Since starlight carries energy, any light that is absorbed (as opposed to scattered) by a dust grain tends to increase the temperature of the dust grain. This absorption of energy is balanced by the energy that the grain reradiates into space. The wavelength of the radiation from the grain depends on its temperature. Normal interstellar dust, at a temperature of a few tens of degrees kelvin, radiates at very long infrared wavelengths of 100 um or more. This radiation is faint and difficult to observe. Grains close to stars become hotter and therefore radiate at shorter wavelengths. They rarely emit visible light, however, since most grains are destroyed by being heated above a few hundred degrees kelvin.

Infrared emission from heated grains is most easily observed when there are thick layers of dust in the immediate vicinity of luminous stars. Such a situation commonly occurs in dense clouds associated with the birth of stars in the nuclei of some galaxies. Many late-type stars also show strong infrared emission from dust grains. The fourth-magnitude star ? Cephei, an M 2 la supergiant, is an example. Shortward of 7 ?m its energy distribution resembles that of a 4000 K black-body, as would be expected for a star of this spectral class . Instead of continuing to decrease longward of 7 ?m, however, the flux density increases sharply at 10 ?m and again at 20 ?m. These are wavelengths at which heated dust particles radiate most efficiently ; the presence of these infra¬red ‘bumps’ indicate that there is a shell of warm dust surrounding the star. The dust particles absorb about ten per cent of the light from the star and reradiate this energy as infrared waves. The dust shell is too small to be seen directly, and its presence can only be inferred from its infrared radiation.

A more extreme example of a star surrounded by a dust shell’ provided by the object IRC + 10216. In this case essentially all of the light from the star, a carbon star at a temperature of 2000K is absorbed in the dust shell and reradiated at infrared wavelengths . As a consequence, this object is hardly visible even on red-sensitive plates taken with large telescopes (figure 13.9). At infrared wavelengths, however, it is very luminous and at 20 ?m is one of the ten brightest objects in the sky. Most of the infrared flux comes from a shell of dust 0.4 arc sec in diameter at a temperature of 600 K. The distance to IRC +10216 may be about 200pc. If so, then the dust cloud has a diameter of 80 AU (the same as Pluto’s orbit around the Sun) and a luminosity of 25 000 L0.

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