William F. Denning.

Telescopic Work for Starlight Evenings online

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gives the separating power. Thus, 4·33 inches separates 1
″.” But a
good deal depends upon the character of the seeing and upon other
conditions. A large aperture will sometimes fail to reveal a difficult
and close _comes_ to a bright star when a smaller aperture will
succeed. This is due to the position of the bright diffraction-ring,
which in a large instrument may overlap the faint companion and obscure
it, while in a small one the ring falls outside and the small star is
visible[48]. Dawes concluded that “tests of separation of double stars
are not tests of excellence of figure,” and he gave much valuable
information with regard to micrometers and double-star observations
generally in the ‘Monthly Notices,’ vol. xxvii. pp. 217-238. This paper
will well repay attentive reading.

_Number of Stars._—In the northern hemisphere there are about 5000[49]
stars perceptible to the naked eye. This is less than an observer
would suppose from a casual glance at the firmament, but hasty ideas
are often inaccurate. The scintillation of the stars and the fact that
many small stars are momentarily glimpsed but cannot be held steadily
have a tendency to occasion an exaggerated estimate of their numbers.
Authorities differ as to the total of naked-eye stars. Sir R. S. Ball
says “the number of stars which can be seen with the unaided eye in
England may be estimated at about 3000.” Gore gives 4000. Backhouse
mentions 5600 as visible in the northern hemisphere. Argelander, who
has charted 324,188 stars between 2° S. of the equator and the N.
pole, gives the following numbers of stars up to the 9th magnitude:—

1st. 2nd. 3rd. 4th. 5th.
20 65 190 425 1100

6th. 7th. 8th. 9th.
3200 13,000 40,000 142,000

With every decrease in magnitude there is a great increase in numbers,
and if this is extended to still smaller magnitudes down to the 15th
or 16th we can readily understand that there exist vast multitudes
of fainter stars. Paul Henry, of the Paris Observatory, says there
are about 1,500,000 stars of the 11th mag., and Dr. Schönfield, of
Bonn, gives 3,250,000 as of the 11½ mag. It is probable that by means
of photography and the largest telescopes considerably more than 50
millions of stars may be charted, but this is a mere approximation.
Roberts has photographed 16,206 stars within an area of four square
degrees in a very rich region of the Galaxy near η Cygni, and Gore
computes that were the distribution equal to this over the whole
firmament the number of stars would reach 167 millions. He also
remarks that in the Paris photographs of the Pleiades, 2326 stars are
shown in a space covering about three square degrees, and this gives
for the entire sky a total of 33 millions. It is, however, manifest
that unusually plentiful spots in the heavens cannot be accepted as
affording a criterion of the whole.

_Magnitudes._—According to Argelander’s figures, above quoted, each
magnitude exhibits a rise of about 300 per cent. But authorities rarely
agree as to scale, as the following comparison between Sir J. Herschel
and Struve will show:—

H. S.
4·0 3·6
4·5 4·1
5·0 4·6
5·5 5·05
6·0 5·5
6·5 5·95
7·0 6·4
7·5 6·85
8·0 7·3
8·5 7·7
9·0 8·1
9·5 8·5
10·0 8·8
10·5 9·1
11·0 9·3
11·5 9·6
12·0 9·8
12·5 10·0
13·0 10·18
13·5 10·36
14·0 10·54
14·5 10·71
15·0 10·87
16·0 11·13
17·0 11·38
18·0 11·61
19·0 11·82
20·0 12·00

Argelander’s magnitudes come between those of Herschel and Struve.
Such disagreements are perplexing to observers, and it is fortunate
that in regard to the naked-eye stars we are now furnished with
a more consistent and accurate series of magnitudes. Photometric
determinations of the light of 4260 stars not fainter than the 6th
mag., and between the N. pole and 30° S. declination, were made at
Harvard College Observatory, and similar measures of 2784 stars
between the N. pole and 10° S. declination were effected at the Oxford
University Observatory, and the results published in 1885. The two
catalogues are in very satisfactory agreement, the accordances within
one tenth of a mag. being 31 per cent., within one quarter of a mag.
71 per cent., and within one third of a magnitude 95 per cent. The
photometers used in the two independent researches were constructed on
very different principles, and the substantial agreement in the results
indicates that “a great step has been accomplished towards an accurate
knowledge of the relative lustre of the stars” (‘Monthly Notices,’ vol.
xlvi. p. 277).

_The Milky Way._—On dark nights when the Moon is absent and the air
clear, a broad zone of glimmering, filmy material is seen to stretch
irregularly across the heavens. It may be likened to a milky river
running very unevenly amongst the constellations, and showing many
curves and branches along its course. On very favourable occasions the
unaided eye glimpses many hundreds of glittering points on this light
background. A field-glass reveals some thousands, and shows that it is
entirely composed of stars the blended and confused lustre of which
occasions that track of whiteness which is so evident to the eye. In a
good telescope stars and star-dust exist in countless profusion, and
great diversity is apparent in their numbers and manner of grouping. In
certain regions the stars are concentrated into swarms, and the sky is
aglow with them; while in others there are very few, and dark cavernous
openings offer a striking contrast to the silvery sheen of surrounding
stars. There are many of these void spaces in Scorpio, and a circular
one in Sagittarius R.A. 17^h 56^m, Dec.-27° 51′ has been particularly
remarked. These inequalities of grouping may be easily recognized with
the naked eye, especially in Cygnus, where bright star-lit regions
frequently alternate with dark void spaces. In the southern sky
there is a noteworthy instance. Near the brilliant stars of Crux and
Centaurus and closely surrounded by the Milky Way there is a large
black vacancy very obvious at a glance, and so striking to ordinary
observers that it is known as the “Coal-sack,” a name applied to it by
the early navigators of the southern seas.

The course of the Milky Way may be described generally as flowing
through Auriga, the club of Orion, feet of Gemini, western part of
Monoceros, Argo Navis, Crux, feet of Centaurus, Circinus, Ara, where
it separates into two branches, the western of which traverses the
northern part of the tail of Scorpio, eastern side of Serpens, Taurus
Poniatowski, Anser, and Cygnus. The eastern branch crosses the tail
of Scorpio, the bow of Sagittarius, Antinous, Aquila, Vulpecula, and
then enters Cygnus, where it reunites with the other branch. It thence
passes through Cepheus, Cassiopeia, Perseus, and enters Auriga. In
breadth it varies greatly, being in some places only 4° or 5°, whereas
in others it reaches 20°. It is, of course, best visible when twilight
is absent, but it is sometimes very plain, even at midsummer, for at
this season some of its more conspicuous sections are favourably placed
for observation. It is supposed that fully nine tenths of the total
number of stars in the firmament are included within the borders, of
the Milky Way.

Some of the ancient philosophers, including Democritus, formed just
conceptions as to the real nature of this appearance. Though they
lacked instruments wherewith to observe the stars forming it, they yet
saw them with the eye of reason. But very vague and incorrect notions
prevailed in early times, when superstition was rife, as to many
celestial phenomena. Some of the ancient poets and learned men refer
to the Galaxy as the path by which heroes ascended to heaven. Thus we
read in Ovid:—

“A way there is in heaven’s extended plain,
Which when the skies are clear is seen below,
And mortals, by the name of Milky, know;
The ground-work is of stars, through which the road
Lies open to great Jupiter’s abode.”

_Scintillation of the Stars._—The rapid variations of light known
as the “twinkling” of the stars received notice from many ancient
observers, including Aristotle, Ptolemy, and others, and they severally
endeavoured to account for it, but not in a manner altogether
satisfactory. At low altitudes bright stars exhibit this twinkling or
scintillation in a striking degree, but it is much less perceptible in
stars placed at considerable elevations. Sirius, the brightest star in
the sky, is a noted twinkler. His excessive lustre and invariably low
position are conditions eminently favourable to induce this effect.
But the planets seldom exhibit scintillation in a very marked degree.
The light of Jupiter and Saturn is steady, even when these planets
are close to the horizon. Mercury, however, twinkles most obviously,
and Venus and Mars, when low down, are often similarly affected,
especially in stormy weather when the air is much disturbed. Hooke,
in 1667, concluded that the scintillation was due “to irregular
refractions of the light of the stars by differently heated layers
of atmosphere.” M. Arago said it arose “from the peculiar properties
possessed by the constituent rays of light, of moving with different
velocities through the strata of the atmosphere, and of producing what
are called interferences.” More recently, M. Montigney has conducted
some interesting researches into this subject, and he believes “that
not only is twinkling caused, to a great extent, by the deviations
of portions of a star’s light altogether away from us by variable
layers of atmosphere, but it is also affected, both in frequency and
in the colours displayed, by the nature of the light emitted by the
individual star.” The planets are little subject to scintillation, as
they present disks of sensible size, and thus are enabled to neutralize
the effect of atmospheric interferences. It is curious, however, that
the steadiness of telescopic images does not appear to be much improved
at high altitudes, and that the phenomenon of scintillation still
operates powerfully as observed from mountainous stations. In February
1888, Dr. Pernter, of the Vienna Academy of Sciences, found “that the
scintillation of Sirius was actually greater at the top of Sonnblick,
10,000 feet high, than it was at the base of the mountain, and he
formed the opinion that scintillation has its origin in the _upper_
strata of the atmosphere and not in the lower as usually assumed.”
It would appear from this that lofty situations do not possess all
the advantages claimed for them in regard to the employment of large
telescopes.

_Star-Disks._—The stars as observed in telescopes are shorn of the
false rays apparent to the naked eye, and they are seen with small
spurious disks. That the disks are spurious is evident from the fact
that the larger the telescope employed, the smaller the star-disks
become. And moreover, when a star is occulted by the Moon, it
disappears instantaneously. There is no gradual diminution of lustre;
the star vanishes with great suddenness. Bright stars, like Aldebaran
or Regulus, have been watched up to the Moon’s limb, and observers
have been sometimes startled at the abruptness with which they were
blotted out. An appreciable disk could not be withdrawn in this
instantaneous manner; it would exhibit a perceptible decadence as the
Moon increasingly overlapped it, but no such appearance is observed.
On the occasion of the occultation of Jupiter on Aug. 7, 1889, the
planet’s diameter was 41″·4, and the disappearance occupied 85
seconds. Now had Aldebaran or Regulus a real disk of only 1″ it would
prevent their sudden extinctions, and their disappearances would be
spread over perceptible though short intervals of time[50]. But there
is every reason to conclude that the actual disks are to be represented
by a small fraction of 1″, so that the largest instrument and the
highest powers fail to reveal it. In this connection, Mr. Gore, in
his ‘Scenery of the Heavens,’ p. 152, says:—“Let us take the case of
α Centauri, which is, as far as is known at present, the nearest fixed
star to the Earth. The distance of this star is about 25 billions of
miles. From comparisons made between its light and the Moon, it has
been found that its intrinsic brilliancy must be about four times that
of the Sun. Supposing its greater lustre is due to its greater size—a
not improbable supposition—it would subtend, if placed at the Sun’s
distance, an angle twice as great, or about 1°, and hence we find that
the angle subtended at its distance of 25 billions of miles would be
about 1/76th of a second of arc, which the most powerful telescope yet
constructed would be incapable of showing as a visible disk.”

_Distance of the Stars._—The distances of the outer planets Uranus and
Neptune, mentioned in an earlier chapter of this work, are sufficiently
large to amaze us; but the distances of the stars may be said to be
relatively infinite. For many years the problem of stellar distances
defied all attempts to resolve it. At length, in 1838-39, Bessell,
Henderson, and Struve obtained results for three stars—viz. 61 Cygni,
α Centauri, and α Lyræ,—which practically settled the question. More
recent measures of stellar parallax, while correcting the earlier
values, have virtually corroborated them; though the figures adopted
can only be regarded as approximations, owing to the difficult and
delicate nature of the work. The binary star α Centauri appears to be
the nearest of all; it has a parallax of 0″·75, and its distance from
us is equal to 275,000 times the distance of the Sun. Light traversing
space at the rate of 187,000 miles per second would occupy 4-1/3 years
in crossing this interval. In the Northern hemisphere 61 Cygni is the
nearest star, with a parallax of 0″·44 and a distance of about 470,000
times the Sun’s distance. Light would take more than seven years in
reaching us from this star, α Lyræ has a parallax of 0″·15, equal to
nearly 22 light-years. α Crucis shows a very small parallax (0″·03),
and its distance is excessively remote—equal to about 108 light-years!

_Proper Motion of Stars._—A very slight motion affects the places
of many of the so-called fixed stars. This must, after the lapse of
long intervals of time, materially alter the configuration of the
constellations. But the change is a very gradual one, and must operate
through many centuries before its effects will become appreciable in
a general way. The greatest proper motion yet observed is that in
regard to two small stars (one in Ursa Major and the other in Piscis
Australis), which amounts to about 7″ annually. Another motion has
been recognized, viz. in the line of sight. Dr. Huggins made the
initiatory efforts in this research by measuring the displacement
of the F line in the spectrum of Sirius. The work has been actively
pursued at the observatories of Greenwich and Rugby, and with
interesting results. While certain stars exhibit a motion of approach,
others display a motion of recession. Thus Vega, Arcturus, and Pollux
are approaching us at the rate of about 40 miles per second; while
Rigel is receding at the rate of 17 miles per second, Castor at the
rate of 19, Regulus 14, Betelgeuse 25, and Aldebaran 31. Sirius, in
the years from 1875 to 1878, was receding from us at the rate of 22
miles per second; but this decreased in subsequent years, and in
1884-85 the star was approaching with a motion of about 22 miles per
second. In 1886 and 1887 this rate was increased to about 30 miles per
second, as observed both at Greenwich and Rugby. This confirms the idea
that Sirius is moving in an elliptical orbit. Similar observations,
in regard to the variable star Algol, have revealed that changes of
velocity are connected with its changes of lustre. Before minimum the
star recedes at the rate of 24½ miles per second, while after minimum
the star approaches with a speed of 28½ miles per second (‘Monthly
Notices,’ vol. 1. p. 241).

_Double Stars and Binary Systems._—Telescopic power will often reveal
two stars where but one is seen by the naked eye. Sometimes the
juxtaposition of such stars is merely accidental; though they are
placed nearly in the same line of sight the conjunction is an optical
one only, and no connection apparently subsists between them. In other
cases, however, pairs are found which have a physical relation, for one
is revolving round the other; and these are termed _binary_ stars. Sir
W. Herschel was the first to announce them, from definite observations,
in 1802. Of double stars more than 10,000 are now known; many of these
are telescopic, but the list includes some fine examples of naked-eye
stars.

[Illustration: Fig. 62.

Double Stars.

β Orionis. γ Leonis. α Ursæ Minoris. γ Virginis.
δ Cygni. γ Arietis. γ Andromedæ. δ Serpentis.
]

Double stars are excellent telescopic tests. A very close pair affords
a good criterion as to the defining capacity of an instrument; while
a pair more widely separated and of greatly unequal magnitude, like
that of α Lyræ, offers a test of the light-grasping power. But in these
delicate observations, as, indeed, in all others, the character of the
seeing exercises an important and variable influence. A double star
that is well shown on one night becomes utterly obliterated on another,
owing to the unsteadiness and flaring of the image. On such occasions
as the latter one is reminded of the “twitching, twirling, wrinkling,
and horrible moulding” of which Sir John Herschel complained, and
which unfortunately forms a too common experience of the astronomical
observer. A close double, of nearly equal magnitudes, requires a steady
night, such as is suitable for planetary details; but a wide double
consisting of a bright and a minute star rather needs a very clear sky
than the perfection of definition. Certain doubles, such as θ Aurigæ, δ
Cygni, and ζ Herculis, are often more easily seen in twilight than on a
dark sky; and some experienced observers, conscious of this advantage,
have secured excellent measures in daylight. Mr. Gledhill says:—“Such
stars as γ Leonis and γ Virginis are best measured before or very soon
after sunset” (‘Observatory,’ vol. iii. p. 54).

_List of Double Stars._

[Abbreviations in col. 9:—β., Burnham; T., Tarrant; S., Schiaparelli;
L., Leavenworth; E., Engelmann; P., Perrotin; Hσ., H. Struve; M., Maw.]

+— - +-—————————————-+————-—————————-+——————-+———————+——————+——————+—————+
| | |Position, 1890.| |Posit- | | | |
|No.| Name of Star. +——————-+——————-+ Mags. | ion- |Dis- |Epoch.|Obser-|
| | | R.A. | Dec. | | Angle |tance.| | ver.|
+— - +-—————————————-+————-—————————-+———————+——————-+——————+——————+—————+
| | | h m | ° ′ | | o | ″ | | |
| 1.|δ Equulei |21 9·1|+ 9 34 | 4½ 5 | 189·9 | 0·25 |1887·7| β. |
| | | Most rapid binary known. Period 11½ years |
| | | (Wrublewsky). Disc. 1852 by O. Struve. |
| 2.|Piazzi 109 | 1 51·0|+ 1 20 | 7 7 | 206·3 | 0·28 |1888·1| S. |
| | | An excessively close and difficult object. Binary.
| 3.|β Delphini |20 32·4|+14 13 | 3½ 5½| 310·1 | 0·29 |1888·6| β. |
| | | A rapid binary. Period 26 years (Doubjago). |
| | | Disc. 1873 by Burnham. |
| 4.|γ^2 Andromedæ | 1 57·1|+41 48 | 5 6 | 277·6 | 0·35 |1884·8| L. |
| | | Distance in Oct. 1889 less than 0″·1, and very |
| | | difficult with 36-inch (Burnham). |
| 5.|γ Coronæ Bor. |15 38·1|+26 39 | 4 7 | 126·6 | 0·38 |1887·5| S. |
| | | A close binary. Period 95½ years (Doberck). |
| | | Colours greenish-white and purple. |
| 6.|55 Tauri | 4 13·6|+16 16 | 6½ 8 | 76·4 | 0·43 |1887·6| S. |
| | | A binary. Difficult object with a 10-inch. |
| 7.|λ Cassiopeiæ | 0 25·7|+53 55 | 6½ 6½ | 146·9 | 0·45 |1887·3| T. |
| | | Another close binary. Distance of components |
| | | shows little change. |
| 8.|ζ Boötis |14 35·9|+14 12 | 4 4 | 293·4 | 0·51 |1887·5| S. |
| | | A binary pair, of equal mags. Period 127 |
| | | years (Doberck). |
| 9.|42 Comæ Bor. |13 4·7|+18 7 | 5½ 6 | 189·6 | 0·55 |1889·1| L. |
| | | A close binary, of short period; about 25¾ years.|
| | | Disc, in 1827 by O. Struve. |
|10.|λ Cygni |20 43·1|+36 8 | 5 7½ | 70·6 | 0·63 |1888·8| Hσ.|
| | | A binary. The distance between the components |
| | | is increasing. |
|11.|ζ Coronæ Bor. |15 18·7|+30 41 | 5½ 6 | 178·5 | 0·63 |1886·5| T. |
| | | A well-known binary, of short period; 41½ years |
| | | (Doberck). |
|12.|ω Leonis | 9 22·6|+ 9 32 | 5½ 7 | 96·8 | 0·70 |1889·1| L. |
| | | A close pair, but not difficult. Binary. Period |
| | | 114½ years (Doberck). |
|13.|15 Lyncis | 6 47·8|+58 34 | 5 6 | 5·9 | 0·77 |1890·3| M. |
| | | A probable binary, the position and distance |
| | | exhibiting a gradual increase. |
|14.|ι Orionis | 5 1·9|+ 8 21 | 5½ 7 | 193·9 | 0·99 |1889·0| L. |
| | | Triple. A low power shows many stars here. |
|15.|ζ Cancri, A.B. | 8 5·9|+18 0 | 5 6 | 40·3 | 1·05 |1889·2| L. |
| | | A triple star. A.C. Pos. 134°·4; Dist. 5″·36; |
| | | Mag. 7; 1878·3 (Hall). |
|16.|ν Scorpii, A.B.|16 5·6|-19 10 | 4 7 | 9·3 | 1·08 |1886·5| T. |
| | | A quadruple star, forming one of the finest |
| | | systems in the sky. |
|17.|π Cephei |23 4·4|+74 47 | 5 7½ | 32·5 | 1·16 |1888·7| Hσ.|
| | | Binary. Becoming more difficult with decrease of |
| | | distance. Yellow and purple. |
|18.|ε Arietis | 2 52·9|+20 54 | 5½ 6 | 202·2 | 1·28 |1889·7| L. |
| | | Distance increasing. Good dividing-test for a |
| | | 4-inch aperture (T.). |
|19.|λ Ophiuchi |16 25·4|+ 2 13 | 4½ 5½ | 42·6 | 1·55 |1888·4| L. |
| | | Binary, but period not yet ascertained with |
| | | accuracy. Yellow and bluish. |
|20.|ζ Herculis |16 37·1|+31 48 | 3 6½ | 65·8 | 1·68 |1890·7| M. |
| | | A fine, rather close binary. Period 34½ years |
| | | (Doberck). Single in 1865. Yellow and red. |
|21.|ξ Ursæ Maj. |11 12·3|+32 9 | 4 5 | 222·7 | 1·63 |1889·3| S. |
| | | One of the first-computed binaries. Period |
| | | 63 years (Breen). Excellent object. |
|22.|δ Cygni |19 41·5|+44 52 | 3 8 | 317·7 | 1·66 |1885·5| T. |
| | | A well-known binary. Period 376·7 years (Gore). |
| | | Test for 4½-inch. Pale yellow and sea-green. |
|23.|33 Orionis | 5 25·5|+ 3 12 | 5 6 | 32·8 | 1·81 |1887·1| T. |
| | | Just visible in a 3-inch. White and pale blue. |
|24.|θ Aurigæ, A.B. | 5 52·2|+37 12 | 3 8 | 2·5 | 1·98 |1885·1| T. |
| | | A similar pair to δ Cygni, though the distance |



Online LibraryWilliam F. DenningTelescopic Work for Starlight Evenings → online text (page 25 of 32)