adiation, leaving behind it portions of its mass, which
condensed to form the planets. Still gaseous, though now denser than
water, it continues to pour out the heat on which our existence
depends, as it shrinks imperceptibly toward its ultimate condition
of a cold and darkened globe.

Laplace's hypothesis has been subjected in recent years to much
criticism, and there is good reason to doubt whether his description
of the mode of evolution of our solar system is correct in every
particular. All critics agree, however, that the sun was once enormously
larger than it now is, and that the planets originally formed part
of its distended mass.

Even in its present diminished state, the sun is huge beyond easy
conception. Our own earth, though so minute a fragment of the primeval
sun, is nevertheless so large that some parts of its surface have
not yet been explored. Seen beside the sun, by an observer on one
of the planets, the earth would appear as an insignificant speck,
which could be swallowed with ease by the whirling vortex of a
sun-spot. If the sun were hollow, with the earth at its centre,
the moon, though 240,000 miles from us, would have room and to
spare in which to describe its orbit, for the sun is 865,000 miles
in diameter, so that its volume is more than a million times that
of the earth.

[Illustration: Fig. 19. Gaseous prominence at the sun's limb, 140,000
miles high (Ellerman).

Photographed with the spectroheliograph, using the light emitted
by glowing calcium vapor. The comparative size of the earth is
indicated by the white circle.]

But what of the stars, proved by the spectroscope to be self-luminous,
intensely hot, and formed of the same chemical elements that constitute
the sun and the earth? Are they comparable in size with the sun? Do
they occur in all stages of development, from infancy to old age?
And if such stages can be detected, do they afford indications
of the gradual diminution in volume which Laplace imagined the
sun to experience?

[Illustration: Fig. 20. The sun, 865,000 miles in diameter, from
a direct photograph showing many sun-spots (Whitney)

The small black disk in the centre represents the comparative size
of the earth, while the circle surrounding it corresponds in diameter
to the orbit of the moon.]

STAR IMAGES

Prior to the application of the powerful new engine of research
described in this article we have had no means of measuring the
diameters of the stars.