A NEW SOLAR THEORY. 167 



pci'hap.s produce iiiore ot less heat than is i-ecpiired for compensation. 

 It is, indeed, inconeeiV-able that the conditions of contraction can 

 remain tlie same throughout the lifetime of a star. The spectroscope 

 has revealed the fact that the photospheres of diHerent stars exhibit 

 widely different stages as regards temperature. Ther(^ are doubtless 

 suns hotter than ours, and others considerably cooler. And we may 

 confidently assume that the various conditions of temperature now 

 recognized in the different types of star-specti'a represent the i)ha8es 

 which successively apjx'ar in the evolution of each of these bodies from 

 its orioin as a far-extended nebula down to its complete obscuration. 

 In the life of each of these stars there will be a period when its tem- 

 perature is on the ascent, and when, consequently, the heat-generating 

 effect of the contractile force exceeds the loss by outward radiation, 

 as well as another period when the declining temperature of the star 

 indicates an excess of the heat-dissipating over the heat-producing 

 forces. Which of these conditions, at the present moment, prevails 

 on our sun can so far be only ;i matter of conjecture. In this respect, 

 therefore, an assumption has to be made. The following inquiry 

 applies to the case of a star on which the generation of energy by con- 

 traction falls short of the loss of energy l)v radiation. Whether the 

 results of this investigation may be applied to the case of our sun 

 must, then, depend on the further question whether the sun really 

 belongs to those stars the temperature of which is declining. So far 

 as I know, this latter opinion is at present held by the great majority 

 of astrophysicists. 



If on a star the loss of energy exceeds the production, the kinetic 

 energv of its molecules, and consequently its absolute temi)erature, 

 must decrease. Hence if the temperature of a layer, ^/,,, at a certain 

 distance, p,,. from the center was Tj at the epoch Z^, it will be T^ at a 

 later epoch 4, where T2<Ti. Now let a^ bo the level of the photo- 

 sphere — or the level of maximum incandescence, and therefore also 

 of maximum radiation — at the e})Och f^. In consequence of deficient 

 contraction the temperature of this layer must decrease, and the mate- 

 rials composing it must cool down, so that, at the subsequent epoch 

 4, the level of maximum incandescence will have shifted toward a 

 layer m., nearer to the star's center, where the temperature is still 

 sufficiently high to maintain the incandescent state of all the particles. 

 The space between a^ and cl, will then be occupied by particles in a 

 less luminous state which act as an absorbing screen on the radiation 

 emanating from a.,. Whatever fraction of the total radiation which 

 originally left the photosphere at a„ is thus stopped in its outward 

 progress will be in part absorbed bv, and in part reflected from, the 

 intervening particles of the layer a^d.^, and there can be no doubt that 

 some at least of this arrested energv will ultimatel}^ be thrown back 

 to r<.^ from which it started. The layer a^a., must therefoic act on the 



