re 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. The process of shrinkage and rise of temperature goes on so
long as they remain in the state of a perfect gas. But as soon as
contraction has increased the density of the gas beyond a certain
point the cycle reverses and the temperature begins to fall. The
bluish-white light of the star turns yellowish, and we enter the
dwarf stage, of which our own sun is a representative. The density
increases, surpassing that of water in the case of the sun, and
going far beyond this point in later stages. In the lapse of millions
of years a reddish hue appears, finally turning to deep red. The
falling temperature permits the chemical elements, existing in a
gaseous state in the outer atmosphere of the star, to unite into
compounds, which are rendered conspicuous by their characteristic
bands in the spectrum. Finally comes extinction of light, as the
star approaches its ultimate state of a cold and solid globe.

[Illustration: Fig. 27. The giant star Antares (within the white
circle), notable for its red color in the constellation Scorpio,
and named by the Greeks "A Rival of Mars" (Hubble).

The distance of Antares, though not very accurately kno