SURFACE AND ATMOSPHERE OF THE SUN — LINDBLAD 175 



sphere that the solar prominences extend widely outward, in a great 

 variety of sizes and shapes. Outside the chromosphere the solar co- 

 rona extends in its streamlined structure resembling the lines of force 

 about a magnetized sphere. As we shall see later, the temperature 

 in the high solar atmosphere increases outward, reaching very high 

 values in the thin matter of the corona. 



The nuclear processes responsible for the release of energy in the 

 central regions of the sun are mainly the combination of two protons 

 into a nucleus of heavy hydrogen, deuterium, and a certain cycle 

 of atomic permutations involving nuclei of carbon, nitrogen, and 

 oxygen, which is maintained by impacts between protons and the 

 nuclear isotopes O^ and C^^ of carbon and N^* and N^^ of nitrogen. 

 The net result of the cycle, which has been demonstrated especially 

 by H. A. Bethe, is the combination of four hydrogen atoms into one 

 atom of helium. It has been proved that the two processes mentioned 

 are entirely sufficient to explain the rate of radiation of the sun. The 

 total lifetime of the sun, during which the r.iain mass of hydrogen is 

 transformed into deuterium and helium, has been estimated to be of 

 the order of magnitude of 10 thousand million years. The sun is an 

 enormous atomic pile, and the same must be true for the stars in gen- 

 eral, though the atomic processes are not necessarily the same for stars 

 of all types. 



The energy released from the core paves its way to the surface of 

 the sun by a very complicated process of successive absorption and 

 emission and is steadily transformed in this way into radiation of 

 a successively lower temperature, until it reaches the photospheric 

 layers at the surface, which have an effective temperature of 5,713°. 



The main part of the mass of the sun is in radiative equilibrium, 

 Vvdiich means that each element of the mass absorbs and emits the 

 same amount of energy. An exception is probably the central core, 

 where the rapid increase of the rate of release of inner-atomic energy, 

 when we approach the center, makes conditions unstable so that there 

 will occur a turbulent motion with steady interchange of matter be- 

 tween different levels. At the surface of the sun the conditions of 

 radiative equilibrium still prevail. The well-known phenomenon that 

 the intensity of the solar disk decreases from the center to the edge, 

 and much more strongly in violet than in red light, is explained quan- 

 titatively to a high degree of accuracy by the theory of radiative 

 equilibrium. 



It is evident, however, that the conditions of equilibrium are not 

 perfect in the surface layers of the sun. The solar surface, when 

 viewed under great magnification, shows a complicated network of 

 fine grains, a phenomenon that is called the granulation of the solar 

 surface. The grains have a diameter of the order of 500 kilometers 



