Figure 18.1 shows a large region of space, a much greater expanse of universal real estate than anything we have studied thus far. The bright regions are congregations of innumerable stars, some of whose properties we have just studied in Chapter 17. However, the dark areas are not simply "holes" in the stellar distribution. They are regions of space where interstellar matter obscures the light from stars beyond. Their very darkness means that they cannot be easily studied by the optical methods used for stellar matter. There is, quite simply, very little to see!
Figure 18.1 A wide-angle photograph of a great swath of space, showing regions of brightness (vast fields of stars) as well as regions of darkness (obscuring interstellar matter). The field of view is roughly 30° across.
From Figure 18.1 it is evident that the dark interstellar matter is distributed very unevenly throughout space. In some directions the obscuring matter is largely absent, allowing astronomers to study objects literally billions of parsecs from the Sun. In other directions there are small amounts of interstellar matter, so the obscuration is moderate, preventing us from seeing objects more than a few thousand parsecs away, but still allowing us to study nearby stars. Still other regions are so heavily obscured that starlight from even relatively nearby stars is completely absorbed before reaching Earth. (See also Interlude 18-1.)
The matter among the stars is called the interstellar medium. It is made up of two components—gas and dust—intermixed throughout all space.
Interstellar gas is made up mainly of individual atoms, of average size 10-10 m (0.1 nm) or so. The gas also contains some small molecules, no larger than about 10-9 m across. Regions containing such small particles are transparent to nearly all types of electromagnetic radiation, including ultraviolet, visible, infrared, and radio waves. Apart from numerous narrow atomic and molecular absorption lines, gas alone does not block radiation to any great extent.
Interstellar dust is more complex. It consists of clumps of atoms and molecules—not unlike chalk dust and the microscopic particles that make up smoke, soot, or fog. Light from distant stars cannot penetrate the densest accumulations of interstellar dust any more than a car's headlights can illuminate roadside objects in a thick Earth-bound fog. Comparisons of the dimming of starlight in interstellar space (over and above the effects of the inverse-square law) with the scattering of light in terrestrial fog indicate that the typical size of an interstellar dust particle—or dust grain—is about 10-7; m. The grains are thus comparable in size to the wavelength of visible light, and about 1000 times larger than interstellar gas particles.
The ability of a particle to scatter a beam of light depends on both the size of the particle and the wavelength of the radiation involved (see More Precisely 7-1). As a rule of thumb, only particles having diameters comparable to or larger than the wavelength can significantly influence the beam, and the amount of scattering produced by particles of a given size increases with decreasing wavelength. Consequently, dusty regions of interstellar space are transparent to long-wavelength radio and infrared radiation but opaque to shorter-wavelength optical, ultraviolet, and X-ray radiation. This dimming of starlight by interstellar matter is called extinction.
Because the interstellar medium is more opaque to short-wavelength radiation than to radiation of longer wavelengths, light from distant stars is preferentially robbed of its higher-frequency ("blue") components. Hence, in addition to being generally diminished in overall brightness, stars also tend to appear redder than they really are, an effect known as reddening.
As illustrated in Figure 18.2, extinction and reddening change a star's apparent brightness and color, but absorption lines in the original stellar spectrum are still recognizable in the radiation reaching Earth, so the star's spectral class can be determined. Astronomers can use this fact to study the interstellar medium. From a main-sequence star's spectral class astronomers learn its true luminosity and color. 
(Sec. 17.6) They then measure the degree to which the starlight has been affected by extinction and reddening en route to Earth, and this, in turn, allows them to estimate both the numbers and the sizes of interstellar dust particles along the line of sight to the star. By repeating these measurements for stars in many different directions and at many different distances from Earth, astronomers have built up a picture of the distribution and properties of the interstellar medium in the solar neighborhood.
Figure 18.2 Starlight passing through a dusty region of space is both dimmed and reddened, but spectral lines are still recognizable in the light that reaches Earth.
The temperature of the interstellar gas and dust ranges from a few kelvins to a few hundred kelvins, depending on its proximity to a star or some other source of radiation. Generally, we can take 100 K as an average temperature of a typical dark region of interstellar space. Compare this with 273 K, at which water freezes, and 0 K, at which atomic and molecular motions all but cease. (More Precisely 3-1) Interstellar space is very cold.
Gas and dust are found everywhere in interstellar space—no part of our Galaxy is truly devoid of matter. However, the density of the interstellar medium is extremely low. The gas averages roughly 106 atoms per cubic meter—just 1 atom per cubic centimeter—although densities as great as 109 atoms/m3 and as small as 104 atoms/m3 have been found. Matter of such low density is far more tenuous than the best vacuum—about 1010 molecules/m3—ever attained in laboratories on Earth. Interstellar dust is even rarer. On average, there is only 1 dust particle for every trillion or so atoms—just 10 -6 dust particles per cubic meter, or 1000 per cubic kilometer. Some parts of interstellar space are so thinly populated that harvesting all the gas and dust in a region the size of Earth would yield barely enough matter to make a pair of dice.
How can such fantastically sparse matter diminish light radiation so effectively? The key is size—interstellar space is vast. The typical distance between stars (1 pc or so in the vicinity of the Sun) is much, much greater than the typical size of the stars themselves (around 10-7 pc). Stellar and planetary sizes pale in comparison to the vastness of interstellar space. Thus, matter can accumulate, regardless of how thinly spread. For example, an imaginary cylinder 1 m2 in cross section and extending from Earth to Alpha Centauri would contain more than 10 billion dust particles. Over huge distances, dust particles accumulate slowly but surely, to the point at which they can effectively block visible light and other short-wavelength radiation. Even though the density of matter there is very low, interstellar space in the vicinity of the Sun contains about as much mass as exists in the form of stars.
Despite their rarity, dust particles make interstellar space a relatively dirty place. Earth's atmosphere, by comparison, is about a million times cleaner. Our air is tainted by only one dust particle for about every billion billion (1018) atoms of atmospheric gas. If we could compress a typical parcel of interstellar space to equal the density of air on Earth, this parcel would contain enough dust to make a fog so thick that we would be unable to see our hand held at arm's length in front of us.
The composition of interstellar gas is reasonably well understood from spectroscopic studies of interstellar absorption lines formed when light from a distant star interacts with gas along the line of sight. (Sec. 4.2) The gas absorbs some of the stellar radiation in a manner that depends on its own temperature, density, and elemental abundance. The absorption lines thus produced contain information about dark interstellar matter, just as stellar absorption lines reveal the properties of stars. Because the interstellar absorption lines are produced by cold, low-density gas, astronomers can easily distinguish them from the much broader absorption lines formed in the star's hot lower atmosphere (see Section 18.3). (Sec. 4.4)
In most cases, the elemental abundances detected in interstellar gas mirror those found in other astronomical objects, such as the Sun, the stars, and the jovian planets. Most of the gas—about 90 percent—is atomic or molecular hydrogen; some 9 percent is helium, and the remaining 1 percent consists of heavier elements. The abundances of several of the heavy elements, such as carbon, oxygen, silicon, magnesium, and iron, are much lower in interstellar gas than in our solar system or in stars. The most likely explanation for this finding is that substantial quantities of these elements have been used to form the interstellar dust, taking them out of the gas and locking them up in a form that is much harder to observe.
In contrast to interstellar gas, the composition of interstellar dust is currently not very well known. We have some infrared evidence for silicates, graphite, and iron—the same elements that are underabundant in the gas—lending support to the theory that interstellar dust forms out of interstellar gas. The dust probably also contains some "dirty ice," a frozen mixture of ordinary water ice contaminated with trace amounts of ammonia, methane, and other chemical compounds. This composition is quite similar to that of cometary nuclei in our own solar system. (Sec. 14.2)
Curiously, astronomers know the shape of interstellar dust particles better than their composition. Although the minute atoms in the interstellar gas are basically spherical, the dust particles are not. Individual dust grains are apparently elongated or rodlike, as shown in Figure 18.3. We can infer this because the light emitted by stars is dimmed and partially polarized, or aligned, by the dust.
Recall from Chapter 3 that light consists of electromagnetic waves composed of vibrating electric and magnetic fields (Sec. 3.2). Normally, these waves are randomly oriented. and we say the radiation is unpolarized. Stars emit unpolarized radiation from their photospheres. Under some circumstances, however, the electric fields can become aligned—all vibrating in the same plane as the radiation moves through space. We then say the radiation is polarized. Polarization of starlight does not occur by chance. If the light detected by our telescope is polarized, it is because some interstellar matter lies between the emitting object and Earth. The polarization of starlight, then, provides another way to study the interstellar medium.
Figure 18.3 A diagram of a typical interstellar dust particle. The average size of such particles is only 1/10,000 of a millimeter, yet space contains enough of them to obscure our view in certain directions.
On Earth we can produce polarized light through a Polaroid filter, which has specially aligned elongated molecules that allow the passage of only those waves having electric fields oriented in some specific direction (see Figure 18.4a). Other waves are absorbed and so do not pass through the filter. The alignment of the molecules determines which waves will be transmitted. In interstellar space, dust grains can act like the molecules in the Polaroid filter. If the starlight is polarized, astronomers can conclude that the interstellar dust particles must have an elongated shape (by analogy with the elongated molecules of the Polaroid filter) and that these molecules are aligned, as shown in Figure 18.4(b). Only then can the dust preferentially absorb certain waves, leaving the remainder (the ones we observe) polarized.
The alignment of the interstellar dust is the subject of intense research among astronomers. The curent view, accepted by most, holds that the dust particles are affected by a weak interstellar magnetic field, perhaps a million times weaker than Earth's field. Each dust particle responds to the field in much the same way that small iron filings are aligned by an ordinary bar magnet. Measurements of the blockage and polarization of starlight thus yield information about the size and shape of interstellar dust particles, as well as about magnetic fields in interstellar space.
Figure 18.4 (A) Unpolarized light waves have randomly oriented electric fields. When the light passes through a Polariod filter, only waves whose electric fields are oriented in a specific direction are transmitted, and the resulting light is polarized. (b) Aligned dust particles in interstellar space polarize radiation in a similar manner. Observations of the degree of polarization allow astronomers to infer the size, shape, and orientation of the particles.
