70 Recent Advances in Stellar Astronomy
Another, and more important, check upon the estimates
of temperature can be found in the luminous efficiency of
the stars—that is, the ratio of their emission of light to
their whole out-put of radiant energy, that is, of heat.
It is only in recent years that apparatus delicate enough
to measure the heat of the stars has been devised. The
most successful observations have been those of Coblentz
at the Lick Observatory in 1914. With an exceedingly
delicate thermopile, placed at the focus of the three-
foot Crossley Reflector, and connected with a galvano-
meter of extreme sensibility, he obtained not merely indi-
cations, but measures, of the heat radiation from white
stars as faint as the third magnitude and red stars at the
limit of visibility to the unaided eye. More than eighty
stars in all were measured, distributed among all the
principal spectral classes. From these results it is found
that stars of the same spectral class all give about the
same amount of heat in proportion to their light, but
from one class to another the proportion is very differ-
ent. The whiter stars, of classes B, A, and F, give the
most light in proportion to their heat, and are very simi-
lar to one another. But, for the same visual radiation,
stars of Class G send out about 70 per cent, more heat,
those of Class K more than twice as much, and the M-
stars six times as much. The extreme case is Alpha
Herculis, which has a spectrum of very pronounced M
type, and gives out more than thirty times the heat, in
proportion to its light, which an А-star does—or only
three per cent, as much light, in proportion to the total
heat.
Now it is well known to all illuminating engineers that
the luminous efficiency of artificial sources of light in-
creases very rapidly with the temperature—which is why