236 



Dis(;ovi:i<v 



perhaps, a hun<lrt.'cl times as brij^lit as the Sun, and 

 varying little in brightness from one class of spectrum 

 to another ; the other of smaller brightness, which 

 falls off rapidly with increasing redness." 



Obviously the existence of these two types of stars 

 was difficult to account for on the generally accepted 

 theory of stellar evolution ; it seemed remarkable that 

 there should be some red stars large and diffuse and 

 others small and dense, if all red stars were to be re- 

 garded as " effete suns hastening rapidly down the road 

 to final extinction," as Vogel believed. Later came 

 Dr. Harlow Shapley's work on eclipsing binary stars. 

 His investigations brought out the surprising result 

 that some stars of the second or solar type were less 

 dense than some stars of the first. Manifestly this 

 result contradicted the accepted hypothesis on which 

 all second-type stars were considered to be older, and 

 therefore denser, than stars of the first class. 



Accordingly, Professor Russell was driven to put for- 

 ward a new theory, or, rather, to revive Lockyer's 

 hypothesis, considerably modified. " The order of 

 increasing density," said Russell, " is the order of 

 advancing evolution. . . . The giant stars, then, repre- 

 sent successive changes in the heating up of a body and 

 must be more primitive the redder they are ; the dwarf 

 stars represent successive stages in its later cooling, 

 and the redder of these are the furthest advanced." 

 The sequence of evolution, then, begins and ends with 

 Class M, and Class B — the helium type — occupies the 

 middle place. The B or helium stars are on the 

 crest of the evolutionary curve, at the meridian of 

 stellar life. 



Thus, on Russell's theory, a star in its earlier stages 

 is of great volume, and comparatively low temperature 

 — about 3,coo' C. — and its density is almost incredibly 

 low — ss.ws^ of the density of the Sun. As the star 

 contracts, it grows hotter and denser. The brightness, 

 however, remains more or less constant, the shrinking 

 area being just about compensated for by the increasing 

 surface-brightness. This process of contraction, ac- 

 companied by increase of temperature, goes on until 

 the star reaches the B stage, when its temperature is 

 about 20,000° C. and it shines with a blue light. After 

 this stage is reached and the star attains a certain 

 density, it falls off rapidly in temperature, and becomes 

 a cooling dwarf. It is at first a star of the Sirian and 

 next of the Solar type, and then finally is transformed 

 into a feebly luminous star. According to this theory, 

 then, our Sun is a fairly large dwarf star, of decreasing 

 brightness and temperature. 



In putting forward his hypothesis, Professor Russell 

 pointed out that the relative scarcity of the B stars — 

 the giants of maximum temperature — is to be accounted 

 for by the fact that only stars of very large mass are 

 able to attain to the high temperature of the B type 



stars. " Only these stars would pass through thtt whole 

 series of the spectral classes from M to B and back again 

 in the course of their evolution. Less massive bodies 

 would not reach a higher temperature than that corre- 

 sponding to a spectrum of Class A, those still less 

 massive would not get above Class F, and so on." " It 

 is now easy to understand why there is no evidence of 

 the existence of luminous stars of mass less then one- 

 tenth that of the Sun. Smaller bodies presumably do 

 not rise, at maximum, to a temperature high enough 

 to enable them to shine perceptibly from the stellar 

 standpoint, and hence we do not see them. The fact 

 that Jupiter and Saturn are dark, though of a density 

 comparable to that of many of the dwarf stars, confirms 

 this view." 



When the theory was first promulgated, the diffi- 

 culties in the waj' of its acceptance seemed insuperable. 

 In 1915 Professor Eddington pointed out that " it 

 plays havoc with a great deal that has hitherto seemed 

 orderly and intelligible," and he specified the progres- 

 sion of average velocity from the earlier to the later 

 types. This objection, however, has not the force 

 which it possessed six years ago. The work of Edding- 

 ton himself, and of Kapteyn, Adams, and others indi- 

 cates that the increase of velocity with spectral type is 

 simply a particular manifestation of a wider generalisa- 

 tion, namely, the increase of speed with diminishing 

 absolute magnitude, and probably, therefore, wth 

 mass. Russell's words in 1914 have certainly been 

 justified by the progress of research : " A correlation of 

 mass and velocity . . . seems more probable than one 

 between temperature and velocity, or velocity and age." 



Perhaps no new hypothesis in the whole history of 

 astronomical science has been so signally and decisively 

 confirmed within a few years as the evolutionary theory 

 of Professor Russell. First of all, the work of Hertz- 

 sprung indicated that even among the " early types " 

 two classes of stars were probably to be distinguished. 

 Striking confirmation was obtained by Dr. \V. S. 

 Adams in the course of his determinations of stellar 

 pandlaxes by the new spectroscopic method at Mount 

 Wilson Observatory. In iqi6 Adams announced that 

 " two groups of M stars are indicated clearly by an 

 examination of the intensities of the h\-drogen lines: 

 in the first the hydrogen lines are very strong ; in the 

 second they are very faint. . . . Connecting links 

 over a range of seven magnitudes are entirely lacking." 

 At the same observatory, Dr. Harlow Shapley, now of 

 Harvard, discovered in the distant star-clusters red 

 and yellow stars which, to be visible at such vast 

 distances from our system, must be giants. He also 

 noticed that in some of the clusters the brightest 

 stars were red, a result which is explicable only on 

 Russell's theory. 



Perhaps the most striking of all the pieces of evidence 



