a disk rotating on its central axis. The ring-theory of
Laplace is practically abandoned. The spiral nebula is evidently the
standard type, and the condensing nebula must conform to it. In this
we are greatly helped by the current theory of the origin of spiral
nebulae.

We saw previously that new stars sometimes appear in the sky, and the
recent closer scrutiny of the heavens shows this occurrence to be fairly
frequent. It is still held by a few astronomers that such a cataclysm
means that two stars collided. Even a partial or "grazing" collision
between two masses, each weighing billions of tons, travelling (on the
average) forty or fifty miles a second--a movement that would increase
enormously as they approach each other--would certainly liquefy or
vaporise their substance; but the astronomer, accustomed to see cosmic
bodies escape each other by increasing their speed, is generally
disinclined to believe in collisions. Some have made the new star plunge
into the heart of a dense and dark nebula; some have imagined a shock of
two gigantic swarms of meteors; some have regarded the outflame as the
effect of a prodigious explosion. In one or other new star each or any
of these things may have occurred, but the most plausible and accepted
theory for the new star of 1901 and some others is that two stars had
approached each other too closely in their wandering. Suppose that,
in millions of years to come, when our sun is extinct and a firm crust
surrounds the great molten ball, some other sun approaches within a few
million miles of it. The two would rush past each other at a terrific
speed, but the gravitational effect of the approaching star would tear
open the solid shell of the sun, and, in a mighty flame, its molten and
gaseous entrails would be flung out into space. It has long been one
of the arguments against a molten interior of the earth that the sun's
gravitational influence would raise it in gigantic tides and rend the
solid shell of rock. It is even suspected now that our small earth
is not without a tidal influence on the sun. The comparatively near
approach of two suns would lead to a terrific cataclysm.

If we accept this theory, the origin of the spiral nebula becomes
intelligible. As the sun from which it is formed is already rotating
on its axis, we get a rotation of the nebula from the first. The mass
poured out from the body of the sun would, even if it were only a small
fraction of its mass, suffice