discovi:hy 



235 



tionar}' sequence from extensive diffused nebulosities 

 through irregular nebuhe, " nebulae a little brighter in 

 the middle " and " much brighter in the middle," to 

 planetary nebulae and nebulx " nearly approaching 

 to the appearance of stars." He expressed the view 

 that " ever>- succeeding stage of the nebulous matter 

 is the result of the action of gravitation upon it while 

 in a foregoing one, and by such steps the successive 

 condensation of it has been brought up to the planetary 

 condition. From this the transit to the stellar form 

 requires but a very small additional condensation of 

 the nebulous matter." Herschcl's nebular theory, 

 which seemed for a time to have been refuted by the 

 resolution of many nebulas into closely-compressed 

 clusters of stars, was triumphantly vindicated by the 

 dramatic discover}" made by the late Sir \\'illiam 

 Huggins in 1864. In that year the newly-invented 

 spectroscope revealed to Huggins the actual existence 

 of the " shining fluid " which many had been inclined 

 to dismiss as a figment of Herschcl's imagination. 

 And with the detection of genuinely gaseous nebulae, 

 the concept of stellar evolution was finally rehabilitated. 

 The reinforcement of the telescope by the spectro- 

 scope enabled the later nineteenth-century astronomers 

 to do what had been beyond the power of Herschel — to 

 classify the stars according to their physical condition. 

 Sccchi, the Italian astronomer, made in the sixties his 

 epoch-making survey of the spectra of the brighter 

 stars, as a result of which he divided them into four 

 general types — white stars, yellow stars, and two classes 

 of red stars. At first this classification was regarded 

 as more or less empirical and arbitrary, but latterly 

 Secchi came to the view that it " represented real 

 physical conditions varied by the temperatures pre- 

 vailing on the different stars." In 1865 the German 

 astronomer Zollner, of Leipzig, casually suggested that 

 yellow and red stars were simply white stars in different 

 stages of cooling, and this idea formed the basis of the 

 elaborate classification of the stars which we owe to 

 his distinguished pupil Vogel, afterweirds of Potsdam. 

 " A rational classification of the stars according to 

 their spectra," Vogel wTote in 1874, " is probably only 

 to be obtained by proceeding from the standpoint that 

 the phase of development of the particular body is in 

 general mirrored in its spectrum." Vogel retained the 

 general framework of Secchi's classification, holding 

 that the white stars, such as Sirius and Rigel, weie the 

 hottest and youngest stars, yellow stars such as the 

 Sun and Capella being obviously older and cooler. 

 He viewed the red stars as stUl older and much cooler, 

 as " effete suns hastening rapidly down the road to 

 final extinction." Vogel revised his classification in 

 1895. in order to make room for the newly-discovered 

 group of blue or helium stars, which he considered to 

 be still hotter and more immattire than the suns of 



the Sirian variety. In the main, Vogel's scheme and 

 its underlying evolutionary assumption was adopted 

 by Huggins, and formed the basis of the elaborate 

 " Draper classification," carried through at Harvard, 

 U.S.A., under the direction of the late Professor 



E. C. Pickering. This classification is based on the 

 observed spectral lines, and is at the present time in 

 general use. In this scheme, the various classes of 

 stars are designated by the letters OB.\FGKMN — types 

 O, B. and A corresponding roughly to the Sirian type, 



F, G, and K to the solar type, and M and N to the 

 two classes of red stars. This sequence, OBAFGKMN^ 

 was, until about ten years ago, believed by the great 

 majority of astronomers to represent the order of 

 stellar evolution. The association of the early-type 

 stars with nebulae was strongly confirmatory of this 

 assumption. In 1910 came the discovery, independently 

 announced by Professor Kaptcyn, of Groningen, and 

 Professor Campbell, of the Lick Observatory, that the 

 average radial velocities of stars of the later types were 

 greater than those of the earlier types. This was 

 believed to be the final confirmation of the Harvard 

 order of evolution. 



There had been, however, grave doubts in certain 

 quarters as to the accuracy of the generallj' accepted 

 theorj'. Ritter, of Aix-la-Chapelle, for instance, had 

 regarded the red stars as composed of two separate 

 groups, stars increasing and stars decreasing in tem- 

 perature. This view was adopted by the late Sir 

 Norman Lockyer, who placed stars of Secchi's third 

 type on the ascending scale, and stars of the fourth on 

 the descending. He also recognised two groups of 

 solar stars, and looked on the white stcirs not as the 

 youngest orbs, but as stars in their prime. Lockyer 's 

 views met with little acceptance at the time, probably 

 because of their association with his meteoritic hypo- 

 thesis of cosmogony. It was not until the beginning 

 of the present century that serious doubts began to 

 enter the minds of most astronomers. At length, in 1912, 

 Mr. K. T. A. Innes, the director of the Union Observa- 

 tory in Johannesburg, suggested that the true evolu- 

 tionary order is the reverse of the Harvard sequence. 



In 1905 Hertzsprung, of Potsdam, pointed out that 

 the stars appeared to belong to one or other of two well- 

 defined classes — large diffuse stars and comparatively 

 small stars of greater density. To these two classes 

 he gave the names of " giants " and " dwarfs." This 

 discovery was made independently by Professor H. N. 

 Russell, now director of the Princeton Observatory, 

 Xew Jersey. From data concerning the masses and 

 brightness of the stars compiled in the course of 

 investigations of stellar parallax, Russell recognised 

 the existence of giants and dwarfs among the red stars. 

 " There are," he wrote in December 1913, " two great 

 classes of stars — the one of great brightness averaging. 



