Page:Popular Science Monthly Volume 86.djvu/447
MEASURING HEAT FROM STARS 443
from all the stars which at any moment can fall upon 1 sq. cm. of the earth's surface is so small that it would have to be absorbed and con- served continuously for a period of 100 to 200 years in order to raise the temperature of 1 gram of water 1 degree centigrade. Evidently the incoming stellar radiation can contribute but little in retarding the cooling of the earth. For the measurements of nocturnal radiation, which is usually a loss of terrestrial radiation into space, indicate that for a lampblack surface the outgoing radiation may be as high as 0.1 the incoming solar radiation.. The emissivity of the materials forming the earth's surface may be much lower than this value. Nevertheless, it is much greater than can be compensated for by a continuous incom- ing of stellar radiation. The temperature of the earth must therefore ultimately tend towards the absolute zero of temperature.
V. Some Astroeadiometric Problems that Await Solution"
A complete . statement can not be made of the problems in stellar radiation that require investigation. They stare one in the face which- ever way one turns. Eealizing the inadequacy of the radiometric appa- ratus now available one must sit with eyes closed in order not to become impatient with existing conditions, both as regards the production of radiometric apparatus sufficiently sensitive to make the measurements and as regards financial assistance which is necessary to carry on the work. It is not a work that can be " cleaned up " in a season. It will require years of painful, nerve-racking toil in order to accomplish any- thing of worth.
While it is hoped that the present investigation will make available to the astronomer one more instrument for the investigation of celestial objects, it is desirable to emphasize that, from the insensitive nature of the instrument, the astronomical application can not be very wide as compared with the spectrograph. However, its physical properties are such that, in a limited field, it can be employed in attempting the solution of some of the most fundamental questions in astrophysics. Take for example the question of the emissivity of blue stars as com- pared with red stars. The general conclusion appears to be that blue stars are at a higher temperature than red stars, and that the emissivity of the red stars is higher than that of the blue stars. The higher emissivity of the red stars would be attributable to a marked change in the distribution of energy in the spectrum, brought about by a change in the physical condition of the stellar surface.
"With the rather insensitive radiometric outfit used in the present investigation it was shown that the total radiation received from a red star is two to three times that of a blue star of the same photometric brightness. These observations should be extended. Another field of astroradiometric research is the measurement of the radiation from