It is then supposed that the contracting mass of gas would begin to
rotate and to throw off gigantic streamers, which would in their turn
form centres of condensation. The whole structure would thus form a
spiral, having a dense region at its centre and knots or lumps of
condensed matter along its spiral arms. Besides the formless gaseous
nebulæ there are hundreds of thousands of "spiral" nebulæ such as we
have just mentioned in the heavens. They are at all stages of
development, and they are visible to us at all angles--that is to say,
some of them face directly towards us, others are edge on, and some are
in intermediate positions. It appears, therefore, that we have here a
striking confirmation of the nebular hypothesis. But we must not go so
fast. There is much controversy as to the nature of these spiral nebulæ.
Some eminent astronomers think they are other stellar universes,
comparable in size with our own. In any case they are vast structures,
and if they represent stars in process of condensation, they must be
giving birth to huge agglomerations 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._