stars in general have been formed directly from the irregular
nebulae, without the intervention of the planetaries. The
planetary nebula seem to be exceptional cases, but to this
point we shall return later.

It is quite possible, and even probable, that gaseous masses
have not in all cases passed directly to the stellar state. The
materials in a gaseous nebula may be so highly attenuated, or
be distributed so irregularly throughout a vast volume of
space, that they will condense into solids, small meteoric
particles for example, before they combine to form stars. Such
masses or clouds of non-shining or invisible matter are thought
to exist in considerable profusion within the stellar system.
The nebulosity connected more or less closely with the brighter
Pleiades stars may be a case in illustration. Slipher has
recently found that the spectra of two small regions observed
in this nebula are continuous, with absorption lines of
hydrogen and helium. This spectrum is apparently the same as
that of the bright Pleiades stars. Slipher's interpretation is
that the nebula is not shining by its own light, but is
reflecting to us the light of the Pleiades stars. That this
material will eventually be drawn into the stars already
existing in the neighborhood, or be condensed into new centers
and form other stars, we can scarcely doubt. The condensation
of such materials to form stars large enough to be seen from
the great distance of the Pleiades cluster must generate heat
in the process, and cause these stars in their earliest youth
to be substantially as hot as other stars formed directly from
gaseous materials. It is possible, also, that the spiral
nebulae will develop into stars, perhaps each such object into