Every atom that is not in a star in a particular galaxy can be considered as part of that galaxy's interstellar matter. The study of this matter is one of the most important branches of modern astrophysics. Because interstellar matter is not as easy to see as stars, its existence was recognized only recently. By observing the spectrum of the Orion nebula, William Huggins found in 1864 that the nebula consists of luminous interstellar gas. In the early days of astronomical photography, E. E. Barnard recognized the existence of dark matter, now called interstellar dust, by its effect in absorbing the light of stars behind it. In 1904, Johannes F. Hartmann found an absorption line in the spectrum of the star delta Orionis, showing the interfering presence of nonluminous interstellar gas. He recognized that this line, caused by ionized calcium, did not arise in the star because it did not share the periodic changes in wavelength of all the other lines in the star's spectrum, caused by the changing velocity of its orbital motion. Large amounts of interstellar gas, composed mostly of atoms but also containing ions and molecules, are now known to be present in our galaxy. Interstellar dust also exists, although in a smaller amount than the gas, because hydrogen and helium, the two most abundant elements in the universe, form only transparent gases; hydrogen can form dust particles only when combined with less abundant elements, such as carbon (C), nitrogen (N), oxygen (O), magnesium (Mg), silicon (Si), and iron (Fe). INTERSTELLAR MATTER IN OUR GALAXY Nearly all the information about interstellar matter is obtained from studying its interaction with radiation through the emission, absorption, or scattering of light at optical, ultraviolet, infrared, and radio wavelengths. Atoms, molecules, and ions emit and absorb radiation at discrete wavelengths, causing spectral lines, and dust particles with sizes ranging from .0001 cm to .000001 cm emit and absorb radiation at all wavelengths, resulting in a continuous spectrum. Considerably larger particles the size of baseballs could be present in interstellar matter, but because they are much less effective per unit mass in emitting or absorbing radiation, they would be essentially undetectable. On theoretical grounds, however, such large particles are thought not to exist. The only indirect way of detecting the presence of interstellar matter, independently of its interaction with radiation, is by its gravitational effect on the motions of stars. Because all forms of mass have this same effect, interstellar matter and faint, low-luminosity stars are not easily distinguished by this method. The best estimate is that approximately 2 or 3 percent of the mass of our galaxy is interstellar matter, and the rest stars. Some galaxies contain much more interstellar matter than our galaxy; others contain much less. Atoms, Ions, and Molecules Interstellar matter in cool regions of space can be observed by the absorption lines it causes in the spectra of distant hot stars. Interstellar absorption lines usually cannot be observed in the spectra of cooler stars, because absorption lines of atoms and ions in the stars' atmospheres blend with the interstellar lines. The strongest interstellar absorption lines observed in the optical spectral region are those of ionized calcium and sodium, and weaker lines are observed of ionized titanium, calcium, iron, and potassium. These are not the most abundant atoms and ions in interstellar space, but they are the most abundant atoms and ions that have interstellar absorption lines in the optical part of the spectrum accessible to Earth-bound instruments. Because of the low density of interstellar matter--one atom per cu cm, or 10 to the power of minus 24 g per cu cm--atomic, ionic, and molecular collisions are infrequent, and the interstellar atoms and ions are almost always in their ground level, the lowest electronic energy level. The only absorption lines that can be observed, therefore, are those which arise from ground levels, and sodium and ionized calcium are the two most abundant interstellar substances that have lines from the ground level in the ordinary region. Interstellar matter also is observed by the radiation it emits if it is heated or ionized. Hot stars emit large amounts of high-energy ultraviolet radiation. If a hot star is located in a cloud of interstellar matter, its ultraviolet radiation ionizes the nearby interstellar gas and transfers energy to it, causing the ionized gas to emit the characteristic emission-line spectrum of a galactic nebula with ionized hydrogen, known as an H II region. Most abundant atoms and ions, such as H, O, O+, C, and C+, have their spectral lines in the ultraviolet spectral region. Ultraviolet radiation from hot stars can be measured using telescopes in rockets or artificial satellites above the Earth's atmospheres. The telescopes these orbiting observatories carry are small, and only a few of the brighter, nearer stars have been measured. These ultraviolet spectra, nevertheless, show the presence of many absorption lines including the interstellar absorption lines of molecular hydrogen. Previously, only the molecules \TCh\t, CN, and CH+ were seen to have interstellar absorption lines in the ordinary optical region. Even at the very low temperature (about 100 K) of typical neutral interstellar material, very low-lying energy levels are collisionally excited and radiate. Neutral hydrogen (H) has an excited hyperfine-structure level close above its ground level, which causes an emission line with a wavelength of 21 cm, in the radio-frequency spectral region. Observations of this line have provided information on the distribution and amount of neutral interstellar gas in this galaxy. Hydroxyl (OH) was the first interstellar molecule detected (1963, 1964), by several groups of radio astronomers, in the radio-frequency spectral region. It has four characteristic spectral lines with wavelengths near 18 cm. OH and CO (carbon monoxide), which has lines at 2.6 mm and 1.3 mm, are abundant interstellar molecules and are observed throughout the galaxy. They are found in dense clouds of interstellar matter, because only in such clouds do the atoms collide frequently enough to combine to form molecules. In clouds surrounding the center of the Galaxy the presence of sodium hydroxide (NaOH) has been revealed by emission lines at 2.98 and 3.97 mm. In very dense interstellar clouds many more complicated interstellar molecules have been observed by their radio-frequency and far-infrared emission lines. These include water, formaldehyde, methanol, ethanol, cyanoethyne, and many others, including molecules with 13 or more atoms. Perhaps half of the interstellar material in our Milky Way galaxy occurs within molecular clouds. Dust Particles Interstellar dust particles can be detected by the extinction of starlight. Extinction includes both absorption, in which light is actually destroyed by conversion into heat, and scattering, in which light's direction is changed, as by small water droplets. In both cases some light from a star shining through a cloud of interstellar particles does not reach the observer; for interstellar particles scattering is probably the more significant factor in the optical spectral region. Very dense clouds of interstellar dust particles are seen as dark features silhouetted against the bright star field or nebula behind them. Examples are the dark nebulae discovered by Barnard, the "Gulf of Mexico" in the North American Nebula, and the dark Horsehead Nebula in Orion. For small particles the extinction increases as the light's wavelength decreases, and light transmitted by a cloud of particles tends to have more long-wavelength radiation than incident light--that is, it tends to be redder (see \Tinterstellar reddening\t). This reddening effect of small particles is very pronounced on Earth when the rising or setting Sun is seen through a long path in the Earth's atmosphere, containing many particles in the form of dust, smoke, or haze. A star's intrinsic color is known from its spectral type, which depends, like the color, primarily on its temperature. By finding a star's spectral type, measuring its actual color, and comparing this measurement with its intrinsic color, the amount of dust between the observer and the star can be measured. From the detailed way in which the amount of extinction produced by interstellar dust varies with wavelength, together with knowledge of the composition of interstellar matter, it can be gathered that the interstellar particles probably are a mixture of dielectric particles, composed largely of solid water, methane, ammonia, with some metal impurities, silicate particles containing magnesium silicate and calcium silicate, and probably also graphite or silicon-carbide particles. An absorbed layer of molecular hydrogen may exist on the surfaces of interstellar particles, but most of their mass consists of elements heavier than hydrogen or helium. Because interstellar particles are not spherical but have elongated shapes, such as ellipsoids or rods, they may also polarize transmitted light. Such polarization is observed to be weakly present in many reddened stars and shows that the interstellar particles are elongated and also roughly aligned in regions of the galaxy, probably by large-scale magnetic fields. Interstellar matter in our galaxy is distributed so that most of it is in or very close to the galactic plane. To a first approximation, interstellar matter can be thought of as forming a layer about 700 light-years thick and 60,000 light-years in diameter. Interstellar matter is not uniformly distributed within this layer, but tends to occur in various-sized clouds or condensations. In the inner part of the galaxy, closer to the galactic center, the clouds tend to be denser, and more of the interstellar matter is in the form of molecules. In the outer part of the galaxy, more of it is in atomic form. Only a small fraction of interstellar gas is ionized. INTERSTELLAR MATTER IN OTHER GALAXIES Interstellar matter can also be observed in other galaxies. The most direct method is the observation of individual galactic nebulae, or H II regions, that can be photographed in many nearby galaxies. The spectra of these nebulae show that they are identical with ionized interstellar-matter clouds in our galaxy. Dust can also be recognized by its extinction effects in other galaxies, particularly in spiral galaxies seen nearly edge-on, because of the strong concentration of interstellar matter to the plane of the galaxy in which it is located. Radio measurements have detected the 21-cm emission line of interstellar hydrogen from many other galaxies. In general, elliptical galaxies contain much less interstellar matter than our galaxy. They do not show any galactic nebulae, dust, or 21-cm line radiation. Most irregular galaxies contain far more interstellar matter than our galaxy; they are rich in galactic nebulae, dust, and 21-cm radiation. Spiral galaxies are intermediate. Photographs of spiral galaxies show that interstellar matter is strongly concentrated in spiral arms. The main defining features that make spiral arms visible on these photographs are the galactic nebulae and the high-luminosity stars, which recently formed from interstellar matter and are therefore mostly still immersed in it. Spiral arms in our galaxy are not easily recognized, because of the Earth's position close to the galactic plane. By identifying distant galactic nebulae and stellar associations, however, and by determining their distances accurately from the spectral types and magnitudes of their stars, interstellar matter within a few thousand light-years of the Sun is found to be arranged in spiral arms, very similar to the spiral arms directly observed in other spiral galaxies. Optical measurements cannot be pushed to larger distances in our galaxy's plane, because of the very strong extinction caused by dust in interstellar matter. Radio measurements, however, which are not subject to appreciable extinction, can be used to detect interstellar hydrogen throughout the galaxy by its 21-cm radiation. Radio measurements also show the presence of galactic nebulae, which emit highly excited radio-frequency lines of hydrogen throughout the galaxy. The pattern of velocities measured by the shifts in wavelengths of the emission lines demonstrates that the gas lies in spiral arms everywhere except near the galaxy's center. The physical mechanism responsible for the distribution of interstellar matter in a spiral pattern is not fully understood. It is certainly connected with the galaxy's rotation, which has a rate that varies with distance from the center of the galaxy. Large-scale magnetic fields may be involved in the spiral arms' production and maintenance. Another possibility is the density-wave theory, which attributes the spiral arms entirely to the interstellar gas's kinematical and gravitational effects. Scientists are currently seeking causes for the unusual motions of galaxies and galaxy clusters in terms of the gravitational influences of unobserved matter--the so-called missing mass problem (see \Tcosmology\t). These gravitational effects could be due to great amounts of cold, undetected interstellar matter or larger bodies (see \Tbrown dwarf\t). In fact, new forms of interstellar matter continued to be revealed, such as the galaxy-pervading clouds of very cold dust detected in 1983 by the Infrared Astronomy Satellite (\TIRAS\t) and since labeled infrared cirrus. Studies of distant quasars have also indicated the probable existence of many fairly compact masses of interstellar matter that diffract radio waves coming from the quasars. Some astronomers think these masses--in the size range of the Earth's orbital path around the Sun--may turn out to be up to 1,000 times as numerous as the stars are in our galaxy. In the late 1980s astronomers observed luminous arcs, more than 300,000 light-years long, around distant galaxy clusters. The perfect arcs were at first taken to be physical objects of unknown nature. They were later determined to be optical effects. The galaxy clusters were acting as \Tgravitational\t \Llens\les, producing distorted images of still more distant galaxies. INTERSTELLAR MATTER AND STELLAR EVOLUTION Interstellar matter is the material from which new stars form. Dense condensations in interstellar clouds are the nuclei that become gravitationally unstable and contract, forming a star or a group of stars (See \Tstellar evolution\t). Young stars, such as high-luminosity O and B stars, are usually found in or close to interstellar matter. The only exceptions are high-velocity luminous stars, which have moved far from the interstellar gas clouds in which they formed. At the end of their lifetimes, most stars return some of their mass to interstellar space. For example, a star of approximately solar mass discards an outer shell, which briefly becomes a planetary nebula; the stellar remnant, after a short, high-luminosity episode, becomes a white dwarf. A more massive star, at the end of its lifetime, becomes a supernova, consisting of a rapidly expanding shell of gas and a stellar remnant that probably becomes a neutron star. The planetary-nebula and supernova shells are slowed down by colliding with interstellar matter and merging with it. Probably other evolving stars, such as red giants, lose mass more gradually by "stellar winds," or flow of matter into space. Interstellar matter is a reservoir in which mass from evolving or evolved stars, often enriched in heavy elements by nuclear reactions, is captured and ultimately partly formed into new stars. Donald E. Osterbrook Bibliography: Bally, John, "Interstellar Molecular Clouds," Science, April ll, 1986; Bok, Bart J. and Priscilla F., The Milky Way, 5th ed. (1981); Carrington, A., and Ramsey, D.A., eds., Molecules in Interstellar Space (1982); Dame, T. M., "The Molecular Milky Way," Sky & Telescope, July 1988; Dalgamo, Alexander, "The Molecules of Outer Space," New Scientist, June 23, 1990: Duley, W.W., and Williams, D.A., Interstellar Chemistry (1984); Millar, Tom, and Williams, David, "Chemistry between the Stars" New Scientist, April 11, 1985; Osterbrock, Donald E., Astrophysics of Gaseous Nebulae and Active Galactic Nuclei (1986); Verschuur, G.L., Interstellar Matters (1988); See also: \Tinfrared radiation\t; \Tnebula\t; \Tplanetary nebula\t.