Two things can happen to light rays when they meet a dust par¬ticle ; they can be either absorbed into the grain or scattered back into space. The relative importance of these two processes depends on the albedo of the grain, which is defined thus:

albedo = Light scattered/ light scattered + light absorbed

The albedo of a grain depends on its size, its shape and its composition, and also on the wavelength of the radiation being scattered. For interstellar dust grains at visible wavelengths the albedo is about 0.5, so that approximately equal amounts of light are absorbed and scattered. Starlight that has been scattered no longer travels in a straight line from its parent star. When observed from the Earth, therefore, such light appears to shine from the patch of interstellar dust where it was last scattered rather than from the star itself. The dust therefore appears to be shining.

Scattered starlight makes up about-a quarter of the light of the Milky Way. Usually the scattered light is too diffuse for either the star or the dust cloud to be identified, but if a bright star lies close enough to a dense patch of dust a REFLECTION NEBULA may be seen. A good example of reflection nebulae is shown in figure 13.5. The Pleiades cluster consists of a group of over 3000 young stars situated at a distance of some 120pc from the Sun hi the constellation of Taurus. The brightest stars in this group, though not the nebulosity surrounding them, are visible to the naked eye. Spectroscopic analysis confirms that the light from the nebulae has the same characteristic absorption lines as the B-type stars illuminating them. However, since blue light is scattered more than red light, the nebulae appear bluer than their illuminating stars.

The blue colour of reflection nebulae is a useful method of distinguishing them from ionized hydrogen (H+ regions) which they sometimes resemble. A more clear-out distinction, however, can be made on the basis of the spectrum of the light from the nebula, A reflection nebula, has a continuous spectrum with stellar-type absorption lines, whereas the light from an H+ region is predominantly in the form of emission lines. Two further differences between the two types of nebula are that the light from reflection nebulae is often polarized whereas that from H+ regions is not and that reflection nebulae are usually associated with B-type stars, H+ regions with 0-type stars.

A reflection nebula can sometimes reveal the presence of a hidden star. The object was discovered in 1974 as a result of an infrared survey of the sky. The spectrum of the light from the two bright blobs indicates that they are being illuminated by an F-type supergiant star lying in the dark lane between the blobs and hidden from earth by a very thick layer of dust.

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