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UNIVERSITY OF CALIFORNIA PUBLICATIONS
THE PLANETARY NEBULAE
By HEBER DOUST CURTIS
As a line of research closely related to the spectrographic investigations of the planetary nebulae which have been carried on at Mount Hamilton during the past three years, a considerable proportion of the time available for using the Crossley Reflector has been devoted to securing a series of photographs of all the planetary nebulae north of 34° south declination. The work was undertaken in the belief that a collection of illustrations showing the forms assumed by the planetary nebulae would be of considerable value as contributing to theories of the structure and life history of these bodies.
In attempting to reproduce these small objects by the ordinary processes of photo-engraving, serious difficulties are met with at once. The larger members of the class can be easily and satisfactorily enlarged, but it is very difficult to secure an adequate representation, by direct photographic enlargement, of the many planetaries which do not exceed thirty seconds of arc in their greatest dimension. On the scale of the negatives taken with the Crossley Reflector, one inch equals 983'.'7, so that thirty seconds of arc is only three hundredths of an inch, and in such a case a ten-fold enlargement will give an image on the printed page only three-tenths of an inch in diameter. In addition, when a minute object is enlarged on this scale, the finer details of structure visible on the original negatives are generally completely masked by the effects of the grain of the plate, and suffer the further obliteration unfortunately inherent in all copying and photo-engraving processes. In many of the brighter planetaries an added difficulty is found in the tremendous differences in brightness between the central portions and the very much fainter outer parts. In some cases the details in the immediate vicinity of the nucleus are so bright that an exposure of a few minutes is sufficient to "burn out" these portions completely, while exposures of an hour or longer may be necessary to record the fainter outlying structures. In such cases no single photograph of the object will give an adequate representation of all the details of the nebula.
After a number of experiments with direct enlargements, composite crayon drawings were adopted as the most satisfactory method of depicting the details of structure in the smaller planetaries. For each planetary a series of negatives of widely different exposure times was secured, so planned as to give a legible photographic record of both the brighter nuclear portions and the fainter peripheral details. The range of exposures was determined by trial, and frequently varies, on the same object, from a few seconds to two hours. The drawings are composite in that I have endeavored to combine in a single sketch all the details visible in all the negatives of a given object. In making these drawings, the dimensions of the nebula and the positions of the more prominent structural features were first determined on a measuring
engine; these dimensions and positions were then laid down on the scale adopted for the sketch. Finally, the general structure and the finer details were filled in, by free-hand drawing, from a careful intercomparison of the different negatives of the object. While the resulting effect is admittedly merely pictorial, and lacks the accuracy of a photograph, the great difficulty of securing satisfactory direct enlargements of these minute objects is undoubtedly a sufficient justification for the adoption of the method employed. In many cases the sketches give a better idea of the sum total of the structural features of a planetary than an enormously enlarged photograph could have done, and they may then be regarded as subserving sufficiently well the main purpose of this paper—to give an accessible and legible conspectus of the forms and structures of the planetary nebulae.
While the drawings have been made with care, it is scarcely necessary for me to emphasize that too much must not be expected from them. Considerable personal equation must inevitably enter into the record of the faintest details, which are often on the limit of visibility. It is manifest that the accuracy of a photographic reproduction can never be reached, and that such drawings can never supplant reference to the original negatives in all questions involving movement, change, or the relative intensities of structural features. In an interesting paper Wood1 has shown that there is so little difference in the reflecting power of white paper or canvas and the same materials covered with the inks or pigments used by printers and painters, that the greatest contrasts of light and shade which can be secured in any painting, drawing, or photographic print will fall very far short of giving a truthful representation of the actual gradations of illumination in any ordinary landscape. In like manner, it is impossible to show the enormous differences in brightness which actually exist in the brighter and fainter portions of many planetaries; such differences can be only approximately indicated, and the fainter features are necessarily and designedly accentuated in the illustrations. In my first drawings I attempted to preserve closely the gradations in intensity, but found that the resulting process blocks obliterated the delicate gradations, and that the faintest features, to show at all in the illustration, must necessarily be unduly accentuated. It is also self-evident that could these objects be photographed with instruments of much greater focal length, a wealth of minute structural detail would doubtless be shown in those more obscure features which, on the scale of the Crossley photographs, appear as mere diffuse areas and wisps.
For several of the planetaries of moderate size I have given also a direct photographic enlargement at the right, and on nearly the same scale as the drawing (indicated by the subscript a to the figure number), from which an idea may be derived as to the advantages, and the limitations as well, of this method of composite drawings. Direct photographic enlargements alone have been given for the largest planetaries; the scale in these cases is given in the descriptive text. The scale of the drawings is indicated below the nebula. I have, in general, omitted in the drawings any stars which are apparently projected on the image of the planetary, though I have made a few exceptions to this rule. The positions given are for 1900. In all the reproductions the orientation is north at the top and east at the left, and the position angles are, as usual, measured from the north through the east.
No attempt has been made to include in the descriptions any reference to the observer by whom the various planetaries were discovered, as such data are accessible in the three catalogues of the N. G. C. and other sources. The planetary character of the following eight nebulae has been discovered since the publication of the catalogue sources mentioned above.
N.G.C. 246; 0" 42'PO, —12° 25'. Noted by Reynolds in Helwan Observatory Bulletin, No. 9, as somewhat resembling the Owl Nebula. Spectrum found of planetary type by Messrs. Campbell and Paddock.
J. 320; 5" OmO, +10° 35'. Discovered by Jonckheere, Observatory, 39, 134, 1916.
J. 900; 6" 20">1, +17° 51'. Discovered by Jonckheere, A. N., 194, 47, 1913.
i Proc. Boy. Inst., 20, Part I, 180, 1913.
N.G.C. 650-1; l" ,36mO; 4-51° 4'
Enlarged 5.8 times from a negative of 4" exposure. Central star of magn. 16. Quite irregular, but evidently to be included as one of the larger members of the planetary class. The central and brighter portion of the nebula is an irregular, patchy oblong 87" X 42" in p. a. 40°; from the ends of which faint, irregular, ring-like wisps extend; total length 157" in p. a. 128°. Brightest patch at southern end of central part. Rel. Exp. 20.
N.G.C. II 1747; l" 50n>3; +62° 50'
Exposures lm to 1". Central star about magn. 14. Nearly round disk 13" in diameter, with an indistinct ring effect, strongest on north and south and fading out along an axis in p. a. 90°. Brighter parts of ring show in 3"1 on S23. Rel. Exp. 15. • •;