There is another method of estimating the thickness with which
stars are sown through space, and hence the extent of the
universe, the result of which will be of interest. It is based on
the proper motion of the stars. One of the greatest triumphs of
astronomy of our time has been the measurement of the actual speed
at which many of the stars are moving to or from us in space.
These measures are made with the spectroscope. Unfortunately, they
can be best made only on the brighter stars--becoming very
difficult in the case of stars not plainly visible to the naked
eye. Still the motions of several hundreds have been measured and
the number is constantly increasing.
A general result of all these measures and of other estimates may
be summed up by saying that there is a certain average speed with
which the individual stars move in space; and that this average is
about twenty miles per second. We are also able to form an
estimate as to what proportion of the stars move with each rate of
speed from the lowest up to a limit which is probably as high as
150 miles per second. Knowing these proportions we have, by
observation of the proper motions of the stars, another method of
estimating how thickly they are scattered in space; in other
words, what is the volume of space which, on the average, contains
a single star.
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