We
have shown that stars of every magnitude, bright and faint, show a
tendency to crowd towards this belt. It is, therefore, remarkable
that no such tendency is seen in the case of those stars which
have proper motions large enough to be accurately determined. So
far as yet appears, such stars are equally scattered over the
heavens, without reference to the course of the Milky Way. The
conclusion is obvious. These stars are all inside the girdle of
the Milky Way, and within the sphere which contains them the
distribution in space is approximately uniform. At least there is
no well-marked condensation in the direction of the galaxy nor any
marked thinning out towards its poles. What can we say as to the
extent of this sphere?
To answer this question, we have to consider whether there is any
average or ordinary speed that a star has in space. A great number
of motions in the line of sight--that is to say, in the direction
of the line from us to the star--have been measured with great
precision by Campbell at the Lick Observatory, and by other
astronomers. The statistical investigations of Kaptoyn also throw
much light on the subject. The results of these investigators
agree well in showing an average speed in space--a straight-ahead
motion we may call it--of twenty-one miles per second.
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