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N.G.C. 729.3; 22" 24n>2; —21° 21'
Enlarged 1.9 times from a negative of 4" 10' exposure time. Scale, 1 inch = 815. The central star is magn. 11 photographically. A beautiful and remarkable object, about 15' in length in p. a. 125°, and 12' across at right angles to this direction. Around the central star is a very faint oval 6' X 4.6' in p. a. 35°. This planetary appears to consist of two turns of a helix (compare N.G.C. 6543). Easily the "giant" of the planetary class, and probably the largest ring-form object known. Intrinsically rather faint. Rcl. Exp. 150.
N.G.C. 7354; 22" 36TM6; +60° 46'
Exposures 1"' to l1' 50"'. Central star of magn. 16. A somewhat irregular oval ring, fading out at each end, 22" X 18" in p. a. 27°. Outside this is a ring or disk of much fainter matter, rather more circular in form and 32" across from east to west. This outer portion likewise is fainter at the ends of the major axis, and shows brighter streaks at the east and west edges. Relatively faint; the inner ring just shows in 5"' on S27. Rel. Exp. 15.
N.G.C. 7662; 23" 21'Pl; -f41° 59'
Exposures 1• to 2* 51"'. According to Barnard, Monthly Notices, 68, 465, the central star is variable from magn. 12 to less than magn. 16, with a period of about 27 days. Series of short exposures taken 1899, Sept. 5; 1908, July 8; 1914, July 26; 1916, Sept. 28, agree in showing the nucleus as nearly 11% magn. photographically, with no certain evidence of variation. The inner ring is somewhat irregular, with a fainter portion at the west, 17" X 14" in p. a. about 45°. The nucleus lies closer to the eastern side of the ring. The outer and fainter oval is 32" X 28" in the long exposures, with minute, wedge-shaped ansae in p. a. 21-201°. Exceedingly bright; the inner ring shows faintly in 5* on S27. Rel. Exp. 0.2. ;;
N.G.C. 2371-2; 7" 19m3, +29° 41'. Recognized as of planetary form by Curtis from Crossley photographs;
bright-line spectrum observed by Messrs. Campbell and Moore. N.G.C. 2610; 8" 28'P8, —15° 48'. Noted as annular, and spectrum probably gaseous, by Reynolds in Eelwan
Observatory Bulletin, No. 15. Planetary type of spectrum confirmed byAlessrs. Campbell and
Paddock. N.G.C. 6058; 16" l^O, +40° 58'. Recognized as of planetary form by Curtis from Crossley photographs;
bright-line spectrum found by Messrs. Campbell and Moore. N.G.C. 7139; 21" 43n>5, +63° 19'. Spectrum found of planetary type by Curtis with the slitless spectroscope. N.G.C. 7293; 22" 24TM2, —21° 21'. Recognized as of planetary form by Curtis from Crossley protographs;
bright-line spectrum found by Messrs. Campbell and Moore.
Dimensions, unless otherwise stated, are to the outside of rings or formations. Because of the added record of slightly fainter contiguous matter, the dimensions of bright structures will frequently be very materially increased if measures are made on negatives of longer exposure times; I have accordingly occasionally given the results of measures made on both short and long exposures. Strictly speaking, a series of sketches would be necessary in certain objects, to give an adequate idea of their appearance under different lengths of exposure, and to indicate the distribution of the different sorts of matter within the nebula. Wolf-' and Burns3 have called attention to the varying size of the monochromatic images of the Ring Nebula in Lyra as shewn in objective spectrograms, and elsewhere in this volume Professor 'Wright shows speetrographic evidences of remarkable differences in distribution or condition of the various sorts of matter composing certain of the planetaries.
Where the magnitude of the central star is given to tenths, the magnitude is visual, and has been derived from Annals H. C. 0., 56, No. VI, or from various catalogue sources. In many cases such magnitudes very evidently refer to the total effect, and will differ widely from the true magnitude of the central star. Estimates of the photographic magnitude of the central star have been made from the photographs, and are to the nearest magnitude only; such estimates are in some eases merely approximations, owing to the difficulty of making trustworthy estimations of magnitude where very bright nebular matter surrounds the central star.
Iu the belief that an approximation to the relative brightness of the planetaries will be of great service to others working in this field, I have endeavored to give an idea of the relative brightness of the nebular matter in different planetaries in two w'ays; first, by indicating the time necessary to show the brightest features on the various brands of Seed plates employed (abbreviated to S30, S27, S23, and L.S.), and secondly, by a comparison with the Nebula in Orion. The portion used for comparison was a bright patch of nebular matter whose center is about 45" north preceding the Trapezium; this patch shows well in 10• on S27. A series of exposures was made of the Orion Nebula on Seed 27 and Seed 23 plates, and an estimate Was then formed of the time necessary to record the brightest portion of the planetary in comparison with that needed for a corresponding density in the selected portion of the Orion Nebula. The phrase "Rel. Exp. 150" will then indicate that the brightest portion (always referred to bona fide nebular matter) of the planetary under consideration requires 150 times the exposure needed by the standard area in the Orion Nebula; "Rel. Exp. 0.1" would similarly indicate that an exposure of but one-tenth is needed, etc.
2 Vicrteljahrschrift Astron. GeseU., 43, 283, 1908.
3 Lick Obs. Bull, 6, 92, 1910.
The positions of the seventy-eight planetaries are plotted in Figure 79. The circles of varying diameters indicate the relative apparent sizes of the objects; where a planetary is elongated a line has been drawn indicating the orientation of the major axis. The diagram confirms in a striking manner the well known fact that the planetary nebulae, as a class, are essentially a galactic phenomenon. The remarkable concentration in the southern branch of the Milky Way in the region from 18h to 20h right ascension is also clearly indicated. It has been noted by Perrine* that two-thirds of the planetaries are situated within 50° of the ellipsoidal vertices, with a decided preference for the ellipsoidal region at 18\ The orientations of the major axis appear to be entirely at random, and uninfluenced by the galactic plane.
A further deduction of considerable interest may be made from the distribution as shown by the diagram. It will be noted that the smallest objects of the class are almost invariably in or very close to the Milky Way. On the other hand, the larger planetaries, and the giants of the class, while somewhat more frequent in the vicinity of the galactic plane, are, on the whole, fairly uniformly distributed over the entire sky. If these bodies are distributed at random throughout the structure of the galaxy, those which are nearest to us would, in numerous instances, appear to be situated at considerable distances from the galactic plane, and these nearer objects would also average largest in their apparent size. The approximately regular distribution of the giants of the planetary class over the entire sky thus gives very strong support to the inherently probable assumption that the planetaries of greatest apparent size are, on the average, the closest to us. The larger planetaries would then appear to be well worth inclusion in parallax programmes, and many of them have central stars of sufficient brightness for the purpose.
In that portion of the Milky Way which extends approximately from 17h to 20b in right ascension, and from 35° south to 35° north declination, there are eighteen planetaries smaller than 20" in diameter, while in the antipodal region at 6h there are but three. The preponderance
* Perrine, C. D., Preliminary Examination of the Planetary Nebulae for Preferential Motion, Astrophysical Journal, 46, 175, 1917.
of small planetaries in the former region would appear to give some ground for the belief that the galactic region in which they occur is considerably more remote than the region near 6h. It is not impossible, however, that the marked concentration of small planetaries near 19h is merely fortuitous.
While the details of the forms assumed by individual planetaries are frequently of bewildering complexity, there are certain characteristics which are common to many members of the group; the occurrence of a central star is such a salient feature. Throughout this paper reference will be made to the planetaries by their numbers in Dreyer's New General Catalogue, and in the same author's First and Second Index Catalogues, with the omission of the letters N.Q.C. To facilitate reference to the illustrations, the number of the illustration will be placed within parentheses after the number of the nebula.
Fifty-five of the seventy-eight planetaries have a central star; these (arranged in the order of right ascension) are as follows:
There are eight very minute planetaries only two or three seconds of arc in diameter; these may possibly possess a central star, though their small size makes it impossible to distinguish it. These planetaries are:
6644 (43) II 4846 (50) 6807 (56) II 4997 (67) II 4732 (44) 6790 (53) 6833 (59) II 5117 (73)
No central star can be made out in the following thirteen planetaries:
1952 (11) 2440 (19) 6565 (37) 6884 (63) 7027 (72) II 2165 (14) II 4634 (29) II 4776 (45) 6886 (64) II 5217 (75) J 900 (15) 6537 (35) 6741 (47)
Four planetaries are binuclear, trinuclear, or too irregular to classify; the condensations in these are not stellar in the shortest exposures.
1952 (11) J 900 (15) 2440 (19) 7027 (72)
Ten of the seventeen planetaries in the last two groups, in which no central star can be detected, are very small, being less than 10" in their greatest dimension, so that a faint central star could easily remain invisible. Two, 2440 (19) and 7027 (72), are quite unlike the typical planetary; the same is true of 1952 (11), the "Crab" Nebula; although this object shows a bright-line spectrum, it should probably be classed with the diffuse nebulosities rather than with the planetary nebulae. J900 (15) has two bright lobe-like nuclei but 2'.'8 apart; the nebular matter is very bright and a faint central star might thus escape detection. To sum up: fiftyfive planetaries show a central star; eight others are so minute as to be indistinguishable from a star on the scale of the Crossley plates; in twelve small planetaries less than 10" in diameter no central star is apparent, but might easily be masked by the bright nebular matter in these minute objects; four larger objects show no central star, and three of these are anomalous forms. From a consideration of these facts there will be but slight chance for error if the definite rule be laid down that a typical planetary nebula has a central star.