Introduction

The present chapter will be devoted to explaining how comets may come to be formed and also become dynamically associated with the sun. In brief, the proposed process is that of the accretion of interstellar dust through the gravitational action of the sun during passages through galactic dust clouds. It will be shown how this can result in the development of suitably compact aggregations of dust particles initially describing almost parabolic orbits round the sun. Estimates of the sizes and masses of these clusters of particles can be made mathematically from the theory of the mechanism, and also a rough calculation of the total number of such cometary aggregations likely to be brought into existence during the traverse of a single cloud. But before proceeding to the detailed discussion of the process, the analysis of which is somewhat intricate, it will be convenient to give first a short account of the evidence for interstellar dust, and the nature of its properties and distribution within the galaxy, since these things bear directly on the hypotheses of the theory.

Interstellar dust

The presence of obscuring matter in interstellar space seems first to have been recognized by Barnard, and that in fact it exists in the form of fine dust was established by Slipher from a study of the so-called reflexion nebulae often associated with dark regions. There are also numerous other observational factors that taken together have gradually confirmed the presence of such dust and further evidence is still accumulating. The frequent occurrence of dark lanes and patches, most of which are far too small and irregular in shape to be explicable as arising from the distribution of stars, provides direct evidence of material having strong obscuring power, a property highly characteristic of dust. Mass for mass, the more finely divided dust is, the greater area per unit mass can it screen, so long as the sizes of the subdivided particles do not fall much below the wave-length of the light concerned. For example, a centimetre cube of material could in the first place screen an area of 1 sq. cm. from light travelling perpendicular to one of its faces. But if it were divided into cubes 10-2 cm. in length and these all placed side by side, they would produce an area of 100 sq. cm.