William F. Denning.

Telescopic Work for Starlight Evenings online

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In small telescopes it looks like a rather faint, round mass of
nebulosity, somewhat brighter in the middle than at the edges. In
Lord Rosse’s telescope it shows many details, including a spiral
arrangement and two dark spots in the middle inclosing bright, eye-like
condensations. The margin is fringed with protuberances, and from its
peculiar aspect this object has been called the “Owl” Nebula. Diameter
between 155″ and 160″. It may readily be picked up 2-1/4° S.E. of β
Ursæ Majoris. It yields a gaseous spectrum.

In Draco at R.A. 17^h 58^m 36^s, Dec. +66° 38′ there is a pretty small,
but exceedingly bright planetary nebula. With a low power it looks like
a star out of focus, but a high power expands it into a well-defined
planetary disk. As observed in Lord Rosse’s 3-ft. reflector on Sept.
17, 1873, this nebula exhibited “a round, well-defined disk of a full
blue colour, light very equable, diameter 22″·4, surrounded by an
extremely faint nebulosity.” This is an excellent object of its class.

_Spiral Nebula_ (M. 51). Discovered by Messier on Oct. 13, 1773.
It is situated in Canes Venatici, and 4° S.W. from ζ Ursæ Majoris.
An ordinary instrument will reveal it as a double nebula, and the
two parts will be seen to differ greatly in size. Messier gave the
distance separating them as 4′ 35″. Sir J. Herschel drew this object
as a bright, centrally condensed nebula, surrounded by a dark space
and then by a luminous ring divided through nearly one half of its
circumference. Closely outlying this he placed a bright round nebula.
Lord Rosse’s 6-foot showed something very different. In April 1845 its
spiral character was discovered; coils of nebulosity were observed
tending in a spiral form towards the centre, and the outlying nebula
was seen to be connected with it. Some striking drawings have been
published of this object. Those by Sir J. Herschel and Lord Rosse
differ essentially, and would scarcely be supposed to represent the
same nebula; but when we reflect that the instruments used were
respectively of 18 inches and 72 inches aperture, the cause of the
disparity becomes evident.

Another fine example of a spiral nebula is M. 99, in the northern
wing of Virgo, and 8° E. of β Leonis. This object was discovered by
Mechain; its spiral form of structure was detected by Lord Rosse in
1848. Diameter 2½′ Like M. 51 it gives a continuous spectrum and is
resolvable into stars.

[Illustration: Fig. 64.

1. Nebula with bright centre.

2. Planetary Nebula.

3. Ring-nebula in Lyra.

4. Star-cluster in Hercules.]

_The Crab Nebula in Taurus_ (M. 1). Discovered by Bevis in 1731, and
situated 1° N.E. of ζ Tauri. Its diameter is 5½′ by 3½′. An early
drawing with Lord Rosse’s telescope shows it with numerous radiations;
whence it was termed the Crab Nebula, from the supposed resemblance:
but later observations have given it quite another form. In 1877 there
was no trace of the nebulous arms: it appeared as a well-defined, oval
nebula with some irregularities of structure. This object is very plain
in small telescopes, and may be readily picked up from its proximity
to ζ Tauri; but in such instruments it is void of detail, and merely
presents a pale, oval nebulosity. It has not been clearly resolved,
though it has a mottled appearance, indicating a stellar composition,
in large apertures.

_The Dumb-bell Nebula_ (M. 27). Discovered by Messier in 1779, and
situated in Vulpecula—a region very rich in small stars. Diameter about
7′ or 8′. Its general form resembles a dumb-bell or hour-glass; hence
its name. Struve, Lord Rosse, and others have seen many stars in the
nebulous mass, but the latter is not resolvable. I have seen seven
stars in the nebula with a 10-inch reflector. Its peculiar shape is
perceptible in a small instrument. This object frequently serves to
illustrate books on Astronomy; but the drawings by Sir J. Herschel,
Lord Rosse, and others are curiously discordant, and show how greatly
differences in telescopic power may affect the observed appearance of
an object.

_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. 0^h 42^m·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. 20^h 41^m 7^s, 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. 13^h 38^m, 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.|Date of |Position 1890. | |
|No. |Discovery. +——————-+——————-+ 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. |
+————+————————————-+——————————————-+———————————————————————————-—-+

Abbreviations:—F., faint; S., small; R., round; M., middle; N.,
nucleus, E., extended; v., very; b., brighter; n., north; s., south;
f., following; p., pretty, preceding; m., much; g., gradually; *,
star; N.G.O., New General Catalogue.

No. 8 is placed centrally within a curious semicircle of stars, thus:—

[Illustration: Fig. 65.]

I.—CLUSTERS OF STARS.

+————————-+——————-+——————————————————+———————————————————————————————+
| | | Position, 1890. | |
| No. | No. +————————-+————————+ |
| N.G.C., | M., | | | Description. |
| 1888. | 1781. | R.A. | Dec. | |
+————————-+——————-+—————————+————————+———————————————————————————————+
| | | | | |
| | | h m | ° ′ | |
| 225. | | 0 37·1 | +61 3 | Stars 9th-10th mags. Between |
| | | | | γ and κ Cassiopeiæ. |
| 869. | | 2 11·3 | +56 38 | In Perseus. Stars 7th-14th |
| | | | | mags. |
| 1039. | 34. | 2 35·0 | +42 18 | A fine group, chiefly of 9th |
| | | | | mag. stars. |
| 1912. | 38. | 5 21·3 | +35 44 | Stars of various mags. In |
| | | | | Auriga. |
| 1960. | 36. | 5 29·0 | +34 4 | Stars of 9th-11th mags. Near |
| | | | | 1912. |
| 2099. | 37. | 5 45·1 | +32 31 | Stars and star-dust. 5° S. of |
| | | | | θ Aurigæ. |
| 2168. | 35. | 6 2·0 | +24 21 | Stars of 9th-16th mags, near |
| | | | | ζ Geminorum. |
| 2287. | 14. | 6 42·3 | -20 38 | Visible to naked eye. 4° S. of|
| | | | | Sirius. |
| 2437. | 46. | 7 36·8 | -14 34 | Nebula involved with cluster |
| | | | | of 8th-13th mag. stars. |
| 2477. | | 7 48·4 | -38 16 | Fine group of 12th mag. stars |
| | | | | near ζ Argûs. |
| 2516. | | 7 56·5 | -60 34 | Visible to naked eye. 200 |
| | | | | stars of 7th-13th mags. |
| 2547. | | 8 7·4 | -48 56 | Vis. n.e. Stars 7th-16th mags.|
| | | | | Diameter 20′. |
| 2548. | | 8 8·3 | -5 28 | Stars of 9th-13th mags. In |
| | | | | Monoceros. |
| 2632. | 44. | 8 34·0 | +20 22 | Præsepe. Group of bright stars|
| | | | | vis. n. e. |
| 2682. | 67. | 8 45·2 | +12 13 | Large group of stars of |
| | | | | 10th-15th mags. |
| 4755. | | 12 47·1 | -59 45 | Very large group about κ |
| | | | | Crucis. |
| 6121. | 4. | 16 16·9 | -26 16 | Close to Antares. Group and |
| | | | | line of stars through it. |
| 6603. | 24. | 18 12·0 | -18 28 | Stars of 15th mag. 3° N. of |
| | | | | μ Sagittarii. |
| 6611. | 16. | 18 12·7 | -13 50 | Group of at least 100 stars |
| | | | | of various mags. |
| 6705. | 11. | 18 45·1 | -6 24 | Stars of 11th mag. and |
| | | | | fainter. Fine object. |
| 6838. | 71. | 19 48·8 | +18 29 | Stars of 11th-16th mags. In |
| | | | | Sagitta. |
| 7243. | | 22 10·9 | +49 20 | A clustering of many bright |
| | | | | stars. |
| 7654. | 52. | 23 19·4 | +61 0 | Irregular group of 9th-13th |
| | | | | mag. stars. |
| 7789. | | 23 51·5 | +56 6 | Grand cluster of 11th-18th |
| | | | | mag. stars. |
+————————-+——————-+—————————+————————+———————————————————————————————+

II.—Globular Clusters of Stars.

+————————-+——————-+——————————————————+———————————————————————————————+
| | | Position, 1890. | |
| No. | No. +————————-+————————+ |
| N.G.C., | M., | | | Description. |
| 1888. | 1781. | R.A. | Dec. | |
+————————-+——————-+—————————+————————+———————————————————————————————+
| | | | | |
| | | h m | ° ′ | |
| 104. | | 0 19·1 | -72 42 | Very large; more than 15′ |
| | | | | diameter. |
| 288. | | 0 47·8 | -27 11 | Slightly elliptical. Stars |
| | | | | 12th-16th mags. |
| 362. | | 0 58·5 | -71 26 | Stars 13th-14th mags. |
| | | | | Diameter 4′. |
| 1261. | | 3 9·3 | -55 38 | Large. Stars and star-dust. |
| | | | | |
| 1851. | | 5 10·5 | -40 10 | Very bright and large. Fine |
| | | | | object. |
| 4147. | | 12 4·5 | +19 9 | Pretty large, round. Minute |
| | | | | stars. |
| 4590. | 68. | 12 33·7 | -26 9 | Much compressed group of 12th |
| | | | | mag. stars. |
| 5024. | 53. | 13 7·5 | +18 45 | Fine object. Chiefly 12th |
| | | | | mag. stars. |
| 5139. | | 13 20·2 | -46 44 | Very large; diameter 20′. |
| | | | | At ω Centauri. |
| 5272. | 3. | 13 37·1 | +28 56 | Visible to naked eye. |
| | | | | Diameter 7′. |
| 5634. | | 14 23·8 | - 5 29 | Very bright, considerably |
| | | | | large. Round. |
| 5904. | 5. | 15 13·0 | + 2 29 | Visible naked eye. Stars |
| | | | | 11th-15th mags. Diam. 5′. |
| 5986. | | 15 38·8 | -37 25 | Stars of 13th-15th mags. In |
| | | | | Lupus. |
| 6093. | 80. | 16 10·5 | -22 42 | Stars of 14th mag. Between |
| | | | | α and β Scorpii. |
| 6205. | 13. | 16 37·7 | +36 40 | Visible naked eye. A grand |
| | | | | object, in Hercules. |
| 6218. | 12. | 16 41·5 | - 1 45 | Stars of 10th mag. and |
| | | | | fainter. Diam. 4′. |
| 6254. | 10. | 16 51·4 | - 3 56 | Stars of 10th-15th mags. |
| | | | | Diameter 4′. |



Online LibraryWilliam F. DenningTelescopic Work for Starlight Evenings → online text (page 29 of 32)