f stars like Algol until
Professor Vogel, from the spectroscopic observations made at Potsdam,
found that before every minimum Algol is receding from the sun, while it
is approaching us after the minimum. Assuming the orbit to be circular,
the velocity of Algol was found to be twenty-six miles per second. From
this and the length of the period (2d. 22h. 48m. 55s.) and the time of
obscuration it was easy to compute the size of the orbit and the actual
dimensions of the two bodies. It was even possible to go a step further
and to calculate from the orbital velocities the masses of the two
bodies,[41] assuming them to be of equal density--an assumption which is
no doubt very uncertain. The following are the approximate elements of
the Algol system found by Vogel:--

Diameter of Algol 1,054,000 miles.
Diameter of companion 825,000 miles.
Distance between their centres 3,220,000 miles.
Orbital velocity of Algol 26 miles per sec.
Orbital velocity of companion 55 miles per sec.
Mass of Algol 4/9 of sun's mass.
Mass of companion 2/9 of sun's mass.

The period of Algol has been gradually decreasing during the last
century (by six or seven seconds), but whether this is caused by the
motion of the pair round a third and very much more distant body, as
suggested by Mr. Chandler, has still to be found out.

We have already mentioned that in order to produce eclipses, and thereby
variations of light, it is necessary that the line of sight should lie
nearly in the plane of the orbit. It is also essential that there should
be a considerable difference of brightness between the two bodies. These
conditions must be fulfilled in the fifteen variable stars of the Algol
class now known; but according to the theory of probability, there must
be many more binary systems like that of Algol where these conditions
are not fulfilled, and in those cases no variations will occur in the
stars' brightness. Of course, we know many cases of a luminous star
travelling round another, but there must also be cases of a large
companion travelling round another at so small a distance that our
telescopes are unable to "divide" the double star. This has actually
been discovered by means of the spectroscope. If we suppose an extremely
close double star to be examined with the spectroscope, the spectra of
the two components will be super