About 80 Algol variable stars are known. These are double stars
whose light is constant except during the short time when one
of the components in each system passes between us and the
other component. All double stars would be Algol variables if
we were exactly in the planes of their orbits. That so few
Algols have been observed amongst the tens of thousands of
double stars, is easily explained. The two component stars in
the few known Algol systems are so great in diameter, in
proportion to the size of their orbits, that eclipses are
observable throughout a wide volume of space, and the eclipses
are of long duration relatively to the revolution period. Their
densities are, so far as we have been able to determine them,
on an average less than 1/10th of the Sun"s density. Let us
note well that their spectra, so far as we have been able to
determine them, are of the early types; mostly helium and
hydrogen stars, and a very few of the Class F, intermediate
between the hydrogen and solar stars. There are no known Algols
of the Classes G, K, and M: these stars are very condensed and
therefore small in size, as compared with stars of Classes B
and A; and the components of double stars of these classes are
on the average much denser and therefore smaller in size than
the components in Classes B and A double stars; the components
are much farther apart in Classes G to M doubles than in
Classes B and A doubles; and for these reasons eclipses in
Classes G to M doubles occur but rarely for observers scattered
throughout space. It is difficult to avoid the conclusion that
the components of double stars separate more and more widely
with the progress of time. The conclusions which we have
earlier drawn from visual double stars are in full harmony with
the argument.
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