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UNIVERSITY OF CALIFORNIA PUBLICATIONS
DESCRIPTIONS OF 762 NEBULAE AND CLUSTERS PHOTOGRAPHED WITH THE CROSSLEY REFLECTOR
BY HEBER DOUST CURTIS
Two lists of nebulae and clusters photographed with the Crossley Reflector were published in Lick Obs. Bull., Nos. 219 and 248 (7, 81, 1912, and 8, 43, 1913). Since the date of the latter of these lists (November 1, 1913) photographs of over four hundred additional objects have been secured, and the growing interest in nebular studies renders it advisable that descriptions of these be published. There are manifest advantages, moreover, in collecting in one list all the work in this field accomplished with the Crossley Reflector to date, as no other programme with large-scale reflectors is so extensive. This paper accordingly comprises all photographs of nebulae and clusters made with the Crossley Reflector from 1898, when systematic work was commenced with this instrument at Mt. Hamilton, until February 1, 1918. It includes the objects listed in Bulletins Nos. 219 and 248, and is intended to supplant these lists. The descriptions of the two earlier lists have been revised and amplified, and a number of errors corrected. For many of the objects of the original Crossley Reflector programme better plates have been secured.
Illustrations and descriptions of seventy-eight planetary nebulae (all the known objects of this type north of 34° south declination) are published eLsewhere in this volume as a separate memoir. These planetaries have been included at their proper places in this list, followed simply by the word "planetary"; for data as to the forms, dimensions, brightness, etc., of these objects reference should be made to the paper on the planetary nebulae.
Illustrations of sixty-eight of the nebulae photographed in Director Keeler's original programme were reproduced in Publ. Lick Obs.. Vol. VIII (referred to in this list as "Vol. VIII"); these objects, omitted in Bulletin No. 219, are included in the present list.
There are many cases where spirals seen edgewise or nearly so show interesting evidence of absorption effects. These have been discussed, and seventy-nine illustrations reproduced in a separate paper in this volume, "A Study of Occulting Matter in the Spiral Nebulae." The phrase, "See Abs. Eff." in the description, indicates that an illustration of the nebula will be found in the paper mentioned.
The positions given are those of Dreyer1 brought up to 1900.0; a few obvious errors in the Drever values have been corrected.
i Dreyer, J. L. E., A New General Catalogue of Nebulae and Clusters of Stars, 1888.
, , Index Catalogue of Nebulae found in the Years 1888 to 1894, Mem. B. A. S., 51, 1895.
—, , Second Index Catalogue of Nebulae and Clusters of Stars, containing objects found in
the Years 1895 to 1907, Mem. E. A. S., 59, 1908.
This list contains 762 entries, which may be subdivided as follows, in accordance with the type of the objects:
513 spiral nebulae.
8 "dark" nebulae.
For many of the smaller nebulae the description reads: "No trace of spiral character," or some equivalent expression. This will, I think, mislead no one. I have, in all cases, indicated where a nebula is undoubtedly of the diffuse or the planetary type. Such indications of the lack of discernible spiral character merely mean that no whorls are visible, either because these may be very compactly arranged, or because of the small size of the object, and should not be taken to signify that the nebulae thus described are not bona-fide spirals. It is my belief that all the many thousands of nebulae not definitely to be classed as diffuse or planetary are true spirals, and that the very minute spiral nebulae appear as textureless disks or ovals solely because of their small size. Were the Great Nebula in Andromeda situated five hundred times as far away as at present, it would appear as a structureless oval about 0'2 long, with very bright center, and not to be distinguished from the thousands of very small, round or oval nebulae found wherever the spirals are found. There is an unbroken progression from such minute objects up to the Great Nebula in Andromeda itself; I see no reason to believe that these very small nebulae are of a different type from their larger neighbors.
There is one fairly common type of spiral of which twenty-three examples are included in this list, and traces of the effect are seen in many nebulae. Its main characteristic is a band of matter extending diametrally across the nucleus and inner parts of the spiral. Frequently the whorls in this type form a nearly perfect ring; in other examples the whorls appear to begin at the ends of this cross-arm. The general appearance is that of the Greek letter if>, and I have termed such objects i£-type spirals, for lack of a better name. See figure 2, where several examples of this type are given.
Varying estimates have been made as to the probable total number of the spiral nebulae. Director Keeler, early in the course of his programme of nebular photography and before photographs of many regions were available, estimated that there were 120,000 small nebulae, and regarded this estimate as a very conservative one.2 Perrine, on completing the original Keeler programme, and using the number of new nebulae found in fifty-seven of the one hundred and four regions of that programme, was of the opinion that 500,000 small uneatalogued nebulae were within reach of the Crossley Reflector. He deemed it probable that the total would ultimately be found to exceed 1,000,000.3 Fath,4 from a series of 139 plates in the Kapteyn areas, on which 1031 nebulae were found, estimated that the number within reach of the 60-inch reflector with exposures of one hour on Lumiere Sigma plates (an approximate equivalent to the exposures of the Crossley Reflector programme) was 162,000. Sanford5 made a number of very long exposures with the Crossley Reflector in the effort to determine whether by this means any considerable number of faint nebulae would be found which were not reached in more moderate exposures, and came to the conclusion that such long exposures would make very little change in the number recorded. The great numbers of small spirals found on nearly all my plates of regions distant from the Milky Way long since led me to the belief that Perrine's estimate of half a million was likely to be under, rather than in excess of, the truth. The extent of existing nebular photographic material now makes possible a new determination.
2 Ap. Jour., 11, 325, 1900, and Publ. Lick Obs., 8. s Lick Obs. Bull, 3, 47, 1904. < Astr. Jour., 28, 75, 1913. » Lick Obs. Bull., 9, 80, 1917.
I have made, therefore, a count of the small nebulae occurring in all the regions covered by this list, rejecting thirty-one regions where the exposure was insufficient or the plate was flawed by developer, or was otherwise of very poor quality. The average exposure of the plates is somewhat over two hours; in general, an exposure of one and a half hours is necessary to record the very faint nebulae, and two to three hours is better. Very little is gained by lengthening the exposure beyond three hours; a cleanly developed plate in sharp focus, of one and a half to two hours exposure time, will show the faintest and smallest nebulae much better than a four-hour plate where the focus is slightly out, or the guiding poor. I have not used any counts made by others, having preferred to make the count as homogeneous as possible. For many of the earlier objects of the Keeler programme better plates have since been taken. Where possible, I have checked the count on a duplicate plate, but this has been impossible for the majority of the regions, for which no duplicate plate exists or for which the duplicate plate is of poor quality. Checks made on regions where duplicate plates exist have convinced me of the essential trustworthiness of my counts, and that few spurious objects have been recorded. The results of these counts are given at the end of the descriptions in the list, abbreviated to the form "12 s. n.", i.e., twelve small nebulae were counted in this region, over and above any objects, large or small, which are given an entry in the list. Where a nebula is followed by no data as to number of small nebulae, it means either that several N. G. C. objects are found in the one region and the number of small nebulae is given elsewhere, or that the count was not made because of rejection of the region. The greatest number found on a single plate was 304 (cheeked by duplicate plate); the central portion of this remarkable region is reproduced in figure 3. Twenty-six other regions were counted as well; these are plates of sufficient exposure taken on some of the Kapteyn regions, and other special regions of the sky, which do not contain any object warranting a separate entry in the list accompanying this paper. These regions, with the number of small nebulae counted, are given below:
In all, 439 regions were counted, giving a total of 5698 small, uncatalogued nebulae. To this must be added the 513 spirals described in this list, making the total number of nebulae found in all regions 6211. In the belief that practically all these very small nebulae are spirals I have designedly omitted the diffuse nebulosities and the planetaries.
The exposed area of a Crossley plate is about nine-tenths of a square degree; the edges and corners of this area are, however, so poorly defined, owing to the distance from the optical axis, that only the brighter small nebulae can be picked up in these portions of the plate; the majority of the small nebulae are found in the more central portions having an area of about six-tenths of a square degree, or less. I have assumed 0.75 of a square degree as the average effective area on which the counts were made; this is certainly somewhat in excess, but is an error on the conservative side.
We shall first assume that these 439 regions are uniformly distributed over the sky, and that they may be taken as a fair representation of the whole sky. Their area is 329.25 square degrees. If the proportion shown by these regions holds over the entire sky, we should expect the number of spiral nebulae to be 778,000. Because of the fact that the faintest and smallest members of the class are, in general, discernible only in the central regions of the plate, I consider the figure given an under-estimate, and believe that the total number accessible with the Crossley Reflector with rapid plates and exposures of from two to three hours may well exceed 1,000,000.
It may be argued that a preponderance of plates taken in the regions near the galactic poles would have the effect of unduly increasing the estimated number of the small spirals. This objection was urged by Fath (loc. cit.) in explanation of the difference between his estimate of 162,000 and that of 500,000 by Perrine; on plotting the regions used by Perrine he found that approximately 33% of the regions counted by Perrine were located within 45° of the north galactic pole, while less than 20% of the "Selected Areas" were found in the same region. In order to test this objection, as far as it may be applied to the present counts, I have plotted in
Fig. 1. Distribution of Regions on which Small Nebulae were Counted. The Position of the Galactic Plane is shown by dotted lines.
figure 1 the 439 regions in which counts were made. It will be seen from this figure that the distribution of the regions may well be regarded as an approximately uniform one. There is, it is true, a marked concentration of regions between 12 and 13 hours R. A., in the vicinity of the north galactic pole, but this would seem to be balanced by the similar preponderance of regions in the Milky Way from 17 to 20 hours R. A. If we divide the celestial sphere into two equal areas, one consisting of a zone 60° wide extending 30° on each side of the galactic plane, and the other comprising the two zones of 60° radius about the two galactic poles, we shall find that 49.4% of the regions are located in the first, or galactic, area, and 50.6% in the polar areas (217 regions in the galactic area, and 222 in the polar areas). The balance in the distribution of the regions in the galactic and extra-galactic zones may then be regarded as a nearly perfect one. On the other hand, bearing in mind the well-known concentration of the spiral nebulae in the vicinity of the north galactic pole, and the fact that this programme deals primarily with the regions containing the spirals, it is necessary to investigate further any possible effect which this concentration might have upon the resulting estimate. I find that 36% of the regions used are within 45° of the galactic poles; as these zones comprise 29% of the sky, there is seen to be a slight preponderance. But a large part of the zone about the south galactic pole is not represented by any observed regions, so that the disproportion is greater than is apparent from the percentage just given. Taking only the zone 45° in radius about the north galactic pole, amount