f two stars which revolve around
each other in regular orbits, and are among some of the most interesting
objects in the heavens. About 1000 Binary stars are known altogether.
Their motions, however, are very slow, and only in a comparatively few
cases have the dimensions of their orbits been ascertained. Some of the
Binary stars are Zeta Hercules, which has a period of about 36 years;
Eta Coronae Borealis, which has a period of 43 years; while the
brightest star, Sirius, is also a Binary star, with a period of about 50
years.

_The Milky Way._--The Milky Way is the name given to that band of light
which stretches across the sky at night-time, and forms a zone or belt
that completely circles the celestial sphere.

This belt of light has maintained from the earliest ages the same
relative position among the stars, and, when resolved by powerful
telescopes, is found to consist entirely of stars scattered by millions
across the expanse of the heavens.

The whole zone or belt is composed of nothing but stars, whose average
magnitude, according to Herschel, is about the tenth.

Stars of all magnitudes are, however, found in this zone.

Of the brightest stars, about twelve are found in this region, while the
majority of stars of the second, third, and fourth magnitudes are also
found in or near it.

The great majority of star clusters are also found along the course of
the Milky Way, while many of the irresolvable nebulae seem to congregate
near the poles of this starry region.

The Milky Way is divided in one part of its course by a stream of stars,
which seems to branch off as a separate stream, thus dividing it into
two parts.

All these facts seem to point to the conclusion that the stars of the
universe, instead of being scattered about haphazard in the space, form
a ring or layer, of which the thickness is very small compared with its
length and breadth.

Our own solar system, according to Herschel, occupies a place somewhere
about the middle of the thickness of the zone, and near the point where
it divides into two parts.

Recent observations go to show that there is a tendency of the sun's
apex to drift along the edge of the Milky Way, and this drift seems to
point to a plane of motion of the sun, nearly coinciding with the plane
of the Milky Way.

ART. 118. _Stars and Kepler's Laws._--We have learned in a previous
chapter that the sun is the centre of a system which comprises a retinue
of planets, wi