William F. Denning.

Telescopic Work for Starlight Evenings online

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for mean rays, we have:—

Radius of object-glass in inches × radius of bright ring in seconds =
3·70.

[49] The number visible to different persons varies according to
eyesight. Some observers see thirteen or fourteen stars in the
Pleiades, while others cannot discern more than six or seven.

[50] About 2 seconds. Sir W. Herschel found the diameter of α Lyræ with
a power of 6450 to be 0″·3553. Tycho Brahe, before the invention of
telescopes, estimated the diameter of Sirius as 120″. J. D. Cassini,
with a telescope 35 feet long, found the diameter of the same star 5″.

[51] Dr. Doberck gives some valuable information with reference to the
computation of binary star-orbits in ‘The Observatory,’ vol. ii. pp.
110 and 140.

[52] The star α Canis Minoris (Procyon) was also inferred to be a
binary and to have a similar period. Several close companions appear
to have been discovered (Ast. Nach. no. 2080). But Prof. Hall, using
the 25·8-inch refractor at Washington, says:—“I have never been able to
see any of these companions that would stand the test of sliding and
changing the eyepiece, turning the micrometer, &c., and am therefore
doubtful of their existence. This is an interesting star for the
powerful telescopes of the future.” It has been surmised that the
companion is a non-luminous one, and therefore invisible.

[53] It is remarkable that nearly all the temporary stars have appeared
in the region of the Milky Way.

[54] This expert comet-finder would appear to have more acute,
sensitive vision on faint stars than Burnham (see ‘Monthly Notices’,
vol. xlix. p. 354).




CHAPTER XVII.

_NEBULÆ AND CLUSTERS OF STARS._

Distinction.—Large number of Nebulæ and Clusters visible.—Varieties
of form and grouping.—Distribution.—Early Observations.—Variable
Nebulæ.—Nebulous Stars.—The Magellanic Clouds.—Double
Nebulæ.—Real dimensions of Nebulæ and Clusters.—Round Nebulæ and
Clusters.—Description of Objects.—Further Observations required.—Lists
of selected Objects.


_Distinction._—These objects, though classed together in catalogues,
offer some great distinctions which the observer will not be long
in recognizing. It was thought at one period that all nebulæ were
resolvable into stars[55], and that their nebulous aspect was merely
due to the confused light of remote star-clusters. But modern
telescopes, backed up by the unequivocal testimony of the spectroscope,
have shown that purely nebulous matter really exists in space. The
largest instruments cannot resolve it into stars, and it yields a
gaseous spectrum. The conjecture has been thrown out that it may be
considered as the unformed material of which suns and planets are made.

_Large Number visible._—D’Arrest once said that nebulæ are so numerous
as to be infinite, and his opinion is supported by the rapid increase
in the number known. Let us make a comparison. Messier inserted in
the _Connaissances des Temps_ for 1783 and 1784 (published in 1781) a
catalogue containing 103 nebulæ and star-clusters. Of these 68 were
new. In 1888 a new edition of Sir J. Herschel’s catalogue of 1864
(revised and extended by Dreyer) was printed by the Royal Astronomical
Society, and this includes 7840 objects![56] The labours of the
Herschels, of Lord Rosse, D’Arrest, Marth, Tempel, Stephan, and Swift
have vastly augmented our knowledge in this branch since the time of
Messier.

_Varieties of Form and Grouping._—A telescope reveals all grades
of condensation in stellar groups. Some consist of rather bright,
scattered stars, and are easily resolved. Others contain more stars,
but they are smaller, and greater power is required to show them.
Others again are condensed into globular clusters needing high powers
and good instruments to disconnect the mass of stars composing them.
Some are faint, and the stars so minute that they are only to be
distinguished from nebulæ in the finest telescopes. As to the nebulæ
properly so called, they exist in all forms. They may be either round,
elliptical, or in the form of a streak. Some are highly condensed in
their centres, others present well-defined circular disks like planets,
and a small proportion are in the form of rings[57]. Many peculiarities
of detail have been remarked, and a curious and complicated spiral
structure has been discovered in certain prominent nebulæ. One of these
has been termed the “Whirlpool” Nebula from its singular convolution of
form. Other objects have received distinctive appellations agreeably
to their appearance. Thus, there is the “Dumb-bell” Nebula, the “Crab”
Nebula, the “Horseshoe” Nebula, &c. Lord Rosse’s 6-foot reflector is in
a large degree responsible for the particular knowledge we possess of
many of these objects. The large mirror commands a grasp of light which
renders it very effective on forms of this character. An instrument of
small diameter is quite inadequate to deal with them. They can be seen,
it is true, and the general shape recognized in the most conspicuous
examples, but their details of structure are reserved for the greater
capacity of large apertures.

_Distribution._—With regard to distribution these objects exhibit the
utmost irregularity, for in certain regions of the heavens they are
found to be very plentiful, while in others they are singularly rare.
Thus, in Virgo, Coma Berenices, Leo, and Ursa Major large numbers of
nebulæ abound, while in Hercules, Draco, Cepheus, Perseus, Taurus,
Auriga, &c., very few are encountered. Taking the 7840 objects in the
New General Catalogue of 1888 it will be found that their distribution
in hours of Right Ascension is as follows:—

R.A. Nebulæ.
0 H. 387
I 428
II 398
III 300
IV 276
V 375
VI 171
VII 196
VIII 230
IX 362
X 404
XI 585
XII 858
XIII 504
XIV 375
XV 212
XVI 230
XVII 259
XVIII 203
XIX 117
XX 153
XXI 188
XXII 275
XXIII 354

The maximum is therefore reached at XII hours, while the minimum is
shown at XIX h. There is a secondary max. at I h., and a secondary min.
at VI h.

_Early Observations._—The nebula in Andromeda appears to have been the
one first discovered, for the distinguished Persian astronomer Al-Sûfi
(who died in 986 A.D.) was undoubtedly acquainted with it. The nebula
is figured upon a Dutch map of the stars nearly 400 years old. In 1612
Simon Marius redetected this object, and appropriately likened its
appearance to that of a “candle shining through a piece of horn.” In
1618 the nebula in Orion was certainly known, for Cysatus of Lucerne
compared it with the head of the fine comet visible in December of that
year. Huygens alighted upon the same object in 1656, and appears to
have been unconscious of its prior discovery. Only six “nebulæ or lucid
spots” were known in 1716, and enumerated by Halley in the ‘Phil.
Trans.’ vol. xxix. These included those of Andromeda and Orion. A third
was situated in the space between the bow and head of Sagittarius.
This is M. 22, and consists of a bright globular cluster of Stars.
The fourth was the fine star-group involving ω Centauri, which Halley
himself found in 1677. The fifth was another fine group in the right
foot of Antinous. This is M. 11, and was discovered by Kirch in 1681.
The sixth was the magnificent globular cluster (M. 13) in Hercules,
discovered by Halley in 1714.

In 1735 the Rev. W. Derham published a list of 16 of these objects, and
in 1761 Lacaille summarized 42 nebulæ and star-clusters which he had
observed in the southern sky. This was followed by Messier’s tables of
45 nebulæ &c. in 1771, and of 103 in 1781[58]. But these contributions,
important though they severally were, sunk into insignificance beside
the splendid results obtained by Sir W. Herschel, who during his
prolonged and systematic sweeps of the heavens picked up no less than
2500 new nebulæ and clusters which he formed into three catalogues
printed in the ‘Phil. Trans.’ as follows:—1786, 1000 objects, 1789,
1000 ditto, 1802, 500 ditto.

_Variable Nebulæ._—It is in the highest degree probable that changes
occur in the physical appearances of certain nebulæ, though the opinion
is not perhaps supported by a sufficient number of instances. Until Sir
W. Herschel began his review of the heavens very few nebulæ were known,
and the information possessed about them was very incomplete. The early
records, obtained with small and inferior telescopes, scarcely admit of
comparison with recent observations, for in matters of detail little
agreement will be found; and this proceeds certainly not so much from
real changes in the objects as from differences due to the variety
of instruments employed, to atmospheric vagaries, and to “personal
equation.” Bullialdus and Kirch in 1667 and 1676 and Le Gentil in 1759
supposed that remarkable changes were operating in the great elliptical
nebula of Andromeda. But G. P. Bond fully investigated the evidence,
and concluded that the variability of the object was by no means
proved. Some observers have represented the nucleus as stellar, while
others have drawn it as a gradual condensation, and Dr. Copeland has
shown that different magnifying powers alter the aspect of the nucleus,
“the lower powers making it more star-like, the higher ones more
soft-looking and extensive.”

Mairan and others entertained the view that the large irregular nebula
in Orion was subject to change. This object received much attention
from Sir W. Herschel, and he concluded that it underwent great
alteration between 1774 and 1811. D’Arrest, from his own researches
and a discussion of other results, expressed himself in 1872 that “the
observed changes in this vast mass of gas seem exclusively to turn out
to be temporary fluctuations of brightness.” Prof. Holden has arrived
at a similar conclusion, and says:—“The figure of the nebula has
remained the same from 1758 till now (if we except a change in its apex
about 1770, which seems quite possible); but in the brightness of its
parts undoubted variations have taken place, and such changes are still
going on”[59] (‘Monograph of the Nebula in Orion,’ p. 225).

Hind discovered a faint nebula, with a diameter of about 1′, on Oct.
11, 1852. It was situated in Taurus, the position being R.A. 4^h 15^m
33^s, Dec. +19° 15′·6 (for 1890), or about 2° W. of the star ε Tauri
(mag. 3·7). D’Arrest, on Oct. 3, 1861, searched for this object, but
found it had quite disappeared! A small round nebula was seen in 1868,
about 4′ preceding Hind’s, but this resisted some later attempts at
observation. In Oct. 1890, Burnham and Barnard, with the 36-inch
refractor of the Lick Observatory, saw two nebulæ here, one a very
small, condensed nebula, with a stellar nucleus, and the other an
exceedingly faint nebulosity about 45″ in diameter (see ‘Monthly
Notices,’ vol. li. pp. 94, 95).

The nebula surrounding the star ζ Argûs has been suspected of
variation, particularly by Abbott, of Hobart Town, Tasmania. Vols.
xxv., xxx., and xxxi. of the ‘Monthly Notices’ contain many references
to, and figures of, this interesting object. But the alleged changes
have not been substantiated, and there seems no reason to doubt that
they were purely imaginary.

The trifid nebula in Sagittarius (M. 20) is supposed by Prof. Holden to
have altered its position with reference to a triple star now situated
in the S. following part of the nebula. Sir J. Herschel, more than
half a century ago, had described this star as placed in the middle of
the vacuity by which the nebula is divided. Dreyer, however, points
out that the drawings of this object differ in many details, and that,
though changes of brightness may have taken place, it is difficult to
understand that the nebula should move so as to envelop the star in
about 1835, “after which no sensible change occurred again, so far as
published observations go.”

The nebula (M. 17) just N. of the bow of Sagittarius was also inferred
by Holden to have shifted its place relatively to the small stars
figured by Lassell in this object; but Dreyer adduces facts which
controvert this assumption. (See ‘Monthly Notices,’ vol. xlvii. pp.
412-420, where much valuable information will be found as to supposed
variable nebulæ.)

On Oct. 19, 1859, Tempel discovered a faint, large nebulosity attached
to the star Merope, one of the Pleiades, and at first mistook it for
a diffused comet. Its position is R.A. 3^h 39^m·6, Dec. +23° 26′
(1890). An impression soon gained ground that this object was variable;
for while Schmidt, Chacornac, Peters, and others saw it with small
instruments, it could not be discerned by D’Arrest and Schjellerup
with the large refractor at Copenhagen. Swift saw the nebula easily in
1874 with a 4½-inch refractor, and has observed it with the aperture
contracted to 2 inches. Backhouse re-observed it in 1882 with a
4-1/4-inch refractor. Yet in March 1881 Hough and Burnham sent a paper
to the Royal Astronomical Society with an endeavour to prove that the
nebula did not exist! They had frequently searched for it during the
preceding winter, but not a vestige of the object could be seen in the
18½-inch refractor at Chicago, and they regarded the supposed nebula
as due to the glow proceeding from Merope and neighbouring stars. But
photography has entirely refuted this negative evidence, and has
shown, not only Tempel’s nebula, but others involving the stars Maia,
Alcyone, and Electra belonging to this cluster. As to the alleged
variations in the Merope nebula, there is every reason to suppose these
were not real.

Proper motion has been suggested in regard to a very small, faint
nebula (N.G.C. 3236) a few degrees following α Leonis. But Dreyer has
disproved this by showing that there was no proper motion between 1865
and 1887, whence “it may be safely inferred that there has been none
since 1830, unless we are to believe, in this and similar cases, that
nebulæ in the good old days moved about as they liked, but have been on
their good behaviour since 1861.”

_Nebulous Stars._—This name was applied by Hipparchus and other ancient
observers to the clusters of stars which, to the naked eye, appear
as patches of nebulous light. Sir W. Herschel, in 1791, showed this
designation to be incorrect, and used it in connection with stars
actually involved in nebulosity. In sweeping the heavens he met with
several instances of this kind. Thus, 3° E.S.E. of ζ Persei he found
a star of the 9th mag. surrounded by a nebula 3′ in diameter. He
picked up another close to the star 63 Geminorum. This is a remarkable
object—a star of the 9th mag. surrounded by two dark and two bright
rings. On Feb. 3, 1864, Lord Rosse’s telescope showed an opening in
the outer bright ring, and the latter seemed connected with the inner
bright ring; so that the object presented the aspect of a spiral nebula
with a star in the centre. The diameter of the whole nebulosity is
45″. Key observed this object with an 18-inch reflector in 1868, and
described it as symmetrical—a central star, with intervening dark and
bright rings. He found a power of 510 the best, for, “like the annular
nebula in Lyra, it bears magnifying wonderfully well.” Since Herschel’s
time many nebulous stars have been discovered. There is one of about
6th mag. in R.A. 8^h 6^m·1, Dec.-12° 36′. The nebulosity round the
star fades away gradually, and its extreme diameter is 157″. There
is a 7th mag. star at R.A. 21^h 0^m 14^s, Dec. +67° 44′ involved in a
very large, faint nebulosity. This is a striking object, and I have
frequently picked it up while comet-seeking. The star has such a
foggy, veiled appearance that on first remarking it the observer thinks
his lenses are dewed, but on viewing neighbouring stars he sees them
sharp and clear on the dark sky, and the contrast is very pronounced.
The nebulous star is much isolated, though in a part of the sky where
small stars abound. This is one of Herschel’s discoveries and No. 7023
of the N. G. C.; Dreyer says he has seen the nebulosity particularly
distinct north and south of the star. In some cases a double star is
involved in nebulosity, and there are instances in which two double
stars are placed within an elliptical nebula.

_The Magellanic Clouds_[60].—These are marked as Nubecula Major and
Nubecula Minor on celestial globes and charts. They form two extensive
aggregations of nebulæ and star-clusters, and are readily visible to
the naked eye in or near Hydrus, and not far from the south pole of the
heavens. They may be likened to detached patches of the Milky Way. Sir
J. Herschel says the Nubecula Major is situated between the meridians
of 4^h 40^m and 6^h and the parallels of 66° and 72° of S. declination,
and extends over a space of some 42 square degrees. The Nubecula Minor
lies between 0^h 28^m and 1^h 15^m and 72° and 75° of S. declination,
and spreads over about 10 square degrees. The composition of these
objects is very complex and diversified, and affords very rich ground
for exploration with a large telescope. Nebulæ exist in profusion and
in every variety, and are intermingled with star-clusters varying in
condensation from the compact globular form to groups more loosely
scattered, and such as we often find in the Milky Way. Nearly three
hundred nebulæ and clusters are included in the major “cloud,” while
more than fifty others closely outlie its borders. In the minor about
forty such objects have been discovered. It is very strange to find
them collected together in this manner; for in other regions of the
firmament they are usually found separated, and certain classes appear
to have their own special zones or localities of distribution. Sir J.
Herschel pointed out that “globular clusters (except in one region of
small extent) and nebulæ of regular elliptic forms are comparatively
rare in the Milky Way, and are found congregated in the greatest
abundance in a part of the heavens most remote possible from that
circle, whereas in the Nubeculæ they are indiscriminately mixed with
the general starry ground and with irregular though small nebulæ.”

_Double Nebulæ._—Instances are not wanting of conspicuous double
nebulæ. M. 51 and 76, near ζ Ursæ and θ Andromedæ, may be classed in
this category. There is a very interesting, though a smaller object
just W. of α and β Geminorum, or in R.A. 7^h 18^m·6, Dec. +29° 43′.
Two bright, round nebulæ are separated by an interval of 28″. These
double nebulæ are usually round, and are sometimes resolvable into
stars. Whether they are physical or mere optical pairs has yet to be
ascertained. So many examples exist that it seems highly probable
they have a real connection, though no motion has yet been certainly
detected to prove they are binary systems. Such motion may, however,
be very slow, and require observations extending over a much longer
interval before it is revealed.

_Real Dimensions of Nebulæ and Clusters._—It may be readily imagined
that these objects are of immense size; for though placed at
distances of the utmost remoteness, they spread over perceptible and
comparatively large areas. Gore remarks that, on the assumption that
the globular cluster in Hercules (M. 13) is 5′ in diameter, and its
parallax one tenth of a second, its real diameter must be 3000 times
the Sun’s mean distance from the Earth, or nearly 280 billions of
miles! He further points out that, though this group contains as many
as 14,000 stars, according to Sir W. Herschel, yet each component may
be separated many millions of miles from the others, owing to the
vast dimensions of the group. Details like these are of course only
approximate, as the distance of a nebula or star-cluster has not yet
been definitely ascertained. The great nebulæ of Orion and Andromeda
must extend over prodigious regions in distance-space; but to quote
figures seems useless, in consequence of our inability to form just
conceptions of such immensity.

_Round Nebulæ and Clusters._—Resolvable nebulæ and clusters are
frequently circular in outline. The central condensation is an
indication of their globular form, though not always so, for many of
these objects become suddenly much brighter in the middle, and show an
apparently stellar nucleus. The material or stars forming the object
cannot therefore be equally distributed. Where it suddenly brightens
there is a great condensation, and in some cases several of these are
evident in the form of bright rings, intensifying as the nucleus is
approached. This irregular aggregation denotes the operation of “a
force of condensation directed from all parts towards the centre of
such systems.” In regard to planetary nebulæ, they cannot be globular
or they would exhibit a brightness increasing from the margin to the
centre. Their even luminosity throughout affords the evidence of a
special structure. Sir J. Herschel thought the planetary nebula (M. 97)
near β Ursæ Majoris must either be in the form of a hollow globe or a
flat circular disk lying perpendicular to the line of vision.

_Description of Nebulæ and Clusters of Stars._—The latter objects are
included in this chapter for several reasons. In a small telescope
nearly all such clusters exhibit the aspect of nebulæ, and they have
been catalogued with them, though, as already explained, some great
distinctions are to be drawn. To the naked eye the cluster Præsepe,
in Cancer, is usually visible as a patch of nebulosity, though on a
very clear, dark night stars may be glimpsed sparkling about the spot,
and a very small glass will suffice to show it as a nest of stars.
This object, and some others of a more difficult character (their
component stars being smaller and more compressed), are tabulated (I.)
at the end of this chapter. A summary (II.) of globular clusters is
also given, together with a list (III.) of nebulæ, a few of which are
resolvable into stars[61]. It must be understood that these selections,
though comprising many notable objects, are by no means exhaustive,
the intention being merely to indicate some typical examples of fine
nebulæ and clusters and of peculiarities of form or appearance, such
as planetary, annular, elliptical, and centrally condensed nebulæ
and loose, compressed, and globular clusters. Some of these objects
deserve individual references, as they present interesting details
to the telescopic observer and come within the reach of moderate
appliances.

_Great Nebula in Andromeda_ (M. 31). This object has often been
mistaken for a comet, for it is readily perceptible to the eye on a
moonless night. It is very large—4° by 2½°, according to Bond, with
a 15-inch refractor. He discovered a pair of dark streaks in the
brightest region of the nebula, and these may be well seen in a 10-inch
reflector. It is really triple; for about 25′ S. of the nucleus there
is a very bright, round, resolvable nebula, discovered by Le Gentil,
and a third, observed by Caroline Herschel, lies rather further to
the N.W. Photographs by Roberts show dark rings dividing the bright
interior parts of the nebula from the outer, and imparting to it a
decided spiral tendency. This nebula has hitherto resisted attempts
to resolve it into stars, though many hundreds have been seen in
the foreground. But its spectrum is continuous, so that its stellar
character is to be inferred.

_Great Nebula in Orion_ (M. 42). Visible to the naked eye just below
a line connecting β and ζ Orionis, and involving θ Orionis. It
exhibits an extremely complicated structure, and many of its irregular
branches and condensations may be discerned in small instruments. Sir
W. Herschel failed to resolve this object into stars with his 4-foot
reflector; but Lord Rosse, in 1844, thought he had effected it with his
6-foot mirror, though the conclusion was premature. The spectroscopic
researches of Huggins have shown this nebula to be composed of
incandescent gases, so that the stars telescopically observed in it are
probably in the foreground and entirely disconnected from the nebulous
mass. Effective photographs have been taken of it by Draper, Common,
and Roberts. It certainly forms one of the grandest objects in the
heavens.

The _Planetary_[62] _Nebula_ (M. 97). Discovered by Mechain in 1781.



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