Interstellar Clouds And The Interstellar Medium

Type of physical science: Astronomy; Astrophysics

Field of study: Galaxies

Although the space between stars is quite empty by terrestrial standards, careful observation yields evidence for the existence of interstellar dust grains and at least one hundred different kinds of molecules and molecular fragments present under a variety of conditions. Study of the interstellar medium provides important information on the life cycles of stars and the history of the cosmos.

Overview

From the terrestrial standpoint, "outer space" begins where Earth's atmosphere ends.

One thinks of the solar system as a group of nine planets together with a collection of smaller bodies, moons, asteroids, comets, and so on, which orbit the sun, passing through an almost perfect vacuum. Similarly, the sun and several hundred billion other stars orbit the center of this galaxy, the Milky Way. The fact that starlight can be seen from stars in this galaxy hundreds or thousands of light-years away would suggest that the space between the stars--the interstellar medium--is essentially empty. Careful observation has shown, however, that dust and a variety of atoms and molecules can be found almost anywhere in the galaxy. Further, the density and temperature of the interstellar medium vary greatly from place to place; in many regions this medium forms "interstellar clouds," which may emit or scatter light or obscure the view of the stars behind them.

By terrestrial standards, interstellar regions are very nearly empty. Even the best vacuums attainable in the laboratory, about one one-hundred-trillionth of an atmosphere, contain about 100,000 molecules per cubic centimeter. This is perhaps the upper limit of density for interstellar matter. The space between stars in a galaxy is so immense, however, compared to the space actually occupied by the stars, that 10 percent or more of a galaxy's mass can be contained in the interstellar medium. The important characteristics of the interstellar medium are the density, or number of particles per cubic centimeter; the chemical composition; and the temperature, which provides a measure of the speed of the molecules in the medium. The interstellar medium is said to be cloudlike in character, in that different regions have different densities. The least dense material, called the intercloud gas, has from one to ten particles per 100 cubic centimeters and an absolute temperature of about 10,000 Kelvins.

The known components of the interstellar medium have been identified either through the electromagnetic radiation they emit or by their effect on the electromagnetic radiation from stars that passes through them. While Earth-based studies of the interstellar medium have had to rely on the microwave region of the electromagnetic spectrum--observed by radio telescopes--and on the visible region, satellite and rocket or balloon-based observations in the infrared, ultraviolet, and X-ray regions have contributed much to the understanding of the interstellar medium. A few basic physical principles provide the basis for the interpretation of the electromagnetic radiation from the interstellar medium. The atoms or molecules of a gas at low density will emit or absorb electromagnetic radiation only at those energies (proportional to the frequency and inversely proportional to the wavelength) that correspond to the difference in energy between allowed quantum mechanical states. If a body of gas lies in front of a star, its chemical composition can be determined from the absorption spectrum, or pattern of dark lines seen against the continuous background spectrum of starlight when viewed through a spectroscope. The temperature of a gas cloud can be determined from the frequency at which the greatest amount of emission occurs. If the gas is in motion toward Earth, the emitted radiation is shifted to slightly higher frequencies and shorter wavelengths. If the gas is moving away from Earth, the emission is shifted to slightly lower frequencies and longer wavelengths. This Doppler shift makes it possible to determine the speed of an interstellar cloud relative to Earth.

By far, the most common chemical elements found in interstellar space are hydrogen and helium. Astronomers distinguish between H I regions, in which the hydrogen exists primarily in the form of isolated atoms, and H II regions, in which the hydrogen exists primarily in ionized form, as separated proton and electrons. H II regions can be quite warm, with temperatures of 10,000 Kelvins or more, and are characterized by a red emission produced by excited hydrogen atoms. Such regions include emission nebulas such as the Trifid nebula, which is one of the most beautiful objects in the sky. H I regions are somewhat cooler than H II regions and are characterized by the emission of radiation at 21-centimeter wavelengths, which can be detected by radio telescopes.

At somewhat lower temperatures, one finds a variety of cloud types in which much of the material exists as molecules rather than as separated atoms. Diffuse clouds have a temperature of about 100 Kelvins, a density of about one hundred particles per cubic centimeter, and contain a mixture of hydrogen atoms and hydrogen molecules, along with some partially ionized carbon atoms, a few types of neutral atoms, and some small molecules, including carbon monoxide (CO) and formaldehyde (H₂CO₃). Dark clouds have a temperature of about 10 Kelvins and density of ten thousand particles per cubic centimeter, with most of the hydrogen appearing in molecular form. A wide variety of molecules are found in dark clouds. So-called giant molecular clouds exist at comparable temperatures and can be up to 400 light-years in diameter.

The range of molecules that have been identified in molecular clouds is quite large.

Compounds of hydrogen, carbon, nitrogen, and oxygen are most common, with sulfur and silicon appearing in a few compounds. The molecular species identified include those such as ethanol (C₂H₅OH), which are stable and even common under terrestrial conditions, as well as those such as the ethynyl radical, C₂H, which are too reactive to be isolated in the laboratory but can exist for a significant length of time at the very low densities present in molecular clouds. The largest molecule identified before 1990 is the thirteen-atom linear molecule cyanopentaacetylene (HC₁₁N).

About 1 percent of the total mass of the interstellar medium exists in the form of microscopic solid particles, generally called "dust" by astronomers. Dust grains are believed to be about one-millionth of a meter or less in size and irregularly shaped. The presence of interstellar dust is indicated by the absorption and scattering of starlight. When a dust cloud is illuminated from the front by a nearby star, one can see the bluish scattered light. Such an object is called a reflection nebula, the best-known example of which is associated with the star group known as the Pleiades. The scattering properties of the interstellar dust grains allow astronomers to make an estimate of their size and shape. Dust particles are more efficient in scattering blue light than red light; thus, distant stars appear somewhat redder than their actual color. Although dust grains have been recovered from the interplanetary space in the solar system, it is not certain that they are representative of interstellar grains. The relatively low concentration of certain elements in the interstellar gas, as compared to the concentration in stellar atmospheres, indicates that these elements--magnesium, iron, and silicon--may be prevalent in the interstellar grains.

One remarkable effect of the interstellar dust is the polarization of starlight that passes through it; that is, the passage of starlight through the interstellar medium has somewhat the same effect as passing light through a polarized sunglass lens. In the case of the lens, the polarizing effect is the result of long, thin light-absorbing molecules, which are held parallel to one another in the polarizing film. The dust grains must therefore be elongated in shape, and it is probable that they are held in a parallel orientation by an interstellar magnetic field, which is relatively constant in direction over large areas of space.

The amount of matter to be found in the interstellar medium varies from one galaxy to another. Elliptical galaxies appear to contain substantially less interstellar matter than spiral galaxies, such as the Milky Way. Irregular galaxies can contain very large amounts. The Magellanic Clouds, satellites of the Milky Way visible in the Southern Hemisphere, are nearly 40 percent interstellar matter. Observations of clusters of galaxies indicate the presence of an intergalactic medium with a density of less than one particle per thousand cubic centimeters and at temperatures of more than 10 million Kelvins. Some heavy elements, including iron, have been identified in this medium.

Applications

The principal reason for astronomers' interest in the interstellar medium is the insight that studies of interstellar matter provide on the formation and subsequent history of stars.

According to the big bang theory, the explosive event that marked the origin of the universe produced hydrogen and some helium, but only traces of any of the heavier elements. Almost all the carbon, nitrogen, oxygen, and other elements found in interstellar molecules, in the planets, and in Earth's atmosphere and bodies were produced by nuclear reactions that took place in the interior of stars, which formed from the interstellar medium and were later returned to it.

Theories of the structure of the galaxy and the life cycles of the stars must account for the present composition of interstellar matter as a result of nucleosynthesis occurring in the stars and the exchange of matter between stars and the interstellar medium.

Much of the interstellar medium exists in the form of very low-density gas at a very high temperature. A density fluctuation, perhaps induced by the "shock wave" from an interstellar explosion, can trigger a collapse of a portion of this medium into a smaller region. At first, a cooling of the material occurs as the energy released by collisions of particles is radiated into space. The collisions of atoms with one another and with the dust grains result in the formation of molecules. In the Milky Way, this process appears to occur primarily in the spiral arms, which is also the location of the most recently formed stars. When the cloud density becomes sufficiently great, the cloud becomes opaque to the passage of electromagnetic radiation, causing the released energy to become "trapped"; the cloud begins to warm and eventually forms a protostar, usually with an associated H II region and emission nebula. The gravitational collapse and the associated warming continue until the temperature becomes sufficiently great to allow nuclear fusion to begin.

Nearly all the nuclei in the universe, other than hydrogen and helium, were formed in the interior of stars. The mechanism of formation is somewhat different for elements containing up to about sixty protons and neutrons (that is, up to the iron-nickel-cobalt group) and for larger nuclei, which are generally much scarcer. For the greater part of a star's lifetime, the principal nuclear reaction is the fusion of hydrogen to form helium. Once the hydrogen has been substantially depleted, further gravitational collapse leads to additional warming, igniting the helium to form carbon, oxygen, and other small nuclei. If the star has sufficient mass, it may go through several additional stages of collapse and ignition, with the formation of still heavier elements. While the core of the star becomes warmer and more compact, the outer regions expand and cool so that eventually the star enters a red giant phase. The temperature in the outer extremes of a red giant are cool enough to allow the formation of molecules and possibly dust grains, which, since they are so far from the stellar core, may be able to escape the relatively weak gravitational field at the stellar surface. Planetary nebulas are H II regions that may represent a late stage in this process.

A number of other processes result in the release of matter from stars back to the interstellar medium. Stars of the sun's mass or smaller typically have coronas, outer gaseous layers with temperatures of about a million Kelvins, in which particles have sufficient speed to escape into space. For very large stars, the radiation pressure of light leaving the star is responsible for the stellar wind, a release of matter from the outer layers of a star. In binary stars systems, the capture of matter from one star by the other can result in a nova, an explosion that ejects much stellar material. The sudden gravitational collapse of a large stellar core results in a supernova, which produces elements heavier than iron and returns them to the interstellar medium.

The one other significant source of nucleosynthesis is the collision of existing nuclei with cosmic rays. Cosmic rays are particles, almost always protons or other nuclei, traveling at immense speeds. The interaction of cosmic rays with the interstellar medium provides a means of studying nuclear reactions occurring at very high energies, including the fragmentation of heavy nuclei, which appears to be the only source of some of the less abundant isotopes.

Context

The existence of interstellar dust and gas has played an important role in astronomers' study of the Milky Way. The belief that the broad band of light in the night sky called the via galacta by the ancient Romans is actually a collection of an immense number of stars of which the sun was a member has been generally accepted by the beginning of the twentieth century. The problem was to determine the shape and size of this collection and the sun's position in it. In the early 1900's, the Dutch astronomer Jacobus Cornelis Kapteyn conducted a survey of the distribution of stars of different magnitudes in different parts of the sky and concluded that the sun appeared to be at the center of this distribution and that the number of stars in a given volume of space diminished with increasing distance from the sun in any direction. This view was challenged, however, in 1917 when the American astronomer Harlow Shapley published a study of the distribution of globular clusters, large groupings of up to hundreds of thousands of stars, and showed that these clusters appeared to be centered on a point several thousand light-years from the sun. The discrepancy between these observations was resolved in 1930 when the Swiss-American astronomer Robert Julius Trumpler showed that the interstellar dust obscured the view of more distant stars and that Shapley's method, based on large collections of stars, was more reliable.

Absorption lines caused by interstellar molecules were first identified in 1904 by the German astronomer Johannes Franz Hartmann, but the first identification of an interstellar molecule, the methylidyne radical, CH, did not occur until 1939. In 1951, the American William Wilson Morgan made the first observations of H II regions in the Milky Way, identifying them with the spiral arms. At the same time, Edward Mills Purcell and Harold Ewen at Harvard University were able to detect the 21-centimeter radiation of atomic hydrogen using the techniques of radio astronomy. By measuring the Doppler shift of the 21-centimeter radiation, astronomers have been able to construct a map of the Milky Way.

Interest in the chemistry of the interstellar medium increased substantially with the discovery in 1963 of the first oxygen-containing species, the hydroxyl radical, OH, and the subsequent discoveries of interstellar water, H2O, and ammonia, NH3, in 1968. Subsequent years saw the discovery of increasingly complex molecules and gave some credibility to the notion that interstellar chemicals may have played some part in the origin of life on Earth or possibly elsewhere in the universe.

Principal terms

DARK CLOUD: an interstellar cloud of sufficient density to block the passage of starlight

DOPPLER SHIFT: the change in frequency or wavelength that results from the relative motion of the observer and the radiation source

ELECTROMAGNETIC RADIATION: energy transported through space in the form of alternating electric and magnetic fields and traveling at the same speed as visible light

EMISSION NEBULA: a cloud of gaseous material warm enough to be observed by its own emitted light

H I REGION: a region in which the element hydrogen exists primarily in the form of neutral atoms

H II REGION: a region in which the element hydrogen exists as separated protons and electrons, necessarily at a higher temperature than an H I region

NUCLEOSYNTHESIS: the formation of heavier atomic nuclei through the fusion of lighter nuclei

REFLECTION NEBULA: a cloud, usually bluish in color, visible by virtue of light scattered from stars closer to the observer than the cloud itself

SPECTROSCOPY: separation of the light or other electromagnetic radiation received from a source into its component frequencies or wavelengths

Bibliography

Asimov, Isaac. ASIMOV'S NEW GUIDE TO SCIENCE. New York: Basic Books, 1984. This comprehensive but very readable volume by the prolific science writer contains a brief but detailed discussion of the discovery of interstellar molecules and the implications of this discovery for the understanding of stellar formation and evolution.

Carbo, R., and A. Ginebreda. "Interstellar Chemistry." JOURNAL OF CHEMICAL EDUCATION 62 (1985): 832-836. This informative short article discusses the reactions by which molecules can form in interstellar space. Contains a number of theories that attempt to explain the relative abundances of interstellar molecules.

De la Contardiere, Philipe, ed. LAROUSSE ASTRONOMY. New York: Facts on File, 1987. This is the English translation of the volume on astronomy from a French series known for its superb illustrations. Chapter 22 is devoted to the interstellar medium. Chapter 23 discusses galaxies and the intergalactic medium. Remarkably detailed, yet written with a minimum of technical jargon.

Dudly, W. W., and D. A. Williams. INTERSTELLAR CHEMISTRY. New York: Academic Press, 1984. Although intended for the more advanced reader, this book provides a fascinating record of the extensive information that has been accumulated about the chemical and physical characteristics of the interstellar clouds.

Kaufmann, William J., III. UNIVERSE. New York: W. H. Freeman, 1988. This comprehensive introductory text devotes three chapters to the life cycles of the stars and the exchange of matter between the stars and the interstellar medium.

Kutter, G. Siegfried. ORIGIN AND EVOLUTION OF THE UNIVERSE. Boston: Jones and Bartlett, 1989. This short astronomy text is unusual because it was written by a professional biologist. Focuses on the evolution of structure in the universe, in effect, treating the formation of stars and nuclei on a par with the development of living organisms.

Sagan, Carl. COSMOS. New York: Random House, 1980. This volume, and the video series of the same name, by the prominent astronomer and popular writer includes a chapter on the lives of the stars. Presents the formation of the chemical elements in a highly entertaining and memorable fashion.

Snow, Theodore P. THE DYNAMIC UNIVERSE. St. Paul, Minn.: West, 1988. An introductory college-level text, intended for the nonspecialist. Devotes a substantial chapter to the interstellar medium, and provides valuable background on spectroscopic methods used by astronomers.

Nuclear Synthesis in Stars

Protostars and Brown Dwarfs

The Evolution of the Universe