By observations with the Potsdam spectograph, Professor Vogel found
that the bright star of Algol pulsated backward and forward in the
visual direction in a period corresponding to the known variation of
its light. The explanation which had been suggested for the star's
variability, that it was partially eclipsed at regular intervals of
68.8 hours by a dark companion large enough to cut off nearly
five-sixths of its light, was, therefore, the true one. The dark
companion, no longer able to hide itself by its obscureness, was
brought out into the light of direct observation by means of its
gravitational effects. Seventeen hours before minimum Algol was
receding at the rate of about 24½ miles a second, while seventeen
hours after minimum it was found to be approaching with a speed of
about 28½ miles. From these data, together with those of the variation
of its light, Vogel found, on the assumption that both stars have the
same density, that the companion, nearly as large as the sun, but with
about one-fourth his mass, revolved with a velocity of about
fifty-five miles a second. The bright star of about twice the size and
mass moved about the common center of gravity with the speed of about
26 miles a second. The system of the two stars, which were about 3¼
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.