11.1 Interstellar Matter

Figure 11.1 Milky Way Photo A wide-angle photograph of a great swath of space, showing regions of brightness (vast fields of stars) as well as regions of darkness (where interstellar matter obscures the light from more distant stars). The field of view is roughly 30° across. (Palomar Observatory/California Institute of Technology)
Figure 11.1 shows a much greater expanse of universal "real estate" than anything we have studied thus far. The bright regions are congregations of innumerable stars, merging together into a continuous blur at the resolution of the telescope. 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.

Gas and Dust

The matter between the stars is collectively termed the interstellar medium. It is made up of two components—gas and dust—intermixed throughout all of space. The gas is made up mainly of individual atoms and small molecules. The dust consists of clumps of atoms and molecules—not unlike the microscopic particles that make up smoke or soot.

Apart from numerous narrow atomic and molecular absorption lines, the gas alone does not block electromagnetic radiation to any great extent. The obscuration evident in Figure 11.1 is caused by dust. Light from distant stars cannot penetrate the densest accumulations of interstellar dust any more than a car’s headlights can illuminate the road ahead in a thick fog. As a rule of thumb, a beam of light can be absorbed or scattered only by particles having diameters comparable to or larger than the wavelength of the radiation involved, and the amount of obscuration (absorption or scattering) produced by particles of a given size increases with decreasing wavelength. The typical diameter of an interstellar dust particle—or dust grain—is about 10-7 m, comparable in size to the wavelength of visible light. 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.

Because the opacity of the interstellar medium increases with decreasing wavelength, 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 appear redder than they really are (Figure 11.2). This effect, known as reddening, is similar to the process that produces spectacular red sunsets here on Earth. As indicated in Figure 11.2, absorption lines in a star’s spectrum are still recognizable in the radiation reaching Earth, allowing the star’s spectral class, and hence its luminosity and color, to be determined. (Sec. 10.7) Astronomers can then measure the degree to which the star’s light has been diminished and reddened en route to Earth.

Figure 11.2 Light Reddening 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.
Density and Composition of the Interstellar Medium

By measuring its effect on the light from many different stars, astronomers have built up a picture of the distribution and chemical properties of interstellar matter in the solar neighborhood.

Gas and dust are found everywhere in interstellar space. No part of our Galaxy is truly devoid of matter, although the density of the interstellar medium is extremely low. Overall, the gas averages roughly 106 atoms per cubic meter—just one atom per cubic centimeter. (For comparison, the best vacuum presently attainable in laboratories on Earth contains about 109 atoms per cubic meter.) Interstellar dust is even rarer—about one dust particle for every trillion or so atoms. The space between the stars is populated with matter so thin that harvesting all the gas and dust in an interstellar region the size of Earth would yield barely enough matter to make a pair of dice.

Despite such low densities, over sufficiently large distances interstellar matter accumulates slowly but surely, to the point where it can block visible light and other short-wavelength radiation from distant sources. All told, space in the vicinity of the Sun contains about as much mass in the form of interstellar gas and dust as exists there in the form of stars.

Interstellar matter is distributed very unevenly. In some directions it 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 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.

The composition of interstellar gas is reasonably well known from spectroscopic studies of interstellar absorption lines. (Sec. 2.5) Generally speaking, it mirrors the composition of 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, nine percent is helium, and the remaining one percent consists of heavier elements. The gas is deficient in some heavy elements, such as carbon, oxygen, silicon, magnesium, and iron, most likely because these elements have gone to form the interstellar dust.

In contrast, the composition of the dust is not well known, although there is some infrared evidence for silicates, carbon, and iron, supporting the theory that interstellar dust forms out of interstellar gas. The dust probably also contains some "dirty ice," a frozen mixture of water ice contaminated with trace amounts of ammonia, methane, and other compounds, much like cometary nuclei in our own solar system. (Sec. 4.2)

Concept Check

If space is a near-perfect vacuum, how can there be enough dust in it to block starlight?