nly exceeding any terrestrial standard, but even any
distance in the solar system. For purely astronomical purposes the most
convenient unit is the distance corresponding to a parallax of 1",
which is a little more than 200,000 times the sun's distance. But for
the purposes of all but the professional astronomer the most convenient
unit will be the light-year--that is, the distance through which light
would travel in one year. This is equal to the product of 186,000
miles, the distance travelled in one second, by 31,558,000, the number
of seconds in a year. The reader who chooses to do so may perform the
multiplication for himself. The product will amount to about 63,000
times the distance of the sun.

[Illustration with caption: A Typical Star Cluster--Centauri]

The nearest star whose distance we know, Alpha Centauri, is distant
from us more than four light-years. In all likelihood this is really
the nearest star, and it is not at all probable that any other star
lies within six light-years. Moreover, if we were transported to this
star the probability seems to be that the sun would now be the nearest
star to us. Flying to any other of the stars whose parallax has been
measured, we should probably find that the average of the six or eight
nearest stars around us ranges somewhere between five and seven
light-years. We may, in a certain sense, call eight light-years a
star-distance, meaning by this term the average of the nearest
distances from one star to the surrounding ones.

To put the result of measures of parallax into another form, let us
suppose, described around our sun as a centre, a system of concentric
spheres each of whose surfaces is at the distance of six light-years
outside the sphere next within it. The inner is at the distance of six
light-years around the sun. The surface of the second sphere will be
twelve light-years away, that of the third eighteen, etc. The volumes
of space within each of these spheres will be as the cubes of the
diameters. The most likely conclusion we can draw from measures of
parallax is that the first sphere will contain, beside the sun at its
centre, only Alpha Centauri. The second, twelve light-years away, will
probably contain, besides these two, six other stars, making eight in
all. The third may contain twenty-one more, making twenty-seven stars
within the third sphere, which is the cube of three. Within the fourth
would probably be found sixty-four stars, this being th