But there can be no doubt
that the diameter of Betelgeuse exceeds 100,000,000 miles, and
it is probably much greater.
The extremely small angle subtended by this enormous disk is explained
by the great distance of the star, which is about 160 light-years.
That is to say, light travelling at the rate of 186,000 miles per
second spends 160 years in crossing the space that lies between
us and Betelgeuse, whose tremendous proportions therefore seem
so minute even in the most powerful telescopes.
STELLAR EVOLUTION
This actual measure of the diameter of Betelgeuse supplies a new
and striking test of Russell's and Hertzsprung's theory of dwarf
and giant stars. Just before the war Russell showed that our old
methods of classifying the stars according to their spectra must
be radically changed. Stars in an early stage of their life history
may be regarded as diffuse gaseous masses, enormously larger than
our sun, and at a much lower temperature. Their density must be
very low, and their state that of a perfect gas. These are the
"giants." In the slow process of time they contract through constant
loss of heat by radiation. But, despite this loss, the heat produced
by contraction and from other sources (see p. 82) causes their
temperature to rise, while their color changes from red to bluish
white.
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