The Sizes and Masses of the Stars 67
stated in simple form. They apply primarily to the be-
havior of a perfect radiator (or “black body” as it is
often called, because such a body would be also a per-
fect absorber of incident radiation, and hence look per-
fectly black), and, as might be expected in any physical
formula, the temperature which appears in it is meas-
ured, not from the arbitrarily chosen zero of our ther-
mometers, but from the absolute zero (—273o centi-
grade, or —460° Fahrenheit) at which a body would
contain no heat energy.
Considering first the total radiation of energy, from
a unit area, and in a unit time, it is found that this is pro-
portional to the fourth power of the absolute temperature.
At 1000° absolute (on the Centigrade scale) the emis-
sion is 1.37 calories per square centimeter per second,
according to the excellent measures of Coblentz, which
should be accurate within a fraction of one per cent. Since
the Sun radiates 1490 calories per second from every
square centimeter of its surface, a simple calculation shows
that, if it were a perfect radiator, its temperature would
be 5740° absolute. An imperfect radiator would emit
less heat at this temperature, and have to be heated hotter
in order to give out the same amount. Hence this figure
may be regarded as a lower limit for the actual surface
temperature of the Sun.
Taking next the amount of light radiated per unit of
surface—that is, the surface-brightness of the body—it
appears that the relation between this and the tempera-
ture is most simply defined by expressing the surface
brightness in stellar magnitudes. We may then say that
the changes in surface brightness are proportional to
those in the reciprocal of the temperature, so that we
have an equation of the form J = A + B/Т, where J is