The arrows shown in Fig. 130 are a feature not usually found with light
curves, but in this case each one represents a spectroscopic
determination of the motion of Algol in the line of sight. These
observations extended over a period of more than two years, but they are
plotted in the figure with reference to the number of hours each one
preceded or followed a minimum of the star's light, and each arrow shows
not only the direction of the star's motion along the line of sight, the
arrows pointing down denoting approach of the star toward the earth, but
also its velocity, each square of the ruling corresponding to 10
kilometers (6.2 miles per second). The differences of velocity shown by
adjacent arrows come mainly from errors of observation and furnish some
idea of how consistent among themselves such observations are, but there
can be no doubt that before minimum the star is moving away from the
earth, and after minimum is approaching it. It is evident from these
observations that in Algol we have to do with a spectroscopic binary,
one of whose components is a dark star which, once in each revolution,
partially eclipses the bright star and produces thus the variations in
its light. By combining the spectroscopic observations with the
variations in the star's light, Vogel finds that the bright star, Algol,
itself has a diameter somewhat greater than that of the sun, but is of
low density, so that its mass is less than half that of the sun, while
the dark star is a very little smaller than the sun and has about a
quarter of its mass. The distance between the two stars, dark and
bright, is 3,200,000 miles. Fig. 129, which is drawn to scale, shows the
relative positions and sizes of these stars as well as the orbits in
which they move.