We cannot examine interstellar dust in a laboratory. Small inter¬planetary particles that reach the Earth or that are intercepted by spacecraft are characteristic only of our Solar System and are prob¬ably quite different from those found in interstellar space. What knowledge we have about the size, shape and composition of interstellar grains is based on our observations of how grains absorb, scatter, polarize and radiate electromagnetic waves, and on our ideas about what kinds of materials are plentiful enough and stable enough to exist in space.

The extinction curve gives us the most information. From it astronomers have deduced that most of the dust grains which cause extinction at visible wavelengths have diameters of about 0.1 ?m. Such grains weigh only 10-18kg each. Even smaller particles are necessary to provide the extinction at ultraviolet wavelengths, but we do not yet have much idea about the total range of grain sizes in interstellar space. We also know almost nothing about the shapes of grains.

The three bumps on the extinction curve indicate that graphite particles, ice crystals and some silicate material, such as magnesium silicate, are probably present. Other substances which have been suggested as constituents of dust grains include magnesium carbonate, silicon carbide, iron, diamond, and various organic molecules including complicated polymers. Different materials may be present in the same
Grain and a favored model consists of a small silicate particle surrounded by a thicker layer of ice

Grains can be formed by condensation from the gas in the outer atmospheres of late-type stars: the dust in the infrared shells of u. Cephei and IRC — 10216 was made in this way. Different sorts of stars make different sorts of grain. An oxygen-rich star, for example, tends to produce silicate grains, a carbon-rich -star graphite or silicon carbide grains. The grains are eventually blown away from the star into interstellar space. New grains are then formed from new material leaving the star’s surface. Not all dust K made this way. Some grains may condense out of gas as it contracts to form new stars, and others grow by steadily accreting extra atoms and molecules from the interstellar medium. Grains are destroyed either by being incorporated into new stars or by being subjected to high temperatures or harsh radiation fields. Whatever the material the grains are made of, and however they form, we do not expect that on average more than one per cent of the mass of the interstellar medium is in the form of dust at any time. The reason for this is that only those atoms in the interstellar medium which are not hydrogen or helium are of much use in making grains. Such elements altogether comprise less than one per cent of the mass of the Galaxy.

Comments are closed.