Observatory,
Illinois, U.S.A. The companion is only of the tenth magnitude, and
revolves around Sirius in a period of about fifty years, at a mean
distance equal to about that of Uranus from the sun. Seen from Sirius,
it would shine only something like our full moon. It must be
self-luminous and not a mere planet; for Mr. Gore has shown that if it
shone only by the light reflected from Sirius, it would be quite
invisible even in the Great Yerkes Telescope.

Procyon is also a binary, its companion having been discovered by
Professor J.M. Schaeberle at the Lick Observatory in 1896. The period of
revolution in this system is about forty years. Observations by Mr. T.
Lewis of Greenwich seem, however, to point to the companion being a
small nebula rather than a star.

The star [e] (Eta) Cassiopeiae (see Plate XIX., p. 292), is easily seen
as a fine double in telescopes of moderate size. It is a binary system,
the component bodies revolving around their common centre of gravity in
a period of about two hundred years. This system is comparatively near
to us, _i.e._ about nine light years, or a little further off than
Sirius.

In a small telescope the star Castor will be found double, the
components, one of which is brighter than the other, forming a binary
system. The fainter of these was found by Belopolsky, with the
spectroscope, to be composed of a system of two stars, one bright and
the other either dark or not so bright, revolving around each other in a
period of about three days. The brighter component of Castor is also a
spectroscopic binary, with a period of about nine days; so that the
whole of what we see with the naked eye as Castor, is in reality a
remarkable system of four stars in mutual orbital movement.

Alpha Centauri--the nearest star to the earth--is a visual binary, the
component bodies revolving around each other in a period of about
eighty-one years. The extent of this system is about the same as that of
Sirius. Viewed from each other, the bodies would shine only like our sun
as seen from Neptune.

Among the numerous binary stars the orbits of some fifty have been
satisfactorily determined. Many double stars, for which this has not yet
been done, are, however, believed to be, without doubt, binary. In some
cases a parallax has been found; so that we are enabled to estimate in
miles the actual extent of such systems, and the masses of the bodies in
terms of the sun's mass.

Most of the spectroscopi