ice the width of that of the planet Neptune. It has been
estimated that they complete a revolution in about eight centuries. The
united mass of the system is about one-half that of the Sun, and in
point of luminosity they are much inferior to that orb.

The star 70 Ophiuchi (fig. 3) may be regarded as typical of a binary
system. The components are five seconds apart, and of the fourth and
sixth magnitudes. Their light period is stated to be twenty years, and
the combined mass of the system is nearly three times that of the Sun.
The pair travel in an orbit from fourteen to forty-two times the radius
of the Earth's orbit; so that when at apastron they are three times as
distant from each other as when at periastron. They complete a
revolution in eighty-eight years.

The accompanying diagram (fig. 4) is a delineation of the beautiful
orbits of the components of Gamma Virginis. These may be described as
elongated ellipses. Both stars being of equal mass, their orbits are of
equal dimensions, and their common centre of gravity at a point
equidistant from each. Any approach to, or recession from this point,
must occur simultaneously with each; they must always occupy
corresponding parts of their orbits, and be in apastron and at
periastron in the same period of time. The ellipse described by this
pair is the most eccentric of known binary orbits, and approaches in
form the path pursued by Encke's comet round the Sun. These orbs
complete a revolution in 180 years, and when in apastron are seventeen
times more remote from each other than when at periastron.

[Illustration: FIG. 4.--The Orbits of the Components of Gamma Virginis.]

From his observation of the motion of Sirius in 1844, Bessel was led to
believe that the brilliant orb was accompanied by another body, whose
gravitational attraction was responsible for the irregularities observed
in the path of the great dog-star when pursuing his journey through
space. The elements of this hypothetical body were afterwards computed
by Peters and Auwers, and its exact position assigned by Safford in
1861.

On January 31, 1862, Mr. Alvan Clarke, of Cambridgeport, Massachusetts,
when engaged in testing a recently constructed telescope of great power,
directed it on Sirius, and was enabled by good fortune to discover the
companion star at a distance of ten seconds from its primary. Since its
discovery, the star has pursued with such precision the theoretical path
previously assign