]
We may thus form a new picture of the two branches of the temperature
curve, long since suggested by Lockyer, on very different grounds, as
the outline of stellar life. On the ascending side are the giants,
of vast dimensions and more diffuse than the air we breathe. There
are good reasons for believing that the mass of Betelgeuse cannot
be more than ten times that of the sun, while its volume is at
least a million times as great and may exceed eight million times
the sun's volume. Therefore, its average density must be like that
of an attenuated gas in an electric vacuum tube. Three-quarters
of the naked-eye stars are in the giant stage, which comprises
such familiar objects as Betelgeuse, Antares, and Aldebaran, but
most of them are much denser than these greatly inflated bodies.
The pinnacle is reached in the intensely hot white stars of the
helium class, in whose spectra the lines of this gas are very
conspicuous. The density of these stars is perhaps one-tenth that
of the sun. Sirius, also very hot, is nearly twice as dense. Then
comes the cooling stage, characterized, as already remarked, by
increasing density, and also by increasing chemical complexity
resulting from falling temperature. This life cycle is probably
not followed by all stars, but it may hold true for millions of
them.
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