78 



KNOWLEDGE. 



[March 1, 1897. 



kind, with its lines of known position, and frequently of 

 known origin, serves as an admirable reference in de- 

 termining the positions and origins of lines seen in other 

 spectra photographed with the same instrument. 



What, then, do we learn when the stellar spectra are 

 "reduced" in this way? First, we have the fact that 

 stars of the Areturus class, which are very numerous, 

 chemically resemble the sun so far as we are in a position 

 to determine ; and our knowledge of this type of spectrum 

 is, therefore, practically limited by what we know of the 

 sun in its most general aspect. 



But, after all, if we measure our knowledge of the 

 chemical composition of the sun's atmosphere by the 

 number of Fraunhofer lines sufficiently accounted for, we 

 know very little indeed. With appliances of the greatest 

 possible delicacy, Prof. Rowland has recently gone over 

 the ground again, confirming in the main the results 

 obtained by his predecessors, and has considerably extended 

 the list of solar elements. Nevertheless, more than sixty 

 per cent, of the two thousand Fraunhofer lines shown in 

 Rowland's magnificent photographs between F and D still 

 rank among the " unknown " lines which remain to be 

 explored, and, if possible, " run to earth." 



There is, however, a possible explanation for some of the 

 unknown lines. Although the spectrum of a substance is 

 perfectly definite at a given temperature, a change of 

 temperature is frequently accompanied by a corresponding 

 alteration in the spectrum, and in such cases there will 

 not be a perfect matching with the solar lines, unless the 

 laboratory spectrum is produced at a temperature more or 

 less approximating to that of the solar region in which the 

 particular vapour is located. It may very well happen, 

 then, that many of the " unknown " Fraunhofer lines 

 really form part of the spectra of familiar substances under 

 untried conditions, a view first advanced by Lockyer in 

 connection with the theory of dissociation. 



With regard to stars, the variations of spectrum with 

 temperature are of special importance, for there is every 

 reason to believe that some stars are hotter than others. 



Tlie spectra of Procyon and /3 Arietis, as shown in our 

 illustration, differ mainly from that of Areturus and the 

 sun in their intensified hydrogen lines and enfeebled 

 metallic ones. Many of the lines in these two stars have 

 accordingly the same chemical origins as the Fraunhofer 

 lines ; that is, hydrogen, iron, magnesium, calcium, and 

 so on. 



The spectrum of y Cygni presents several peculiarities, 

 and has not yet been fully investigated. Its departure 

 from that of a star like Areturus, which it resembles in 

 having a great number of lines, is much more considerable 

 than a casual comparison would suggest, and most of its 

 lines have not yet been certainly identified. 



a Cygni presents us with a still more peculiar spectrum, 

 hydrogen lines, however, being conspicuous. Many of its 

 lines seem to coincide in position with lines of known 

 substances, but they appear with such dift'erences of relative 

 intensity that it would be unwise to draw conclusions 

 untU laboratory experiments have indicated the possibility 

 of similar changes of intensity in the lines of known sub- 

 stances. Excepting hydrogen, calcium, and magnesium, 

 then, the lines of a Cygni remain without sufficient ex- 

 planation. 



The comparatively recent discovery of terrestrial sources 

 of helium has resulted in the identification of many of the 

 lines in such stars as Bellatrix which had previously 

 ranked among the unknown lines. In the photograph of 

 the spectrum of BeUatrix, which is reproduced, the three 

 most intense hues are due to hydrogen, while seven of the 

 other more prominent ones have been traced to helium ; 



f is one of them. All the remaining lines are very feeble, 

 so that stars of this class may be said to give visible indi- 

 cations of little besides hydrogen and helium. 



Another group of stars, of which a Orionis is a good 

 specimen, have spectra showing a great number of lines, 

 together with a number of shaded bands or flutings. With 

 regard to the lines, the identifications are even less 

 advanced than in the case of stars like Areturus ; but 

 Lockyer has shown that the dark flutings probably repre- 

 sent metallic vapours at a comparatively low temperature, 

 while the bright ones, like those in comets, are possibly 

 due to carbon. 



Still another group of spectra is characterized by dark 

 flutings, which have long been known to be either due to 

 carbon or compounds of carbon. Little is known about 

 the lines which also appear. But if the day should come 

 when the " origin ' of every line in the spectrum of any 

 star should be known, we should not even then be in a 

 position to assert that we knew every chemical substance 

 entering into its composition. In the case of our own sun, 

 which may be regarded as a star, our knowledge is 

 enormously extended when the spectra of individual parts 

 are separately investigated by projecting an image of the 

 sun upon the slit, and by taking advantage of solar 

 eclipses. This " taking the sun to bits,'' as Prof. Lockyer 

 terms it, reveals the presence of certain substances round 

 the sun — helium among them — which are not represented 

 at all by Fraunhofer lines, and of the presence of which in 

 the sun we should have remained in ignorance were it not 

 for our very special relation to the sun in point of distance. 

 There is, accordingly, nothing improbable in the idea that 

 similarly situated vapours in stars may quite elude detection 

 by the instrumental means now at our command. 



More than this : the study of the relation existing 

 between the different kinds of stellar spectra — a subject too 

 great to be touched upon in the present article — indicates 

 beyond all question a gradual evolution through successive 

 forms with different temperature conditions. A common 

 composition for all stars is, therefore, suggested, and 

 dift'erences of spectra on this view are caused by differences 

 of temperature, some substances best giving comprehensible 

 indications of their presence at one temperature, and some 

 at another. 



[I should wish to qualify one statement in Mr. Fowler's 

 most excellent article, viz. : that " the relation existing 

 between the different kinds of stellar spectra .... 

 indicates beyond all question a gradual evolution through 

 successive forms with difi'erent temperature conditions." 

 This is certainly one interpretation which can be put upon 

 that relation, but not the sole one. It assumes that the 

 chemical elements known to us here are not only distributed 

 through all celestial bodies, but distributed Ln equal pro- 

 portion through them all ; an assumption which appears 

 tome not only gratuitous but inadmissible. — E. Walter 



Maunder.] 



♦ 



THE FACE OF THE SKY FOR MARCH. 



By Herbert Sadler, F.R.A.S. 



SUNSPOTS are occasionally visible on the solar disc. 

 Conveniently observable mmima of Algol occur 

 at lOh. Om. p.m. on the 7th, at 6h. 48m. p.m. on 

 the 10th, and at lib. 42m. p.m. on the 27th. 

 Mercury is not well situated for observation during 

 the month. 



Venus is now a magnificent object in the evening sky. 

 On the 1st she sets at lOh. 10m. p.m., or four and a half 

 hours after the Sun, with a northern declination of 13° 17', 

 and an apparent diameter of 28", about T*oths of the disc 



