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The Variability of the Nucleus of the Planetary Nebula
This remarkable nebula (N.G.C. 7662) is situated in Andromeda. In an ordinary telescope it looks like a small round disc of a bluish-green colour. In the 4-inch finder of the 40-inch telescope it is only distinguishable from an ordinary star of the 8m or 81m by a slight fuzziness of the image. In the great telescope, however, under good conditions, it is a beautiful object -a slightly elliptical disc with quite sharply defined outlines. Unsymmetrically placed on this is a roughly elliptical broken ring of greater brightness. The interior of this ring is dark, but not black, and in this, approximately central, is ordinarily a faint stellar nucleus. The broken ring (broken on its n.p. side, and with a smaller break opposite this) is of a bright bluish-green colour, while the general disc of the nebula often has the appearance of a warm or faintly reddish tint. The northern part of this inner ring is the brightest part of the nebula. The nucleus, with which I am going to deal specially, is sometimes of a bright yellowish colour, though ordinarily it is faint and apparently colourless. There is a faint star of the 155 or 16 magnitude exactly on the following edge of the outer disc.
Shortly after beginning to observe this nebula in 1897 I suspected that the nucleus was variable (see Astrophysical Journal, xiv. p. 153). Familiarity with the changing aspect of a star involved in nebulosity, due to moonlight, bad seeing, etc., has made me extremely cautious in this matter. I have, however, for several years, established conclusively the fact that the nucleus of 7662 is actually variable to an extent of upwards of three magnitudes. At times it has appeared as a bright yellowish star of about the 12th magnitude; and at other times, equally favourable, it has been either entirely invisible or excessively faint. Though I have not definitely determined the period, the light changes seem to cover about 28 days. The light curve is still undetermined; the normal condition, however, seems to be faint--the nucleus remaining bright for a few days only.
I have collected here the principal observations of the brightness of the nucleus, etc., obtained with the 40-inch telescope in the past ten years.
On 1904 August 6, among other objects, I showed this nebula to Professor H. H. Turner. Though we both looked very carefully, under fairly good conditions, we could not see any trace of the nucleus. It must have been below the 16th magnitude at that time.
In all the observations of this nebula the eyepiece was adjusted for the nebula itself, the focus of which, as I have shown elsewhere (M.N., lx. pp. 255-257, and Astrophysical Journal, xiv. pp. 151-157), is about a quarter of an inch outside that for a star.
Following are the observations of the nucleus, taken from my notes on the nebula with the large telescope.
Observations of the brightness of the Nucleus.
1897 Sept. 17. Julian day 4185, 10h om. The nucleus is 15th or 16th magnitude.
Sept. 28. 4196, 10h 30m. The nucleus is seldom seen on account of poor seeing.
4201, 8h om The nucleus is seen only once in a while. The seeing is bad.
1898 Sept. 20. 4553, 9h 30m. The nucleus is either absent or excessively faint. I think I can occasionally see faint traces of it. From recollection alone, it seems this must be very much fainter than it was last year. The definition is fair.
Sept. 26. 4559, 12h 35m. I think I can see the feeblest trace of the nucleus, but it must be excessively faint. The seeing is fair. 13h om. Can see it faintly with 700 diameters.
1899 July 31. 4867, 14h om. The nucleus is very difficult.
Aug. 1. 4868, 13h 30m. The nucleus is very difficult. The inside of the inner ring is very slightly darker than the general surface of the nebula.
The seeing is
Sept. 25. 4923, 15h 20m.
Can only catch the feeblest glimpses me that it was brighter when the The inner ring is broken in two
The nucleus is very hard to see.
Can once in a while see the nucleus.
The nucleus is seen only feebly.
1900 July 17. 5218, 14h 30m. The nucleus is 12th magnitude, and is very easy. It is yellowish.
July 28. 5229, 14h om. I cannot see the nucleus; possibly could catch feeble glimpses of it. It ought to be visible. I have thought several times that the nucleus of this nebula is variable. It looks strongly that way now.
Aug. 6. 5238, 14h om.
The nucleus is fairly distinct. There is a 15.5m or 16m star exactly on the north edge of the nebula.
Sept. 20. 5283, 10h 10m. The nucleus is extremely faint. The seeing is fairly good.
1901 Aug. 7. 5604, 12h 30m. The nucleus is readily seen. It is yellowish, and of the 12th magnitude.
Aug. 24. 5621, 161 om. 5621, 16 om. Can feebly see the nucleus, but it seems very faint. The sky is poor, but the nucleus ought to be brighter; the sky is too poor to be certain. I have suspected the nucleus of being variable. Cannot see it well enough to measure its position.
Dec. 22. not distinctly enough to measure. 1904 Aug. 6. 6699, 13h 0m.
5741, Ioh om. Can faintly see the nucleus, but
Professor Turner and I looked for the nucleus. We could see no trace of it with 460 or 700. diameters. The seeing was fair, so that we could see the details of the nebula well. The nucleus must be variable.
Aug. 8. 6701, 14h 20m. Have just tried the focus, etc. for the nucleus. Can see nothing of it. Tried eyepieces 460 and
The seeing part of the time was fair.
Aug. 13. 6706, 11h 40m. Can see the nucleus quite distinctly. It is about the 14th magnitude and is quite stellar. I think we ought to have seen it on the 6th if it was as bright as now, Seeing 2 to 3.
Aug. 15. 6708, 12h om. the sky is thick.
Can faintly see the nucleus, though
Aug. 16. 6709, 14h om. Can very faintly see the nucleus; eyepiece 700. I am not sure but that it may be fainter than last night, for the sky, though poor and thick, is better than then. Aug. 22. 6715, 10h 0m. Can see the nucleus very plainly; eyepiece 700. It is 13th or 14th magnitude, Very much plainer than the central star in M 57. It is quite bright. Seeing 3-4.
Aug. 27. 6720, 13h 30m. Eyepiece 700. The nucleus is well seen, about 12 magnitude; the same brightness as star b (more carefully:-It is decidedly brighter than b). It is about midway between a and b in brightness and is very easy. Perhaps slightly yellowish.
Aug. 29. 6722, 14h 10m. By glimpses can see the nucleus. Can just see on account of poor seeing, so the nucleus is not fainter than on the 27th. Seeing very bad.
Sept. 3. 6727, 9h om. Can see the nucleus, but it is not clear. It is about 14th magnitude. Decidedly less noticeable than
the star b. Eyepiece 700.
Sept. 5. 6729, 13h 40m. Cannot see the nucleus, or if it is seen it is excessively faint. I think in the moments of good seeing
I ought to see it. At such times b is quite easy or rather conspicuous. Just in this direction the sky seems fairly clear.
Sept. 10. 6734, 9h 30m. Can feebly catch glimpses of the nucleus, but the sky is thick. I can see the star b, but not very distinctly, on account of thick sky. Eyepiece 700.
Sept. 12. 6736, 11h 45m. The nucleus is very faint, 15th magnitude. Seeing 3. The star b is conspicuous.
Sept. 16. 6740, 10h om (no record of time). The nucleus is easy, say 13 magnitude. It is less than the star b.
Sept. 24. 6748, 12h 30m. Eyepiece 700. Can see the nucleus steadily. When best seen it is brighter than the star b. It is not less than 13th magnitude. It is perhaps as bright as 12 or 12 magnitude. Full moonlight. Looking more carefully, it is decidedly brighter than b. It is not fainter than 12th magnitude. Seeing 3.
Oct. 1. 6755, 9h 30m. The nucleus is seen occasionally.
It is much fainter than the star b. I should say it is 14th magnitude. Eyepiece 700. Seeing 2.
6765, 9h 30m. The sky is thick. Cannot be sure of the brightness of the nucleus. It is faint, but the magnitude uncertain, say 14th or less. Seeing poor = 2.
Oct. 15. 6769, 13h 40m. Cannot see the nucleus. The starb is easy and conspicuous. I do not think the seeing is such as to blot the nucleus out if as bright as it has sometimes been. Eyepiece 700.
Oct. 17. 6771, 7h 35m. Can see the nucleus quite easily, 13th or 14th magnitude. I am sure it is brighter than on the 15th. It seems nearly as conspicuous as the star b. Seeing 2 or 3.
Oct. 22. 6776, 14h 5m. I think I can see the nucleus occasionally, but cannot be certain. If it is seen it is about as bright as the star b, which is faint and not well seen, but the seeing is too poor to decide. The nebula is 5 hours west.
Oct. 29. 6783, 7h 30. I can see the nucleus. It is about 14th magnitude. By glimpses it is fairly well seen but I do not think it is as bright as it has been. Seeing 2. Eyepiece 700.
Oct. 31. 6785, 7h 30m. The nucleus is fairly well seen, about 13 magnitude. It is not as bright as the star b. A little later :-I see it better now. It is 13th magnitude. Eyepiece 700.
Nov. 5. 6790, 10h 50m. Eyepiece 700. I can see the nucleus occasionally. It is faint, perhaps 14th magnitude. The starb is quite easy and bright. I think the conditions are such that the nucleus ought to show up distinctly.
Nov. 12. 6797, 9h 20m. nucleus occasionally fairly well. steadiness it must be as bright as conspicuous as the star b.
I can see the
It is faint, say
Nov. 14. 6799, 6h om. 14th magnitude. I think it ought to be brighter if it is not variable. Though I can see it, it does not appear as stellar as it has appeared at times. There is a half moon shining, which may have something to do with it. Seeing = 3. Nov. 26. 6811, 9h 15m. Eyepiece Eyepiece 700. is faint. It is not brighter than 14th magnitude. the seeing is fairly good. It is far less bright than not well seen.
By moments star b. It is
Dec. 5. 6820, 5h 30m. nucleus, but it is faint. It is between 13th and 14th maguitude, and is much less conspicuous than the star b.
Dec. 10. 6825, 7h om. The sky is too thick and the nebula is dull. I can see the star b faintly, but cannot see the nucleus. The sky is too thick, however, for this observation to count. 1905 Oct 28. 7147, 7h 30m. The nucleus is steadily seen with both eyepieces, perhaps best with eyepiece 460. It is nearly as bright as the star b. Eyepieces 460-700. Seeing 3. Nov. 26. 7176, 8h 35m. The nucleus is very faint, very much fainter than the star b. Seeing 3.
Nov. 28. 7178, 8h 45m. Eyepiece 700. The nucleus is very faint, but I can see it in spite of the bad seeing. It is much fainter than the star b.
1906 Aug. 11. 7434, 14h 40. Eyepiece 460. The nucleus is of about the same brightness as star b. Its appearance is the same with eyepiece 700. It is not less bright than the star b. Seeing poor.
7437, 11h 50m. The nucleus is very faint. I think it is one magnitude less than the star b.
Sept. 4. 7458, 13h 50m. Eyepiece 700. The nucleus is bright and conspicuous, of about the 12th magnitude, and seems to be yellowish. The seeing is not especially good; it comes and
Sept. 8. 7462. 13h 45m. The nucleus is easy. It is magnitude less than the star b. I think it is a little less bright than at the last observation. It is 13th magnitude.
Sept. II. 7465, 13h 50m. Eyepiece 700. I can see the nucleus occasionally very distinctly. It is 13th magnitude. I do not think it is any different from the few last observations. It is much less bright than the star b.
Sept. 22. 7476, 13h 20m. Eyepiece 700. I can feebly see the nucleus. I think it is faint to-night. It is 2 magnitudes or more less than the star b. I think there is no doubt of its variability. The seeing is fair, 2 or 3.
7497, 10h 55m. I can scarcely see the nucleus, and then not distinctly. It is several times fainter than the star b. Eyepiece 700.
Oct. 30. 7514, 7h 30m. The seeing is too poor to tell anything about the nucleus.
7528, 9h 5m. The nucleus is very faint. It is 1 magnitude less than the star b. If as bright as I have seen it sometimes, it ought to be quite bright to-night. Seeing 3, fairly good.
1907 July 2. 7759, 14h 40m. Eyepiece 700. I can see the nucleus fairly well, but the sky white from the moon. Nucleus is 13th magnitude. Seeing 3.
Aug. 11. 7799, 12h om. Eyepieces 460 and 700. I can feebly see the nucleus. It is very faint and pale, and cannot be seen steadily. Cannot see it any better with eyepiece 460. Seeing 3.
Aug. 13. 7801, 11h 35m. Eyepiece 700. I can possibly see traces of the nucleus. It is very faint. The seeing is poor, but the nucleus ought to be seen easily if as bright as it has appeared sometimes. Of course I always test for the best focus on this nebula. With eyepiece 460, with the best glimpses, the inside of the ring is dark, and I can perhaps feebly see the nucleus. Seeing 2.
Aug. 20. 7808, 11h om. Eyepiece 700. Can faintly see the nucleus. Perhaps it is somewhat brighter than at the last observation (1).
Sept. 1. 7820, 10h 55m. Eyepiece 700. Can see the