ing occurred in 1750, they
travelled more rapidly in the last century than they are doing at
present, and, as their orbits are so eccentric that when at apastron the
stars are twice as remote from each other as at periastron, they will
for the next three and a half centuries continue to slacken their pace,
until they shall have reached the most remote points of their orbits,
when they will again begin to approach with an increasing velocity; so
that the time in which an entire revolution can be accomplished will not
be much less than 1,000 years.[8]

As the distance of Castor is unknown, it is impossible to compute the
combined mass of its components. They are very remote, their light
period being estimated at forty-four years. Castor is doubtless a more
massive orb than our Sun, and possesses a higher degree of luminosity.

Alpha Centauri, in the Southern Hemisphere, is the brightest binary, and
also the nearest known star in the heavens; its estimated distance being
twenty-five billions of miles. Both components equal stars of the first
magnitude, and are of a brilliant white colour. Since they were first
observed, in 1709, they have completed two revolutions, and are now
accomplishing a third. The eccentricity of their orbit approaches in
form that of Faye's comet, which travels round the Sun; consequently the
stars, when at apastron, are twice their periastron distance. Their
period of revolution is about eighty-eight years. The mean radius of
their orbit corresponds to a span of 1,000 millions of miles, so that
those orbs are sometimes as close to each other as Jupiter is to the
Sun, and never so far distant as Uranus.[9] Their combined mass is twice
that of the Sun, and the luminosity of each star is slightly greater.

The double star 61 Cygni--one of the nearest to our system--is believed
to be a binary the components of which move in an orbit of more spacious
dimensions than that of any other known revolving pair. Though they have
been under continuous observation since 1753, it is only within the last
few years that any orbital motion has been perceived. Some observers
are disinclined to admit the accuracy of this statement; whilst others
believe that the stars have executed a hyperbolic sweep round their
common centre of gravity and are now separating.

The radius of the orbit in which those bodies travel is sixty-five times
the distance of the Earth from the Sun; which means that they travel in
an orbit tw