Introduction

The central astronomical role of dust is at its most evident in the infrared. Protostars form from dusty clouds of molecular gas; the cool condensing dust around them emits copiously at sub-millimeter and far-infrared wavelengths. Even fully developed stars such as Vega may be found to be surrounded by remnant dust particles (see Habing et al., 1996), causing excess emission at long wavelengths. Near the end of their existence, a new generation of dust is formed by evolved stars, for example, in the atmospheres of asymptotic giant branch objects and in the ejecta of supernovae.

As in the visible region, dust scatters and absorbs light, giving rise to extinction, though its effects are much smaller in the infrared than in the visible. A dramatic example is the Galactic Center, which suffers 30 mag of extinction at V, so that only about one photon in 1012 comes through, whereas AK (2.2 μm) is about 2.5 mag and ∼10% of the photons can penetrate. Figure 5.1 shows the distribution of several types of objects with cool dust in an IRAS color-color diagram.

Dust also polarizes the light from distant stars and some properties of the polarization are found to be related to the extinction.

Even when the extinction is quite moderate, by observing in the infrared, the effects of interstellar absorption, which bedevil the use of the cepheid and RR Lyrae period-luminosity relations for distance determination, may be greatly reduced (e.g., Laney and Stobie, 1993, in the case of cepheids).