Pagina:Scientia - Vol. VIII.djvu/45

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STAR-STREAMS

Other regions of the sky show similar results. The existence in many cases of a secondary maximum hints clearly at the interpretation of the phenomenon, ― the motions of the stars show a preference for two favoured directions. We

Fig. I

have seen that a preference for one favoured direction merely indicates a motion of the system as a whole with regard to the sun, and is not inconsistent with a haphazard distribution of the motions relative to one another; but a preference for two favoured directions of motion is a very significant peculiarity. We can hardly be wrong in considering it to indicate the coexistence of two streams of stars, using the word stream in a general sense; there is of course an element of hypothesis admitted, when we assume that each stream is a more or less independent stellar system.

Acting on this clue, we now assume that instead of dealing with one system of stars having haphazard motions, there are two such systems, these systems having different motions with respect to the sun. The theoretical distribution of the motions of the stars can be calculated for each system separately and by adding the results together, we arrive at the distribution to be expected from the combination. The success of the hypothesis of the two star-streams in accounting for the observed data becomes very striking when a numerical comparison of observation and theory is made. It is found possible, by choosing suitable directions, velocities and relative proportions of the two streams, to build up from the combination a theoretical distribution of motions agreeing