wn, is probably
not far from 350 light-years. Its angular diameter of 0.040 of a
second would thus correspond to a linear diameter of about 400
million miles.]

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.

The existence of giant and dwarf stars has been fully proved by
the remarkable work of Adams and his associates on Mount Wilson,
where his method of determining a star's distance and intrinsic
luminosity by spectroscopic observations has already been applied
to 2,000 stars. Discussion of the results leads at once to the
recognition of the two great classes of giants and dwarfs. Now
comes the work of Michelson and Pease to cap the climax, giving us
the actual diameter of a typical giant star, in close agreement with
predictions based upon theory. From this diameter we may conclude that
the density of Betelgeuse is extremely low, in harmony with Russell's
theory, which is further supported by spectroscopic analysis of
the star's light, revealing evidence of the comparatively low
temperature called for by the theory at this early stage of stellar
existence.

TWO OTHER GIANTS

The diameter of Arcturus was successfully measured by Mr. Pease
at Mount Wilson on April 15. As