40 



KNOWLEDGE. 



[Februaey 1, 1899. 



but judgment should be held in suspense until movements 

 can be detected in the stars, or in the nebulosity, before a 

 definite pronouncement is made that the stars are in 

 reality involved, for the apparently finished stars may be 

 either en the solar side of the nebula or else on the farther 

 side, and owing to the vast distances of the stars and the 

 nebulosity from us we cannot hope to be enabled to decide 

 the question during our lifetime, or, indeed, that it can be 

 decided by the next generation of astronomers, but with 

 reliable data and sufScient interval of time between the 

 epochs, the solution will undoubtedly be obtained. 



The photograph shows the nebula to be cloud-like in 

 character — as a thin fleecy cloud, irregular in form and 

 in outline — with sinuous condensations of the nebulosity in 

 several parts, in some of which are faint stellar condensa- 

 tions that may be the early stages in the development of 

 new stars. 



If the question should be asked : Does spectrum analysis 

 not indicate the physical condition of this and of the other 

 known bodies of similar appearance that exist in space '? 

 the answer would be that the nebulosities are so faint and 

 diffuse in character as to be hopelessly beyond the light- 

 grasping power of any form of spectroscope at present 

 known, whether it be applied to eye observation or to the 

 photographic plate. 



The limit of the power of the spectroscope seems at 

 present to be in the analysis of light (star-light) that is not 

 feebler than that of stars of about the tenth magnitude. 

 It is, therefore, evident that light so feeble as that of stars 

 of the seventeenth or eighteenth magnitude, which may be 

 the light of some of the nebuls here referred to, is im- 

 mensely beyond the range of the spectroscope. We must, 

 therefore, be content in producing the best possible photo- 

 graphic records of the various bodies as they exist in these 

 days, and leave our successors in the future to determine, 

 by the correlation of their own photographs with ours, the 

 solution of the problems which we can now only imperfectly 

 deal with because of the absence of reliable data. 



The nebula extends about 2° 86' in length, in the direction 

 south fulloicing to 7iorth preceding, and in breadth at the 

 widest part about 40'. 



COMPARATIVE PHOTOGRAPHIC SPECTRA' 

 OF THE BRIGHTER STARS. 



JUST a year has passed since we noticed in Knowledge 

 the most important memoir by Miss Antonia C. 

 Maury on the classification of the photographic 

 spectra of six hundred and eighty-one stars. We 

 have now to record the publication of the results of 

 another spectrographic survey having a distinct bearing 

 upon that of Miss Maury's. 



The present research is the work of a single astronomer. 

 Mr. McClean, whose munificent generosity to the science 

 is well known, as shown both in his foundation of the 

 Isaac Newton Studentships, and in the presentation to the 

 Royal Observatory at the Cape of Good Hope of a splendid 

 equipment for spectroscopic and photographic research, 

 has himself carried out a complete survey of the heavens 

 within the limits he thought well to adopt. He began the 

 work first at his own observatory at Tunbridge Wells, using 

 a prium of twelve inches aperture and twenty degrees angle 

 before the object glass of his own photographic refractor. 



* " Comparative Photographic Spectra of Stars to the Three and a 

 half Magnitude" (reprinted from 7ke Philosophical Transactions 

 of the Hoy al Society of London), &nA "Spectra of Southern Star', 

 with Tables and Plates." By Frank McClean, f.e.s. London: 

 Edward Stanford. 



Here he fulfilled rather more than half his programme; then 

 he completed it at the Cape Observatory, using the same 

 prism as that which he had employed at Tunbridge Wells 

 before the object glass of the Cape astrographic telescope. 

 Mr. McClean's method, therefore, was essentially the same 

 as that used in the Draper Memorial research at Harvard 

 College. The apertures differed little — twelve inches as 

 against eleven — the focal length was one hundred and 

 thirty-five inches as against one hundred and fifty-three, 

 and the dispersion Mr. McClean employed, though less 

 than the highest used at Harvard College, was greater 

 than the lowest. The photographs, therefore, were of the 

 same general order, a fact which increases the value and 

 importance of both series. 



The chief differences between the two researches are as 

 follow : — The Harvard College Survey was restricted to 

 stars north of - 80° declination. Mr. McClean's extended 

 over the entire heavens. The former included the spectra 

 of six hundred and eighty-one stars ; the latter, extending 

 only to magnitude '6\, includes two hundred and seventy- 

 six stars. The chiet object of Miss Maury's diacuasion of 

 her material was to systematize afresh the classification 

 of stellar spectra and to push it to further detail ; whilst 

 the primary object with Mr. McClean was to ascertain the 

 distribution, with respect to the Galaxy, of types which 

 had obtained general recognition. 



Having this object in view, Mr. McClean adhered prac- 

 tically to the simplest of all the many systems of stellar 

 classification — that of Secchi, modifying it only by dividing 

 the first type, that of the " hydrogen stars," into three divi- 

 sions. Of these divisions Secchi had himself recognized 

 the existence of the first, as he saw that the Orion stars 

 differed materially in their spectra from the stars like 

 Sirius. The Orion stars form Mr. McClean's first division, 

 together with those of all stars that are rich in the lines of 

 helium. The remaining spectra of Secchi's first type are 

 grouped with Sirius in Division II., or with Procyon in 

 Division III., according as their great hydrogen bands are 

 more marked, or are less marked, than the great calcium 

 band K. 



The number of stars examined being small for the pur- 

 poses of the enquiry, it was absolutely necessary that ttieir 

 classification, either according to their spectral type, or 

 according to their position with regard to the Milky Way, 

 should be of the simplest. The following table gives the 

 result of the enquiry when the stars are grouped together 

 into four equal areas, one on each side of the galactic 

 equator, and one round each of the galactic poles : — 



Galactic Regions. 

 Stellar Types. Upper Upper Lower Lower 



Polar. Equatorial. Equatorial Polar. 



Division I. ... 9 29 41 9 



II. 13 10 6 7 



„ III. ... 16 9 18 5 



IV. 23 16 24 22 



V. ... 4 5 5 5 



Nebulae. 

 Planetary ... 4 10 11 2 



Extended ... 2 8 11 5 



Small as are the number of stars involved, the table is 

 very suggestive. First of all it brings out clearly the 

 preference which both the helium stars and the planetary 

 nebulffi show for the region of the Milky Way; not an 

 accidental similarity, since, as Mr. McClean has forcibly 

 argued, the helium stars are probably in the earliest stage 

 of development from the nebuhe. This disposition of 

 early types to crowd towards the Galaxy appears a strong 

 indication that the latter is a region where the process of the 

 crystallization into definite world-forms of the primeval 



