glomerations of stars--to star clusters at least.
These vast and enigmatic objects do not throw much light on the origin
of our own solar system. The nebular hypothesis, which was invented
by Laplace to explain the origin of our solar system, has not yet met
with universal acceptance. The explanation offers grave difficulties,
and it is best while the subject is still being closely investigated, to
hold all opinions with reserve. It may be taken as probable, however,
that the universe has developed from masses of incandescent gas.

[Illustration: _Photo: Yerkes Observatory._

FIG. 24.--THE GREAT NEBULA IN ORION

The most impressive nebula in the heavens. It is inconceivably greater
in dimensions than the whole solar system.]

[Illustration: _Photo: Lick Observatory._

FIG. 25--GIANT SPIRAL NEBULA, March 23, 1914

This spiral nebula is seen full on. Notice the central nucleus and the
two spiral arms emerging from its opposite directions. Is matter flowing
out of the nucleus into the arms or along the arms into the nucleus? In
either case we should get two streams in opposite directions within the
nucleus.]

THE BIRTH AND DEATH OF STARS

Sec. 3

Variable, New, and Dark Stars: Dying Suns

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