Page 651 - 89 history of astronomy_opt

lessons to be drawn from stellar spectra.
Some say that they know with certainty
that each star begins by being a nebula,
and is condensed and heated by condensa-
tion until it begins to shine as a star; that it
attains a climax of temperature, then cools
down, and eventually becomes extinct. They
go so far as to declare that they know what
class of spectrum belongs to each stage of a
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