taken as our unit. We find the distance of the surface of this
sphere by extracting the cube root of this number, which gives us
500. We may, therefore, say, as the result of a very rough
estimate, that the number of stars we have supposed would be
contained within a distance found by multiplying 400,000 times the
distance of the sun by 500; that is, that they are contained
within a region whose boundary is 200,000,000 times the distance
of the sun. This is a distance through which light would travel in
about 3300 years.

It is not impossible that the number of stars is much greater than
that we have supposed. Let us grant that there are eight times as
many, or 1,000,000,000. Then we should have to extend the boundary
of our universe twice as far, carrying it to a distance which
light would require 6600 years to travel.

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