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| 35 | STAR-STREAMS |
the streaming of the stars in the two favoured directions as due to some other cause which does not postulate a dual origin for the universe. But in either case it is clear that this new presentation differs very markedly from the old idea of a chaos of stars moving at random; and the existence of the two streams is an evidence of structure of a very unexpected kind.
Whilst the statements contained in the foregoing paragraphs are the results of mathematical investigations which could not well be described here, it is possible to indicate in a general way the peculiarity in the distribution of the stars’ motions which led to the detection of the streams. Suppose that in a small region of the sky there are a few hundred stars whose motions have been observed, and that we count how many of these are moving in the various directions. If the group considered consists of stars moving at random, it might at first sight be expected that the observed motions would be in approximately equal numbers in all directions. But this leaves out of account the fact that the group taken as a whole will probably be in motion relatively to the solar system, which is our point of observation. We have already referred to this matter in connection with the solar motion. The fact is that when we speak of a group of stars moving at random, we mean that their motions relative to one another or to the mean of the group are at random; when referred to some independent point such as the sun, the resultant motions cease to be strictly at random for there is, as it were superposed, a systematic motion, namely that of the group as a whole relative to the sun. Let, for example, this motion of the group of stars as a whole be towards the north, each star having in addition its own haphazard motion with respect to the rest; evidently the effect of the group-motion will be that, although stars may be found moving towards all points of the compass, the greatest number will be moving north, the fewest south, the number falling off symmetrically on either side from north to south. But it is possible to advance a stage further. If the motions of the individuals are entirely haphazard (controlled by no law) the tout ensemble will yet be subject to that most paradoxical of laws, the law of chance; and it is possible to predict, at least tentatively, what proportion of stars will be moving in any direction, having given
