ALGOL AND SPICA.
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 241/2 miles a second, while seventeen
hours after minimum it was found to be approaching with a speed of
about 281/2 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.
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