As discovery went on and our conceptions of the
universe were enlarged, it was found that the system of the fixed
stars was made up of bodies so vastly distant and so completely
isolated that it was difficult to conceive of them as standing in
any definable relation to one another. It is true that they all
emitted light, else we could not see them, and the theory of
gravitation, if extended to such distances, a fact not then
proved, showed that they acted on one another by their mutual
gravitation. But this was all. Leaving out light and gravitation,
the universe was still, in the time of Herschel, composed of
bodies which, for the most part, could not stand in any known
relation one to the other.
When, forty years ago, the spectroscope was applied to analyze the
light coming from the stars, a field was opened not less fruitful
than that which the telescope made known to Galileo. The first
conclusion reached was that the sun was composed almost entirely
of the same elements that existed upon the earth. Yet, as the
bodies of our solar system were evidently closely related, this
was not remarkable. But very soon the same conclusion was, to a
limited extent, extended to the fixed stars in general.
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