The Ring-Nebula in Lyra (M. 57). Discovered by Messier between the stars β and γ Lyræ. Diameter 80″ by 60″. This object is bright, though rather small, and it will stand high powers. The dark centre may possibly be glimpsed in a 3-inch refractor; I have seen it readily in a 4-1/4-inch. It was at one period thought to be resolvable, but the spectroscope has negatived the idea, and shown it probably consists of nitrogen gas. A small star near the centre was frequently seen in Lord Rosse’s telescope; but the 36-inch refractor at Mount Hamilton reveals twelve stars projected on or within the ring, and several others have been suspected. There is a faint star exterior to the ring, and following it; this is visible in small telescopes. The space within the ring is not quite dark, and the structure of the nebula is somewhat complicated as seen in large instruments. Another fine instance of an annular nebula may be found 3° preceding the 4th mag. star 41 Cygni, but it is not so large or conspicuous as that in Lyra. Its diameter is 47″ by 41″. Several stars were seen sparkling in it by Lord Rosse, who found the centre was filled with faint light and the N. side of the ring broadest and brightest.
Elliptical nebulæ are well represented by the pair (M. 81 and 82) about 2° E. of δ (22) Ursæ Majoris. They are separated by about 38′ of declination, so that they may be observed in the same field of a low-power eyepiece. The preceding one is very bright and large (8′ by 2′). The following one is a ray or streak of nebulosity (7′ by 3/4′). On May 21, 1871, the great Rosse telescope showed the latter as a most extraordinary object, at least 10′ in length and crossed by several dark bands. Roberts photographed these nebulæ on March 31, 1889. “The negative shows that the nucleus [of M. 81], which has not a well-defined boundary, is surrounded by rings of nebulous or meteoric matter, and that the outermost rings are discontinuous in the N.p. and S.f. directions.” M. 82 is “probably a nebula seen edgeways, with several nuclei of a nebulous character involved, and the rifts and attenuated places in it are the divisions of the rings that would be visible as such if we could photograph the nebula from the direction perpendicular to its plane” (‘Monthly Notices,’ vol. xlix. p. 363). This fine pair may be easily picked up in a small instrument. Another grand object of this class (discovered by Caroline Herschel in 1783) lies in R.A. 0h 42m·2, Dec.-25° 54´, between the stars β Ceti and α Sculptoris.
Globular clusters furnish us with many examples of easily resolved and richly condensed balls of stars. M. 3 (discovered by Messier), M. 5 (discovered by G. Kirch), and M. 13 (discovered by Halley) may be selected as amongst the finest of these objects in the northern hemisphere. They are severally visible to the naked eye, and may be found in a telescope directed as follows:—M. 3, between Arcturus and Cor. Caroli, and nearer the former; M. 5, 7° S.W. of α Serpentis and close to the double star 5 Serpentis; M. 13, one third the distance from ζ to ζ Herculis. They are brilliant objects from 5′ to 7′ diameter. With power 60 on my 10-inch reflector they are spangled with stars, though not fully resolved. Smyth described M. 3 as consisting of about 1000 small stars, blazing splendidly towards the centre. Webb hardly resolved it with a 3-7/10-inch refractor. Another fine object of this class (M. 80) will be encountered midway between α and β Scorpii. Sir W. Herschel described it as the richest and most compact group of stars in the sky, and it is noteworthy from the fact that a new star burst forth near its centre in 1860. There is a magnificent cluster, involving ω Centauri, which Sir J. Herschel considered as “beyond all comparison the richest and largest object of the kind in the heavens.” It is visible to the naked eye as a 4th mag. star, but residents in northern latitudes are precluded from a view of it. Pegasus also supplies us with some fine clusters; Maraldi picked up two in 1746 (M. 2 and 15), and these will respectively be found 5° N. of β and 4° W.N.W. of ε Pegasi. They are to be classed amongst the grandest objects of their kind.
In Cygnus, at R.A. 20h 41m 7s, Dec. +30° 19′, near κ and especially in the region immediately north-east, there exist irregular and extensive streams of faint nebulosity which may be said to form a telescopic milky way, Nebulæ and stars are curiously grouped together, forming a remarkable arrangement which will well repay study. To see these objects satisfactorily, a moonless night, free from haze or fog, should be chosen, and the power should be moderately low, or some of the more feeble nebulous films will be lost. The observer may spend some agreeable hours in sweeping over this region, which is one of the best in a wonderfully rich constellation.
Further Observations.—The fact that Swift has discovered many hundreds of nebulæ during the last few years affords indubitable proof that considerable numbers of these objects still await detection. No doubt they are generally small and faint, but it is necessary they should be observed and catalogued, so that our knowledge in this department may be rendered as complete as possible. New nebulæ are sometimes mistaken for expected comets, and occasionally give rise to misconceptions which would be altogether avoided were our data more exhaustive.
Those who sweep for nebulæ must have the means of determining positions, and a small telescope will be inadequate to the work involved. A reflector of at least 10 inches, or refractor of 8 inches, will be required; and a still larger instrument is desirable, for to cope successfully with objects of this faint character needs considerable grasp of light. The power employed should be moderate; it must be high enough to reveal a very small nebula, but not so high as to obliterate a large, diffused, and faint nebula. In forming his first catalogue of 1000 nebulæ, Sir W. Herschel used a Newtonian reflector of 18·7 inches aperture, power 157, field 15′ 14″; Swift’s recent discoveries were effected with a 16-inch refractor and a periscopic positive eyepiece, power 132, field 33′. With a low power a very extensive field will be obtained, and a large part of the sky may soon be examined, but it will be done ineffectively. It is better to use a moderately high power, and thoroughly sweep a small region. The work is somewhat different to comet-seeking; it must proceed more slowly and requires greater caution, for every field has to be attentively and steadily scanned. If the telescope is kept in motion, a faint nebula will pass unseen. Some of these objects are so feeble that they are only to be glimpsed by averted vision. When the eye is directed, say, to the E. side, a faint momentary glow comes from the W. side of the field; but the observer discerns nothing on looking directly for the object. On again diverting his gaze he receives another impression of faint nebulosity from the same point as before, and becomes conscious of its reality. Frequently, while comet-seeking, I meet with a small indefinite object, the character of which cannot be determined by direct scrutiny. On withdrawing the eye to another part of the field, however, the mystery is solved. If the object is a nebula, it flashes very distinctly on the retina; but if a small cluster, the individual stars are seen sparkling in it. These indirect views are usually so effective that the trouble of applying higher powers is dispensed with.
The glow from a faint nebula or comet often apparently fluctuates in a remarkable manner. Light-pulsations affect it, causing the nebulosity to be intermittently visible. It flashes out and enlarges, then becomes excessively feeble and indeterminate. The changes are not real, but due to the faint and delicate nature of the object, which is only fugitively glimpsed and presents itself differently with the slightest change in the manner of viewing it. Burnham has said there is no such thing as glimpsing an object; but he is wrong. It is the intermediate step between steady visibility and absolute invisibility.
The work of sweeping for nebulæ is much delayed by the comparisons necessary for the identification of objects. The path may be smoothed by marking the known nebulæ on a good chart, like Argelander’s. The observer may then see, by reference, whether the objects he encounters have been picked up before. The labour of projecting all the nebulæ contained in the New General Catalogue would of course be considerable, and the observer will probably find it expedient to select certain regions for examination, and map such nebulæ as are included within their borders.
The discovery of new nebulæ offers an inviting field to amateurs. Vast numbers of these objects have escaped previous observation, for though the sky has been swept again and again, its stores have not been nearly exhausted. Mr. Barnard recently stated that with the powers of the great 36-inch refractor the number of known nebulæ (more than 8000) might readily be doubled! As an example of their plentiful distribution in certain regions it may be mentioned that Mr. Burnham very recently discovered eighteen new nebulæ in a small area of 16′ by 5′·5 near the position in R.A. 13h 38m, Dec. 56° 20′ N. Near the pole of the northern heavens there exist many unrecorded nebulæ, as this region does not appear to have been thoroughly examined with a large instrument. It is often the case that several nebulæ are clustered near together. Whenever a new one is discovered the surrounding space should therefore be carefully surveyed in search of others. The region immediately outlying known objects may also be regarded as prolific ground for new discoveries. After several hours’ employment in the work of searching for nebulæ or comets the eye is enabled to discern faint objects which were invisible at first, as it is in a better condition to receive feeble impressions. While comet-seeking in 1889 and 1890 I discovered ten new nebulæ, all near the N. pole, and their approximate positions are given below:—
| Ref. No. | Date of Discovery. | Position 1890. | Description | |
|---|---|---|---|---|
| R.A. | Dec. + | |||
| h m s | ° ′ | |||
| 1. | 1889, Aug. 26 | 4 29 59 | 75 25·2 | F., S., b. M., ⁎ 12, n.p. |
| 2. | 1890, Nov. 7 | 4 40 19 | 78 7·9 | F., S., R. |
| 3. | 1890, Oct. 19 | 4 46 38 | 68 9·8 | F., S., R., b. M. N., F. double ⁎ s.f. |
| 4. | 1890, Nov. 16 | 5 50 7 | 80 31·0 | v. F., S. |
| 5. | 1890, Nov. 9 | 6 11 45 | 83 1·9 | F., S., R., m. b. M. |
| 6. | 1890, Oct. 17 | 6 59 26 | 85 45·0 | v. F., v. v. S., 12′ s. s. f. N.G.C. 2300 |
| 7. | 1890, Nov. 7 | 7 8 52 | 80 7·4 | v. F., p. S., 22′ s. s. f. N.G.C. 2336. |
| 8. | 1890, Sept.14 | 7 23 24 | 85 30·0 | F., S., E., 46′ s. f. N.G.C. 2300. |
| 9. | 1890, Sept. 8 | 8 21 37 | 86 7·4 | p. F., S., m. b. M., * n. f. |
| 10. | 1890, Aug. 23 | 8 34 30 | 85 54·4 | F., S., R., g. b. M., near preceding. |