The name is absent

II

THE SIZES AND MASSES OF THE STARS

E described yesterday the close relations which
exist between the spectral types, colors, and tem-
peratures of the stars, but we said little or nothing about
the actual temperatures of their surfaces. At the begin-
ning of the twentieth century, there was really very little
that could be said; but since then our knowledge has in-
creased rapidly.

The laws of radiation from heated bodies are quali-
tatively familiar to everyone—especially in these days
when electric heating is a commonplace. If we heat a
body to a moderate temperature, it will radiate energy,
which the hand held near by can appreciate as heat—
though the eye cannot see it, because it is carried by waves
too long, and too slowly vibrating, to affect this organ
of special sense. Raise the temperature higher, and the
heat radiation increases, and light begins to appear, at
first dull and red, then brighter. As the temperature
still increases, the light becomes much brighter and
changes from red to yellow, and at last to white—till fin-
ally our experiment (as in the case of an “overvolted”
lamp) is brought to an end by the melting or volatiliza-
tion of the heated body.

The accepted formulation of these laws in mathemati-
cal language is due to Planck. His formula—tested both
by observation and by thermodynamic reasoning—is rather
complicated to discuss here, but its principal consequences,
so far as they affect astronomical phenomena, may be

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