They show two (possibly more, but
generally two) great spiral arms starting from the central part
and winding out into space. As they are flat or disk-shaped, we
see this structure plainly when they turn full face toward the
earth, as does the magnificent nebula in Canes Venatici. In it,
and many others, we clearly trace a condensed central mass, with
two great arms, each apparently having smaller centres of
condensation, sprawling outward like the broken spring of a
watch. The same structure can be traced in the mighty nebula in
Andromeda, which is visible to the naked eye, and it is said that
more than half the nebulae in the heavens are spiral. Knowing
that they are masses of solid or liquid fire, we are tempted to
see in them gigantic Catherine-wheels, the fireworks of the gods.
What is their relation to the stars?
In the first place, their mere existence has provided a solid
basis for the nebular hypothesis, and their spiral form
irresistibly suggests that they are whirling round on their
central axis and concentrating. Further, we find in some of the
gaseous nebulae (Orion) comparatively void spaces occupied by
stars, which seem to have absorbed the nebulous matter in their
formation.
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