Instead of all moving one way, the galactic stars, as far as their
movements can be inferred, are governed by local influences and
conditions. They appear to travel crosswise and in contrary
directions, and perhaps they eddy around foci where great numbers have
assembled; but of a universal revolution involving the entire mass we
have no evidence.
Most of our knowledge of star motions, called ``proper motions,''
relates to individual stars and to a few groups which happen to be so
near that the effects of their movements are measurable. In some cases
the motion is so rapid (not in appearance, but in reality) that the
chief difficulty is to imagine how it can have been imparted, and what
will eventually become of the ``runaways.'' Without a collision, or a
series of very close approaches to great gravitational centers, a star
traveling through space at the rate of two hundred or three hundred
miles per second could not be arrested or turned into an orbit which
would keep it forever flying within the limits of the visible
universe. A famous example of these speeding stars is ``1830
Groombridge,'' a star of only the sixth magnitude, and consequently
just visible to the naked eye, whose motion across the line of sight
is so rapid that it moves upon the face of the sky a distance equal to
the apparent diameter of the moon every 280 years.
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