Many astronomers believe that in "variable stars" we have another star,
following that of the dullest red star, in the dying of suns. The light
of these stars varies periodically in so many days, weeks, or years. It
is interesting to speculate that they are slowly dying suns, in which
the molten interior periodically bursts through the shell of thick
vapours that is gathering round them. What we saw about our sun seems to
point to some such stage in the future. That is, however, not the
received opinion about variable stars. It may be that they are stars
which periodically pass through a great swarm of meteors or a region of
space that is rich in cosmic dust of some sort, when, of course, a great
illumination would take place.
One class of these variable stars, which takes its name from the star
Algol, is of special interest. Every third night Algol has its light
reduced for several hours. Modern astronomy has discovered that in this
case there are really two stars, circulating round a common centre, and
that every third night the fainter of the two comes directly between us
and its companion and causes an "eclipse." This was until recently
regarded as a most interesting case in which a dead star revealed itself
to us by passing before the light of another star. But astronomers have
in recent years invented something, the "selenium-cell," which is even
more sensitive than the photographic plate, and on this the supposed
dead star registers itself as very much alive. Algol is, however,
interesting in another way. The pair of stars which we have discovered
in it are hundreds of trillions of miles away from the earth, yet we
know their masses and their distances from each other.