We need not now attempt to follow the shrinking of the central
part of the nebula until it becomes a rounded fiery sun. That has
been done in tracing the evolution of a star. Here we have to
learn how the planets were formed from the spiral arms of the
nebula. The principle of their formation is already clear. The
same force of gravitation, or the same pressure of the
surrounding ether, which compresses the central mass into a fiery
globe, will act upon the loose material of the arms and compress
it into smaller globes. But there is an interesting and acute
difference of opinion amongst modern experts as to whether these
smaller globes, the early planets, would become white-hot bodies.
The general opinion, especially among astronomers, is that the
compression of the nebulous material of the arms into globes
would generate enormous heat, as in the case of the sun. On that
view the various planets would begin their careers as small suns,
and would pass through those stages of cooling and shrinking
which we have traced in the story of the stars. A glance at the
photograph of one of the spiral nebulae strongly confirms this.
Great luminous knots, or nuclei, are seen at intervals in the
arms.
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