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telescope. Especially astonishing were the photographs of the Draco Nebula, which showed it to have a delicate spiral structure.
Among the nebulæ thus photographed was N.G.C. 7662. In none of the photographs of this object was the exposure time long enough to bring out all the fainter parts of the nebula. The inner ring came out very strong, showing the breaks just as I had seen them in the telescope and drawn them. There were some threadlike lines (which I have not seen visually) running from the inside of the ring to the nucleus. On these photographs the nucleus does not seem to be central in this inner ring, but is perceptibly displaced to the following side. The main disc of the nebula is decidedly uneven in brightness. The north preceding portion is very faint, as is also the north following side, but less so. A section of the south following edge is very bright, while the opposite side of the nebula is somewhat more luminous than the general surface. A section of the north preceding outline of the nebula is lacking for want of longer exposure. There is evidence in the photographs of changes of brightness in the nucleus.
In all these photographs the plate was placed from to of an inch outside the stellar focus, in conformity with my previous observations, which showed that the focus for the nebula is outside that for a star. See M.N., lx., January 1900, and Astrophysical Journal, xiv., for October 1901.
I hope soon to be able to take up this subject again, with the large telescope, with suitable screen and plates prepared by Mr. Wallace.
Following is a list of the photographs.
1900 Aug. 18. Exposure oh 35m. Exposure stopped by storm. Shows feeble traces of nucleus, inner ring and part of disc. Aug. 21. Exposure 1h 18m. Sky poor. Seeing poor. Faint traces of nucleus. Shows inner ring and bright spot on s.f. edge. Aug. 28. Exposure oh 45m. Bad sky ending in fog. Shows nucleus feebly. Only inner ring feebly shown.
Sept. 3. Exposure 1h 5. Sky poor.
Sept. 3 (same date). Exposure 1h 30m. Seeing fine, sky good. The last one is best. They are both strong and show nearly all of the nebula. The nucleus is strong iu both.
Sept. 20. Exposure 2h 35. Sky very clear. Seeing poor. Shows inner ring and part of entire disc.
Sir John Herschel's Observations.
In a paper containing observations of the nebulæ made with the 20-feet reflector at Slough from 1825 to 1833, read by Sir John Herschel before the Royal Society, November 21, 1833, the following observations of this nebula are recorded :—
"Sweep 183. A fine planetary nebula. Diameter 12"; with 240 beautifully defined, light, rather mottled, and the edges the least in the world unshaped. It is not nebulous, but looks as if
it had a double outline, or like a star a little out of focus. A perfect circle. Has a star near; pos. 68°1, well measured over the centre of the nebula. See fig. 45.
"Sweep 180. vB; R; bluish white; 8 or 10" diam. has no haze at the edges, but if it be not enveloped in an eF nebulosity (perhaps arising from glare), and also the star 13 m., whose pos. is 619 and ▲ R A=40. The light is a very little mottled, and not absolutely planetary. It is a 7 m. (N.B.These satellites of planetary nebulæ ought to be especially attended to.)
"Sweep 204. Diameter in RA 20 of time. The attendant star is double A = 12 m; B=13 m., pos. of A = 58°·5; A RA from centre = 4o'0. The nebula is not perfectly round, light equal,
but a very little hazy at the edges.
"Sweep 189. Exactly R: 25 of time = diam. in R A.
a 13 m. pos. 69°0; dist. 30"; the light of the disc is perfectly equable, and equals a star 8 m.
"Sweep 190. Exactly R; a 8 m; a 13 mnf; pos. = 65°·6 by microm.; dist. = 30". [N.B.-The great discordance in the measured positions of the satellite-star seems to arise from the star being double, and taken for a single one. Whenever this occurs, it is a source of error, and should be most carefully guarded against in all future observations of this interesting object.]
I have carefully examined, with the 40-inch telescope, the immediate vicinity of the star a--the one referred to by Herschel as being double. There are no signs of duplicity, and there is no star nearer than b. This statement rests specially on observations made 1907 Dec. 25-a good night. From the recent measures there seems to be no certain motion in any of the stars a, b, and c, or the nucleus.
Observations with the Rosse Telescope.
In Phil. Trans. for 1850, p. 513, the Rosse observers, using the great reflector, give the following account of their observations of this nebula ::
"Oct. 31, 1848. Has a central spot, at moments very dark. "Dec. 13, 1848. Nothing more, except perhaps that faint external annulus extends further than had been seen before.
"Dec. 14, 1848. Note by Mr Johnstone Stoney -'Three stars near it, somewhat in this fashion; showed it to Sir James South.'
"Dec. 16, 1848. Sketches made by Lord Rosse and Mr Johnstone Stoney.
"Dec. 19. 1848. Drawing confirmed."
A drawing of the nebula is given, plate xxxviii. fig. 13. This
does not show any central star.
In Phil. Trans. for 1861, plate xxx., another drawing appears, fig. 40, which differs very greatly from the first one.
the central star conspicuously. The following additional notes are also given:
"Since the publication in the Transactions for 1850. The outlying portions in the published sketch are parts of spiral branches. Fig. 40, plate xxx. represents it as seen on a very fine night (Sept. 16, 1852), with a freshly polished speculum which defined very sharply. Oct. 2, 1856. All the details in Mr Stoney's drawing very well seen. Oct. 16, 1857 [J.D. 2, 399, 604]. The spiral arms and the in centre distinctly seen.'
The drawing referred to shows a nearly round disc with a brighter spiral arrangement commencing near the edge and ending near the central star. This light spiral is outlined by a dark shading, or by a parallel dark spiral. I do not find any reference to previous observations of the nucleus by the Rosse observers, though, from the last sentence quoted, they seemed to be familiar with the star.
Professor Burnham (Pub. L.O., ii.) measured the central star on two nights with the 36-inch telescope:
1890.78 62°9 51" 84
The central star was 15th magnitude and a 13m. He also gives measures by O. Struve :
Struve could not see the central star. His measures were, therefore, referred to the centre of the nebula.
Volume 13, Annals of the Harvard College Observatory, pp. 80-81, contains some observations of this nebula by Winlock and Searle about 1866. Combining their various measures of the dimensions, etc. of the nebula we get—
P.A. major axis of the nebula 47° ±
Some of the notes in these observations are:-"Edges as definite as Jupiter's. Two other observers also considered the colour to be green, much deeper under illumination than without ... Two edges seen as of a brighter nebula placed over a dimmer one. The brighter and fainter ellipses have major axes slightly differing in direction; the brighter having the position angle of its major axis 10 to 15° the larger (J. W. obs.) 20 to 30° according to a less careful estimate by G. M. S., to whom the northern side appears brighter; appearance to J. W. that of a hollow cylinder with one end turned nearly towards the observer."
Searle and Winlock also measured the position angle and distance of the star a on two nights in the last part of 1866, P.A. 62°*7 Dist. 52" 14
The central star was not seen at any time.
The Probable Period of Variation.
An investigation of the observations seems to give a period for the variability of the nucleus, or central star, of about 28 days, The observation of 1906 Sept. 4, however, does not seem to fit in with this period.
I give below some efforts to determine the period, utilising the early Rosse and Lassell observations in connection with my own. In general the results imply a period of 28 days. It is my intention to follow the variations of the nucleus more closely, and to determine accurately the period and the light curve. I think this is the first case in which the nucleus of a planetary or other nebula has been shown to certainly vary in its light. It is also my intention to follow up some of the other planetary nebulæ, for there are one or more cases where I have suspected variability in the nucleus.
If we take the observation of 1907 Oct. 13 and combine it as below we shall get the following results :—
Taking the observations of 1900 July 17, with an interval of 94 periods, we shall have the period 28'1 days.
If we use the observation of 1901 Aug. 7, with an interval of 80 periods, we shall have for the period 279 days.
If we combine the observation of 1904 Aug. 27, with an interval of 41 periods, the result is period, 279 days.
If we use the observation of 1904 Sept. 24, with an interval of 40 periods, we have period, 27'9 days.
The observation of 1906 Sept. 4, with an interval of 14 periods, gives a period of 289 days.
The observation of 1906 Aug. 11, gives, with an interval of 15 periods, 28-7 days.
In Memoirs of the R.A.S., vol. xxxvi. p. 51, Lassell, gives observations of this nebula with his 4-foot reflector at Multa on 1862 Oct. 23 (Julian day 2,401,437).
"Central point not stellar, but apparently a very minute planetary disk of a bluish colour, . . The central point is bright and conspicuous, not to be overlooked even in a cursory examination."
Assuming that the nucleus was at its maximum brightness at or near this observation, by comparison with the observation of 1904 Sept. 24 (J.D. 2,416,748) when it was at or near maximum, we have an interval of 547 periods, which gives a period of 28'0 days,
If we compare this observation with that of 1906 Sept. 4 (J.D.
2,417,458), 579 periods would have elapsed, which would give a period of 27'7 days.
Or taking the observation of 1907 Oct. 13 (J.D. 2,417,862), when it was evidently close to maximum, there will have elapsed 587 periods from which the period derived is 28'0 days.
For some reason all the comparisons with 1906 Sept. 4 give discordant results.
The Rosse observation of 1857 Oct. 16 (J.D. 2,399,604) was undoubtedly at or near maximum. Combining this with my observation of 1907 Oct. 13, assuming an interval of 652 periods, we shall get a period of 28'0 days.
The accompanying drawing of this planetary nebula (Plate 12) was made with the 40-inch refractor. I think it is a fair representation of the object.
In a paper read before the Royal Dublin Society (Transactions, vol. ii. (Series ii.)) on Feb. 18, 1878, the following additional notes from Lord Rosse are given :
"1848 Oct. 31. Has a central spot, at moments very dark. Nothing more, except perhaps that F external
annulus extends farther than had been seen before.
Dec. 19. Drawing confirmed. [P.T., 1850, plate xxxvii. fig. 13.] This should be plate xxxviii.-(E. E. B.)
1852 Sept. 16. Very fine night, freshly polished speculum which defines very sharply. Drawing made [P.T. 1861, plate xxx. fig. 40]. In the figure in P.T., 1850, the outlying portions are parts of spiral branches.
1856 Oct. 2. All the details of Mr. B. Stoney's drawing very well seen by Lord R. and myself.
1857 Oct. 16. The spiral arms and the star in centre distinctly seen.
1872 Oct. 7. Moderately well defined, an outer F atmosphere; darker in the middle about the spiral.
1873 Aug. 20. Has a *13 m in Pos. 244°8. Dist. 46"6.
Oct. 11. *8 m. Pos. 80°0 (3). Dist. 498"9 (2).
3 st. near . Pos. 14°2. Dist. from limbs 82"1 and 51′′*2.
Certainly no conspicuous nucl. (xxxii. obs.).”