But,
although it may prolong the limited term of life which physicists
formerly allotted to the sun and other stars, it is still felt
that the condensation of a nebula offers the best explanation of
the origin of a sun, and we have ample evidence for the
connection. We must, therefore, see what the nebula is, and how
it develops.
"Nebula" is merely the Latin word for cloud. Whatever the nature
of these diffused stretches of matter may be, then, the name
applies fitly to them, and any theory of the development of a
star from them is still a "nebular hypothesis." But the three
theories which divide astronomers to-day differ as to the nature
of the nebula. The older theory, pointing to the gaseous nebulae
as the first stage, holds that the nebula is a cloud of extremely
attenuated gas. The meteoritic hypothesis (Sir N. Lockyer, Sir G.
Darwin, etc.), observing that space seems to swarm with meteors
and that the greater part of the nebulae are not gaseous,
believes that the starting-point is a colossal swarm of meteors,
surrounded by the gases evolved and lit up by their collisions.
The planetesimal hypothesis, advanced in recent years by
Professor Moulton and Professor Chamberlin, contends that the
nebula is a vast cloud of liquid or solid (but not gaseous)
particles.
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