July i, 1897] 



NA TURE 



207 



attempt has apparently been made to assign chemical origins to 

 the various lines, so that the endeavour to arrive at a natural 

 j and satisfactory system of classification may be regarded as the 



I most important part of the discussion. 



The classification of stars has another object besides that of 

 the mere grouping together of those which have similar spectra. 

 It is generally believed that the various types of spectra represent 

 different stages of stellar evolution, but there are divergences 

 of opinion as to the exact order in which the various types should 

 follow each other. Dr. Vogel still holds, with some slight 

 modifications, to the classification which he suggested in 1874, 

 and believes that all the stars can be arranged along a descending 

 line of temperature. Sir Norman Lockyer, who has adopted the 

 same method of work as Prof. Pickering, and has also obtained 

 large scale photographs of stellar spectra, finds evidence that 

 there are some stars which are getting hotter while others are 

 becoming cooler, so that two series of spectra can be recognised. 



For the Draper Catalogue a somewhat arbitrary and provisional 

 classification was adopted, but this has not been found sufficient 

 to meet the requirements of the more detailed results which are 

 now available. 



Among the stars with line spectra, as previous researches have 

 shown, there are a few sets of lines which occur with various 

 relative intensities in different stars, each set in some degree vary- 

 ing bodily, and the new classification is based chiefly upon the dis- 

 tribution of these sets. As will appear later, the classification 

 adopted by Miss Maury also takes account of the appearance, as 

 well as of the positions of spectral lines, and every care has 

 been taken to eliminate instrumental sources of error. 



Four distinct sets of lines are distinguished. The first includes 

 the lines of hydrogen and calcium, and the remainder are thus 

 described : — 



" Another class of lines frequently mentioned comprises those 

 which are characteristic of the solar spectrum, excluding the 

 lines of hydrogen and calcium. They are called ' solar ' lines, 



(except when referring to lines not contained in the solar spec- 

 trum, in which case they are called metallic lines." 

 " A third class of lines includes those known as ' Orion lines,' 

 from the fact that they are conspicuous in the spectra of many 

 stars belonging to the constellation Orion. . . . 

 " Certain stars, such as a Cygni and 8 Canis Majoris, have 

 spectra in which the majority of the lines, though probably 

 identical in position with lines belonging to the solar spectrum, 

 differ greatly in intensity, while others apparently are not repre- 

 sented in the solar spectrum. The characteristic lines of such 

 stars should perhaps be regarded as forming a class distinct from 

 those already described." 

 ^ Bearing in mind these different classes of lines, the new system 



of classification can readily be understood. Excluding "com- 

 posite" spectra and bright line stars, "the stars were arranged 

 in an apparently progressive series, which in the present case 

 was made to include twenty-two groups. . . . But it also 

 appeared that a single series was inadequate to represent the 

 peculiarities which presented themselves in certain cases, and 

 that it would be more satisfactory to assume the existence of 

 collateral series." 



Three lines of progression are recognised in the earlier stages, 

 and are called " divisions." Stars of division a are characterised 

 by lines having the appearance with which we are familiar in 

 the solar spectrum ; that is, they are fine and sharp, if hydrogen 

 and calcium be excluded. Those of division b are uniformly 

 hazy, as in a Aquilae, but otherwise present no notable 

 differences in relative intensity from corresponding lines which 

 are sharp in division a, so that " there appears to be no decided 

 difference in the constitution of the stars belonging respectively 

 to the two divisions." In division c the hydrogen lines are 

 narrow and sharp and less intense than in the other divisions, 

 while several lines, some of which do not correspond with solar 

 lines, are of unusually great intensity; these are especially 

 marked in a Cygni. 



tlroups and divisions alike proceed by very gradual stages in 

 some parts of the series, and it has frequently been found 

 difficult to assign some of the stars their proper places. 



In consequence of the adoption of the term "group," which has 

 been in use for the last ten years in connection with Sir Norman 

 Lockyer's classification, some confusion may possibly occur, as 

 similarly numberetl groups include different stars. To avoid 

 ambiguity, it will therefore be necessary, in the case of the fir'st 

 seven groups at least, to specify the system of classification in 

 question. In what follows, the Draper groups will be dis- 



NO. 1444. VOL. 56] 



tinguished by the addition of the letter D to the number where 

 necessary. 



Of the twenty-two groups, the first five include stars in which 

 the Orion lines are especially marked ; the sixth contains stars 

 intermediate between this type and the first type of Secchi, to 

 which belong the stars in the seventh to the eleventh groups 

 inclusive. The twelfth group is intermediate between Secchi's 

 first and second types, and the stars included in groups thirteen 

 to sixteen are of Secchi's second type. Groups seventeen to 

 twenty inclusive correspond to Secchi s third type, and groups 

 twenty-one and twenty-two to the fourth and fifth types re- 

 spectively. Besides these, two unnumbered groups are recog- 

 nised, one containing composite spectra, apparently resolvable 

 into two or more, and the other including stars of the Orion 

 type which also show bright lines. Nebulae find no place 

 among the numbered groups, but reference is made to a former 

 paper i^Ast. Nach., vol. 127, p. i), in which it was suggested 

 that the Wolf Rayet stars probably form a connecting link 

 between the spectra of nebulae and those of the Orion type. 



It is not possible within the limits of this notice to indicate 

 the full details of the twenty-two groups with their sub-divisions, 

 but the general course of development which is suggested may 

 be briefly stated. 



In Group I. D, of which t Orionis is a type, the hydrogen 

 lines are comparatively faint, while the Orion lines are strong, 

 and "solar" lines are absent. Passing to Secchi's first type, 

 through Groups II. D to V. D, the Orion lines become fainter 

 and less numerous until in the spectrum of Sirius (Group VII. 

 D) all but two or three are wanting. Meanwhile solar lines 

 have become numerous, and the hydrogen lines reach their 

 maximum intensity. The transition to succeeding groups is 

 very gradual, hydrogen lines thinning out and solar lines be- 

 coming stronger. Arriving at stars like Capella and the sun 

 (Group XIV. D), the intensity of the hydrogen lines is little 

 more than a tenth of that shown in Sirius, and they afterwards 

 continue to decrease, but less rapidly, down to the third type 

 stars (Groups XVII. D to XX. D), where they are incon- 

 spicuous. In the third-type stars banded absorption appears, 

 and becomes more marked in each succeeding group, while the 

 majority of the lines fade out in the later groups. An important 

 feature of the series is the manifestation of extensive absorption 

 in the later groups of second type stars and in those of the 

 third type. 



For the present, the series is regarded as ending with the 

 spectra of the third type, stars following the twentieth group 

 not being considered as having a place in the series exhibiting 

 the gradual development of stellar spectra. 



Spectra of division c are not found after the thirteenth group, 

 and those of division b disappear still earlier, " so that the 

 series tends to become more uniform as it progresses." 



In connection with the new classification, it is remarked (p. 

 11) that " while it will be generally admitted that the series 

 represents successive stages in stellar evolution, it may still be 

 doubted whether the arrangement beginning with the Orion 

 type, and here adopted, is in fact the natural order. It is 

 strongly indicated, however, by the gradual falling off of the 

 more refrangible rays in the successive groups, by the corre- 

 sponding increase in the less refrangible rays, and by the occur- 

 rence of marked absorption at the close of the series. The 

 comparative simplicity of the Orion spectra and the increasing 

 complexity shown throughout the series, lends additional weight 

 to the argument. Finally, the prevalence of the Orion type in 

 great nebulous regions, as in Orion and the Pleiades, indicates 

 very emphatically that stars of this type are in an early stage of 

 development." 



It will be seen that the supposed evolutionary series has been 

 arrived at without reference to temperature considerations. 

 Nevertheless, a gradual reduction of temperature as the series 

 progresses is suggested by the diminishing intensity of the more 

 refrangible rays, so that, in the main series at least, the order 

 is in all probability one of gradually reducing temperature. 



As already remarked, the stars of Secchi's fourth type have 

 been omitted from the supposed evolutionary series of spectra, 

 for the reason that the few lines photographed " have not yet 

 been identified with those of other classes of stars, owing to the 

 total dissimilarity of the spectra." This dissimilarity is stated 

 to extend to the yellow part of the spectrum, and is difiicult to 

 comprehend in the light of the more recent results obtained by 

 Dr. McClean, who has shown that the spectrum of 152 Schj. 

 contains many lines which are apparently identical with lines in 



