The system of the two stars, which were about 31/4
millions of miles apart, considered as a whole, was approaching us
with a velocity of 2.4 miles a second. The great difference in
luminosity of the two stars, not less than fifty times, suggested
rather that they were in different stages of condensation, and
dissimilar in density. It was obvious that if the orbit of a star with
an obscure companion was inclined to the line of sight, the companion
would pass above or below the bright star and produce no variation of
its light. Such systems might be numerous in the heavens. In Vogel's
photographs, Spica, which was not variable, by a small shifting of its
lines revealed a backward and forward periodical pulsation due to
orbital motion. As the pair whirled round their common center of
gravity, the bright star was sometimes advancing, at others receding.
They revolved in about four days, each star moving with a velocity of
about 56 miles a second in an orbit probably nearly circular, and
possessed a combined mass of rather more than two and one-half times
that of the sun. Taking the most probable value for the star's
parallax, the greatest angular separation of the stars would be far
too small to be detected with the most powerful telescopes.
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