174 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1950 



The exact amount of solar radiation is known mainly from the 

 measurements made by the observers of the Smithsonian Institution, 

 Dr. C. G. Abbot and his collaborators, at Washington, D. C, and at 

 various stations on the globe. On each square centimeter of a sphere 

 whose radius is the mean distance of the earth from the sun, falls 1.9 

 calories per minute. The distance of the sun from the earth is 93 

 million miles, and from this we can readily find the enormous amount 

 of energy released by a square centimeter of the surface of the sun, 

 about 1,500 calories per second. By the laws of radiation we may 

 then calculate the effective temperature of the solar surface to be 

 about 5,700°. The mass of the sun is 332,000 times the mass of the 

 earth, and we may conclude that on the average each gram of the 

 solar mass loses 1.4 calories per year. During a few thousand mil- 

 lion years, which we assume to be the age of the sun, the amount 

 becomes tremendous. 



There have been different theories as to the origin of the energy 

 output of the sun. We have had mechanical theories which assume 

 that the source of energy is the kinetic energy of meteorites captured 

 by the sun, or the potential energy lost by the shrinking of the entire 

 body of the sun in a process of contraction. We know now beyond 

 doubt that the main source of energy is of inner-atomic origin. In 

 the interior of the sun the temperature and density must increase 

 immensely toward the center. The mass of the interior consists 

 mainly of hydrogen and helium in fairly equal amounts, whereas the 

 amount of all other elements together is only about 12 percent of the 

 entire mass. All elements will be in a high stage of ionization. The 

 ionized hydrogen atoms, or protons, will fairly often hit each other, 

 as well as the nuclei of other atoms, and if the impact carries suffi- 

 cient energy a nuclear reaction may take place. 



There is in the sun a central core of a temperature of about 25 mil- 

 lion degrees, wliich is sufficient to produce certain nuclear reactions 

 that release a great amount of energy. The intermediate region be- 

 tween the core and the surface of the sun has a great power of absorp- 

 tion on the stream of light generated in the central regions. This 

 light is of very short wavelength, corresponding to X-rays and y-rays. 

 In the regions close to the surface the density decreases rapidly out- 

 ward, till we reach the photospheric layers, from which we receive 

 light directly. The sun has no fixed boundary, and the decrease of 

 density of the gaseous layers is quite continuous. From the photo- 

 spheric layers there is a continuous transition to the sun's "atmos- 

 phere," which we define as the layers where the absorption lines in the 

 solar spectrum are formed. 



The upper part of the sun's atmosphere is the chromosphere. It 

 has in general a very irregular structure, and it is from the chromo- 



