_THE STARS._
Sidereal Work.—Greek alphabet.—Learning the Names of the
Stars.—The Constellation figures.—Means of Measurement.—Dividing
power.—Number of Stars.—Magnitudes.—The Milky Way.—Scintillation
of the Stars.—Star-Disks.—Distance of the Stars.—Proper Motion of
Stars.—Double Stars and Binary Systems.—Variable Stars.—New or
Temporary Stars.—Star Colours.—Groups of Stars.—Further Observations.
“Ten thousand suns appear
Of elder beam; which ask no leave to shine
Of our terrestrial star, nor borrow light
From the proud regent of our scanty day.”
BARBAULD.
The planetary observer has to accept such opportunities as are given
him; he must use his telescope at the particular seasons when his
objects are well presented. These are limited in number, and months may
pass without one of them coming under favourable review. In stellar
work no such irregularities can affect the progress of observations.
The student of sidereal astronomy has a vast field to explore, and a
diversity of objects of infinite extent. They are so various in their
lustre, in their grouping, and in their colours, that the observer’s
interest is actively retained in his work, and we often find him
pursuing it with unflagging diligence through many years. No doubt
there would be many others employing their energies in this rich field
of labour but for the uninteresting character of star-disks, which
are mere points of light, and therefore incapable of displaying any
detail. Those who study the Sun, Moon, or planets have a large amount
of surface-configuration to examine and delineate, and this is ever
undergoing real or apparent changes. But this is wholly wanting in
the telescopic images of stars, which exhibit a sameness and lack
of detail that is not satisfying to the tastes of every observer.
True there are some beautiful contrasts of colour and many striking
differences of magnitude in double stars; there are also the varying
position and distance of binary systems, the curious and mysterious
fluctuations in variable stars, and some other peculiarities of stellar
phenomena which must, and ever will, attract all the attention that
such important and pleasing features deserve. And these, it must be
conceded, form adequate compensation for any other shortcomings. The
observer who is led to study the stars by comparisons of colour and
magnitude or measures of position, will not only find ample materials
for a life-long research, but will meet with many objects affording him
special entertainment. And his work, if rightly directed and accurately
performed, will certainly add something to our knowledge of a branch in
which he will certainly find much delectation.
_Greek Alphabet._—The amateur must, at the outset of his career,
thoroughly master the Greek alphabet. This will prevent many
time-wasting references afterwards, and avoid the doubt and confusion
that must otherwise result. The naked-eye stars in each constellation
have Greek letters affixed to them on our celestial globes and
star-maps.
α Alpha
β Beta
γ Gamma
δ Delta
ε Epsīlon
ζ Zēta
η Eta
θ Theta
ι Iota
κ Kappa
λ Lambda
μ Mu
ν Nu
ξ Xi
ο Omīcron
π Pi
ρ Rho
σ Sigma
τ Tau
υ Upsīlon
φ Phi
χ Chi
ψ Psi
ω Omĕga.
The letters are applied progressively to the stars (generally according
to brightness) in each constellation. The 1st-mag. stars frequently
have a duplicate name. Thus α Leonis is also known as Regulus, and α
Canis Majoris as Sirius, the Dog-star.
_Learning the Names of the Stars._—A knowledge of the stars as they
are presented in the nocturnal sky may be regarded as the entrance to
the more advanced and difficult branches of the science, and forms
the young observer’s introductory lesson. When he has learnt a few of
the principal constellations, and can point them out to his friends,
he already begins to feel more at home with the subject, and regards
it with a different eye to what he did before when the names and
configurations of the stars were alike unknown to him. He no longer
views the heavens as a mysterious assemblage of confusing objects, for
here and there he espies certain well-known groups always preserving
the same relative positions to each other. The unconscious gaze he
formerly directed to the sky has given way to the intelligent look of
recognition with which he now surveys the firmament.
An acquaintance with the leading constellations, and with the names
or the letters of the brighter stars in each, becomes very important
in some departments of observation, and various methods have been
suggested as likely to impress the positions and names on the memory.
The beginner must first be content to get familiar with a few of the
brighter stars, and make these the base for extending his knowledge.
The objects are so numerous that it is impossible his primary attempts
can be anything like complete. He must advance step by step in
his survey, and feel his way cautiously, setting out from certain
conspicuous stars with which he has already become conversant. A
lantern and a series of star-maps are the only aids required, and
with these he ought to make satisfactory progress. The stars as they
are seen in the sky may be compared with those figured in the maps,
and their names and the constellations in which they lie may then be
identified. It is an excellent plan as conducing to fix the positions
indelibly in the memory to construct maps from personal observation,
and to compare these afterwards with the published maps for
identification of the constituent stars. This plan, if repeated several
times, has the effect of impressing the positions of the leading stars
forcibly upon the observer’s mind.
It is not intended to give, in this place, any details as to the
places or distribution of the stars. Without diagrams, such a
description could not be made readily intelligible. To those, however,
who are commencing their studies, I would recommend the northern sky as
the most suitable region to aid their initiatory efforts. For
“He who would scan the figured sky
Its brightest gems to tell,
Must first direct his mind’s eye north
And learn the Bear’s stars well.”
The seven bright stars of Ursa Major are familiar to nearly everyone.
Two of them, called the Pointers, serve to direct the eye to the Polar
star, which, though not a brilliant one, stands out prominently in a
region comparatively bare of large stars. It is important to know the
Polar star, as it is situated near the centre of the apparent motion
of the firmament. When the student has assured himself as to the
northern stars he will turn his attention southwards, and recognize the
beautiful Orion and the curious groups in Taurus. He will also observe,
much further east, the well-known sickle of Leo, and in time become
acquainted with the many other constellations that make the winter sky
so attractive.
[Illustration: Fig. 60.
The constellation Orion.]
_The Constellation Figures._—The observer will soon realize that the
creatures after which the constellations have been named bear no
resemblance to the configuration of the stars they represent. If we
look for a Bear amongst the stars of Ursa, for a Bull amid the stars of
Taurus, or for a flying Swan in the stars of Cygnus we shall utterly
fail to find it. The names appear to have been originally given, not
because of individual likenesses between them and the star-groups
to which they are applied, but simply on account of the necessity
of dividing the sky into parts, and giving each a distinguishing
appellation, so that it might be conveniently referred to. There were
pressing needs for a system of stellar nomenclature, and the plan of
grouping the stars into imaginary figures was the one adopted to avoid
the confusion of looking upon the sky as a whole. There are some who
object to the method of the Chaldean shepherds because the series
of grotesque figures on our star-maps and globes bear no natural
analogies. But it would be unwise to attempt an innovation in what has
been handed down to us from the myths of a remote antiquity, for
“Time doth consecrate,
And what is grey with age, becomes religion.”
[Illustration: Fig. 61.
Diagram illustrating the Measurement of Angles of Position.
(In measuring angles of position the larger star is always understood
as central in the field. The north point is zero, and the angles are
reckoned from this point towards the east. If a star has a faint
component lying exactly east or following it, then the angle is 90°;
if the smaller star is south, the angle is 180°; and so on.)]
_Means of Measurement._—A micrometer becomes an indispensable
instrument to those who make sidereal observations of an exact
character. Without such means it will be impossible to determine
either positions or distances except by mere estimation, and this
is not sufficiently precise for double-star work. With a reliable
micrometer[47] excellent results may be obtained, especially with
regard to the varying angles of binary systems. Frequent remeasurement
of these is desirable for comparison with the predicted places in
cases where the orbits have been computed. In this department of
astronomy the names of Herschel, South, Struve, Dawes, Dembowski,
Burnham, and others are honourably associated, and it is notable that
refracting-telescopes have accomplished nearly the whole of the work.
But reflectors are little less capable, though their powers seem to
have been but rarely employed in this field. Mr. Tarrant has lately
secured a large number of accurate measures with a 10-inch reflector
by Calver, and if care is taken to secure correct adjustment of the
mirrors, there is no reason why this form of instrument should not
be nearly as effective as its rival. Mr. Tarrant advises those who
use reflectors in observing double stars “to test the centering of
the flat at intervals during the observations, as the slightest shift
of the large mirror in its cell will frequently occasion a spurious
image which, if it by chance happens to fall where the companion is
expected to be seen, will often lead to the conclusion that it has
been observed. In addition to this, any wings or the slightest flare
around a bright star will generally completely obliterate every trace
of the companion, especially if close and of small magnitude, and such
defects will in nine cases out of ten be found to be due to defective
adjustment. Undoubtedly for very close unequal pairs the refractor
possesses great advantages over a reflector of equal aperture; in the
case of close double stars the components of which are nearly equal
there appears to be little, if any, difference between the two classes
of instruments; while for any detail connected with the colour of stars
the reflector certainly comes to the fore from its being perfectly
achromatic.” These remarks from a practical man will go far to negative
the disparaging statements sometimes made with regard to reflectors and
stellar work, and ought to encourage other amateurs possessing these
instruments to take up this branch in a systematic way.
_Dividing Power._—This mainly depends upon the aperture, and it was
made the subject of careful investigation and experiment by Dawes,
who found that the diameters of the star-disks varied inversely as
the aperture of the telescope. Adopting an inch as the standard, he
ascertained that this aperture divided stars of the sixth magnitude
4″·56 apart, and on this base he constructed the following table:—
Aperture Least
in inches. separable
distance.
″
1·0 4·56
1·6 2·85
2·0 2·28
2·25 2·03
2·5 1·82
2·75 1·66
3·0 1·52
3·5 1·30
3·8 1·20
4·0 1·14
4·5 1·01
5·0 0·91
5·5 0·83
6·0 0·76
6·5 0·70
7·0 0·65
7·5 0·61
8·0 0·57
8·5 0·536
9·0 0.507
9·5 0.480
10·0 0·456
12.0 0·380
15·0 0·304
20·0 0·228
25·0 0·182
30·0 0·152
Dallmeyer, the optician, confirmed these values by remarking:—“In all
the calculations I have made, I find that 4·33 divided by the aperture
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 |
| | | is wider. |
|25.|70 Ophiuchi |18 0·0|+ 2 32 | 4 6 | 348·7 | 2·16 |1889·3| β. |
| | | Binary. Period nearly 88 years (Gore). Good |
| | | object for a 3-inch. Yellow and purple. |
|26.|ι Leonis |11 18·2|+11 8 | 4½ 7½ | 62·0 | 2·56 |1889·2| L. |
| | | Binary; but distance shows little variation since|
| | | 1839. Yellowish and blue. |
|27.|ε Boötis |14 40·2|+27 32 | 3 5½ | 328·1 | 2·88 |1885·4| T. |
| | | A very interesting object, and visible in a small|
| | | instrument. |
|28.|α Scorpii |16 22·7|-26 11 | 1 8 | 271·7 | 2·92 |1880·0| β. |
| | | This pair forms an atmospheric rather than an |
| | | optical test. |
|29.|γ Ceti | 2 37·6|+ 2 46 | 3 7 | 289·7 | 2·94 |1883·9| P. |
| | | A binary system. Test for a 2½-inch. |
| | | Yellow and blue. |
|30.|α Piscium | 1 56·3|+ 2 14 | 5 6 | 321·9 | 3·03 |1886·9| T. |
| | | A probable binary, but since 1831 not much change|
| | | in position or distance. |
|31.|ζ Aquarii |22 23·1|- 0 35 | 4 4 | 325·8 | 3·08 |1889·9| L. |
| | | A fine binary, with very long period. 1625 years |
| | | (Doberck). |
|32.|ε^1 Lyræ |18 40·7|+39 34 | 4½ 6½ | 15·3 | 3·24 |1877·4|Dob- |
| | | | | | | | |erck.|
|33.|ε^2 Lyræ |18 40·7|+39 30 | 5 5 | 137·6 | 2·45 |1877·4| Hall|
| | | These stars form a wide double (distance 3′ 27″),|
| | | just separable by the naked eye. A 2½-inch |
| | | shows a fine double-double. A 4-inch reveals |
| | | three faint stare between. |
|34.|ε Hydræ | 8 41·0|+ 6 49 | 4 7 | 226·5 | 3·16 |1889·1| β. |
| | | A new _comes_, Pos. 154°·4; Dist. 0″·26; Mag. 6, |
| | | 1889; 36-inch, power 3300! β. |
|35.|γ Leonis, A.B. |10 13·9|+20 24 | 2 4 | 114·6 | 3·51 |1889·3| β. |
| | | A fine binary. Period 407 years (Doberck). |
| | | Readily seen in a 3-inch. |
|36.|δ Serpentis |15 29·6|+10 55 | 3 5 | 189·9 | 3·52 |1886·6|Ball.|
| | | Probably binary. Fine object in small instruments.|
|37.|α Canis Maj. | 6 40·3|-16 34 | 1 10 | 359·7 | 4·19 |1890·3| β. |
| | | Brilliant binary. Period 58·5 years (Gore). |
| | | Colours white and yellow. |
|38.|α Herculis |17 9·6|+14 31 | 3 4½ | 114·5 | 4·58 |1885·5| T. |
| | | A splendid object. Orange and bluish green. |
|39.|ζ Cassiopeiæ | 0 42·4|+57 14 | 4 8 | 184·7 | 4·76 |1888·3| M. |
| | | Binary. Period 195 years (Gruber). Difficult |
| | | object for 2-1/4-inch (Johnson). |
|40.|γ Virginis |12 36·1|- 0 51 | 3 3 | 153·9 | 5·45 |1889·3| L. |
| | | Well-known binary. Period 182 years |
| | | (J. Herschel). Single in 1836. |
|41.|α Geminorum | 7 27·6|+32 8 | 2 3 | 229·4 | 5·68 |1889·2| L. |
| | | Very fine object. Binary; Period doubtful |
| | | (Mädler 232 years, Doberck 1001 years). |
|42.|π Boötis |14 35·6|+16 54 | 4 6 | 104·3 | 6·04 |1885·4| T. |
| | | This pair has exhibited little change in pos. or |
| | | dist. since 1781. |
|43.|α^2 Capricorni,|20 11·9|-12 53 | 3 15 | 149·7 | 6·30 |1879·7| β. |
| | A.B. | Good light-test for 6-inches. Companion double; |
| | | pos. 240°, dist. 1″·5. |
|44.|δ Geminorum | 7 13·5|+22 11 | 3½ 9 | 207·2 | 6·98 |1886·1| T. |
| | | Rather wide pair of unequal mags. Difficult with |
| | | small apertures. |
|45.|γ Arietis | 1 47·5|+18 45 | 4½ 5 | 178·3 | 8·78 |1886·9| T. |
| | | A fine, easy object. Discovered in 1664 by Hooke.|
|46.|ι Ursæ Maj. | 8 51·7|+48 28 | 3 12 | 356·7 | 9·56 |1883·4| E. |
| | | Well seen in a 4-inch, powers 80 and 130. |
| | | Good light-test. |
|47.|β Orionis | 5 9·3|- 8 20 | 1 9 | 202·0 | 9·61 |1887·2| T. |
| | | A fine object for small instruments. Visible in a|
| | | 2-inch refractor. |
|48.|γ^1 Andromedæ | 1 57·1|+41 48 | 3 6 | 62·6 |10·50 |1876·0|Hall.|
| | | A splendid pair, stationary in relative positions|
| | | (see no. 4). |
|49.|γ Delphini |20 41·6|+15 44 | 4 6 | 271·2 | 11·35|1879·7|Hall.|
| | | Estimates of the colour of this pair differ, and |
| | | change is inferred. |
|50.|σ Orionis, A.D.| 5 33·2|- 2 40 | 4 10½| 236·8 | 11·62|1875·2| |
| | | Multiple. Fine group here. Schröter saw 12 stars,|
| | | Struve 18. |
|51.|β Scorpii |15 59·0|-19 30 | 2 5½| 26·7 | 12·72|1879·7| β. |
| | | The brighter star is a close double; Pos. 87°, |
| | | Dist. 0″·73 (Burnham). |
|52.|ζ Ursæ Maj. |13 19·5| +55 30| 2 4 | 150·5 | 14·38|1886·2| T. |
| | | Fine object for small instruments. Other stars in|
| | | the field. |
|53.|α Centauri |14 32·1| -60 23| 1 2 | 202·9 | 17·12|1888·6| S. |
| | | A fine southern binary with Period of 80·3 years |
| | | (Elkin). |
|54.|α Ursæ Min. | 1 18·5| +88 43| 2 9 | 210·1 | 18·60| | |
| | | Good test for a 2-inch. Dawes saw it with |
| | | 1-3/10-inch, Ward with 1-1/4 inch. |
|55.|61 Cygni |21 2·0| +38 12| 5 6 | 121·0 | 20·58|1887·7| S. |
| | | Probably a binary of long period (782½ years, |
| | | Peters; 1159 years, Mann). |
|56.|33 Arietis | 2 34·3| +26 35| 5 8 | 0·3 | 29·76|1879·7| β. |
| | | A distant and easy pair in small instruments. |
|57.|β Cygni |19 26·3| +27 44| 3 7 | 55·1 | 34·32|1879·7| β. |
| | | A beautiful pair, colours golden yellow and |
| | | smalt blue. |
|58.|β Geminorum | 7 38·6| +28 18| 2 14 | 274·9 | 43·00|1877·9| β. |
| | | Disc. by Burnham, who also finds the companion |
| | | double; dist. 1″·4 (1879·2). |
|59.|α′ Capricorni |20 11·9| -12 53| | 219·7 | 44·55|1879·7| β. |
| | | α^1 and α^2 Capricorni (No. 43) form a naked-eye |
| | | double; Pos. 291°, Dist. 373″·4. |
|60.|α Canis Min. | 7 33·6| + 5 30| 1 14 | 317·3 | 44·62|1877·9| β. |
| | | Difficult object; just seen steadily by Dawes |
| | | with 8-1/4-inch refractor. |
|61.|β Lyræ, A.B. |18 46·0| +33 14| 3 7 | 148·9 | 45·20|1886·9| T. |
| | | There are three other faint and distant |
| | | components. | |
|62.|α Lyræ |18 33·2| +38 41| 1 11 | 156·1 | 48·00|1879·7| β. |
| | | Good light-test for a 3-inch. There are other |
| | | more distant companions. |
|63.|α Cassiopeiæ | 0 34·3| +55 56| 2 13½| 280·2 | 61·33|1879·7| β. |
| | | The 36-inch refractor shows a very faint _comes_;|
| | | Dist. 17″·5 (Burnham). |
|64.|α Canis Maj., | 6 40·3| -16 34| 1 13 | 114·9 | 71·39|1877·5|Hall.|
| | A.C. | This faint and distant companion to Sirius was |
| | | disc. by Marth. |
|65.|α Andromedæ | 0 2·7| +28 29| 2 11 | 271·6 | 71·60|1878·6| G. |
| | | A wide double, visible in a 3-inch, but _comes_ |
| | | very faint. |
|66.|α Tauri | 4 29·6| +16 17| 1 12 | 34·1 |114·96|1879·7| β. |
| | | Good light-test for a 3-inch. Very faint _comes_ |
| | | Pos. 109°; Dis. 30″·4 (Burnham). |
+——-+——————————————-+——————-+——————-+——————-+——————-+——————+——————+—————+
The determination of the angles of position and distance of double
stars forms a very important and extensive branch of work in connection
with sidereal astronomy. In cases where double stars preserve
stationary places relatively to each other, there is clearly no need
for frequent re-observation. But in those numerous instances where the
two components form a binary system it is desirable to obtain as many
measures as possible, so as either to verify the calculated orbit or
to furnish the materials for an orbit if one has not been computed
before. Dr. Doberck, whose name is well known in these researches,
mentioned, in 1882, that ample data for purposes of computation had
not been secured for more than thirty or forty binaries out of between
five and six hundred such systems that were probably known to exist.
Sir W. Herschel, in 1803, estimated the period of revolution of α
Geminorum as 342 yrs. 2 mths. and of γ Virginis as 1200 yrs. Orbits[51]
do not appear, however, to have been computed until 1827, when Savery
of Paris showed that the companion of ξ Ursæ Majoris was revolving in
an ellipse with a period of 58-1/4 years. The accomplished Encke also
turned his attention to this work, and adopted a more elaborate method;
and many others have pursued the subject with very interesting and
valuable results. On pp. 302-305 is a selected list of some of the most
noteworthy double and binary stars, arranged according to the distance
between the components.
In compiling the above list, I have used some of the latest measures
available, as most of these doubles are binary systems, and therefore
in motion, so that a few years effect a perceptible difference in the
angles of position and distance of the components. Some of the pairs
are closing up, others are opening, and thus it happens that a binary
star, divided with great difficulty to-day, may become an easy object
some years hence, and _vice versâ_. In fact, as telescopic tests they
are constantly varying.
Before leaving this part of the subject it may be interesting to refer
individually to a few brilliant examples of double stars.
α _Canis Majoris_ (_Sirius_). A red star according to ancient records,
but it is now intensely white. In 1844 Bessel inferred from certain
little irregularities in the proper motion of this star that it
consisted of a binary system with a period of about half a century[52].
Peters confirmed this idea in 1851, and it was observationally verified
eleven years afterwards. On Jan. 31, 1862, Alvan Clark, jun., while
testing a new 18½-inch refractor, discovered a very faint companion
10″ distant. Measures in the few subsequent years proved that the
position-angle was decreasing, while the distance showed a slight
extension. In 1872 it was about 11″·50, but since then the two stars
have been approaching each other, and Mr. Burnham’s measures in April
1890 gave the distance as only 4″·19. It is now, therefore, a very
difficult object, and only visible in large instruments. In England it
is never easy, owing to its southern position, and it has been little
observed, but it is satisfactory to note that the large refractors at
Washington, Princeton, and Chicago, U.S.A., have been often employed
on this object in recent years. Mann gives a period of 51·22 years for
this interesting binary, and places the time of periastron-passage as
1890·55. This differs from Gore’s orbit, quoted in the table.
β _Orionis_ (_Rigel_). A favourite test-object for small instruments.
The companion has been seen with only 1½-inch aperture by experienced
observers familiar with the object, and accustomed to its appearance
in larger telescopes. The beginner may, however, esteem himself
fortunate if he distinguishes the smaller star with 3 inches of
aperture. When he has done this he may afterwards succeed with 2½
inches only, and quite possibly with 2 inches. He can ascertain his
ability in this direction by inserting cardboard diaphragms of the
diameters referred to in the dew-cap of his telescope. This object
is not a binary; the component stars are fixed relatively to each
other, and merely form an optical double. The colours are pale yellow
and sapphire blue. Burnham thought the smaller star was elongated,
as though a very close double, but the 36-inch at Mount Hamilton has
disproved the idea.
α _Lyræ_ (_Vega_). Another well-known object, and one upon which
amateurs are constantly testing their means. The companion star is
extremely faint, and small instruments would have no chance with it
but for its comparatively wide distance from Vega. Were it much nearer
it would be obliterated in the glare. This is a more difficult pair
than that of Rigel, though certain lynx-eyed observers have glimpsed
the minute star with ridiculously small apertures. It is no mean feat,
however, to discern the star with a 3-inch telescope. Webb saw it
more easily with a power of 80 than with 144 on a 3-7/10-inch. There
are many other stars in the same field, though more distant than the
companion alluded to. With power 60 on my 10-inch reflector, I counted
eighteen stars in the field with Vega on Oct. 9, 1889, though the full
Moon was shining at the time. Several faint stars have been alleged to
exist much closer to Vega than the well-known _comes_; but these have
resisted the great American refractors, and it may be safely assumed
that they were ghosts produced by a faulty image.
α _Ursæ Minoris_ (_Polaris_). This double, from its constant visibility
in northern latitudes, from its unvarying brightness, and from the
relatively stationary positions of the stars composing it, forms
an excellent test for small instruments. But it is a comparatively
easy object, and ought to be seen in a 2-inch telescope. With this
aperture the primitive efforts of a young observer will probably be
disappointing. If possible he should first look at the pair through a
3-or 4-inch, and then he will know exactly what he may expect to see
with inferior means. A difficult object is often readily glimpsed in a
small telescope after the eye has become acquainted with it in a larger
one. Experience of this kind is very requisite, and it is by thus
educating the eye that observers are gradually enabled to reach objects
which appeared hopelessly beyond them at their first attempts. The
companion to Polaris, like that of Rigel and Vega, though situated in
nearly the same line of sight is not physically related to the larger
star, the contiguity of the objects being accidental. Some dubious
observations have been made of _comites_ nearer to Polaris than the one
to which we have been adverting; but Burnham does not see these, and we
are forced to conclude that they have no objective existence.
α _Scorpii_ (_Antares_). A fiery-red star, with a rather close, faint
companion. This object being in 26° of S. declination is rarely
seen with advantage in places with latitudes far north. Atmospheric
disturbance usually affects the image in such degree that the smaller
star is merged in the contortions of the larger. This pair is, however,
interesting from the circumstance that it is frequently liable to
occultation by the Moon. A night on which this double star can be
distinctly seen may be regarded as an exceptional one in point of
definition. It appears to have been discovered nearly half a century
ago by Grant and Mitchel.
_Variable Stars._—A proportion of the stars exhibit fluctuations in
their visible brightness. In most cases, however, the variation is
but slight, though there are instances in which the differences are
considerable. The fluctuations are periodical in nature and capable
of being exactly determined. But the character of the variation and
the period are very dissimilar in different stars. Some are of short
period, and their variations occupy a few days only; others, however,
are more gradual, and twelve months or more may represent the complete
cycle of their changes. These alterations of brightness generally
escape the notice of casual observers of the heavens. To them the
stars appear as constant in their relative magnitudes as they are in
their relative positions. But a close observer of the firmament, who
habitually watches and records the comparative lustre of the stars,
must soon discover numerous evidences of change. He will remark certain
stars which alternately grow bright and faint, and, in fact, display
a regular oscillation of brilliancy. In the case of a pair of stars
he may possibly notice that the superior lustre is emitted first by
one and then by the other. The observation of these variables dates
from a period anterior to the invention of the telescope. Nearly three
centuries ago Fabricius remarked that ο Ceti (Mira) suffered
a great diminution of light; for while it was of the 3rd mag. in Aug.
1596, it became invisible in the following autumn. It was re-observed
by Holwarda in 1639, and as he appears to have been the first to
estimate its period, some authors, including Argelander, have credited
him with the discovery. The star has a period of about 331·3 days. Its
variations are somewhat erratic, for at max. it is sometimes only 4th
mag., while at others it is as bright as 2nd mag., and its min. are
equally inconsistent.
β Persei (Algol) is another and perhaps the best known of all the
variable stars. Its changes are very rapid, for it passes through its
various gradations of brilliancy in less than three days. It was first
noticed by Montanari in 1669, though it was left for Goodricke in
1782 to ascertain its period. The normal mag. of the star is 2·2, and
it only shows distinct variation during the five hours which precede
and follow a minimum, when it declines to 3·7 mag. Its period is
shortening, for in 1782 it was 2^d 20^h 48^m 59^s·4, in 1842, 2^d 40^h
48^m 55^s·2, and at present Chandler finds it 2^d 20^h 48^m 51^s. As
to the causes which contribute to these variations, they are invested
in mystery. It has been conjectured that dark spots on the surfaces
of the stars may, by the effects of rotation, introduce the observed
alternations. Another surmise is that the temporary diminutions of
lustre are to be ascribed to the interposition of dark satellites, and
this theory seems tenable in regard to stars of the Algol type. It is
satisfactory to note that a large amount of systematic work is being
done in this important and delicate branch of research. Such stars as
are subject to variation have been classed as follows:—1. Temporary or
new stars; 2. Stars having long and pretty regular variation; 3. Stars
irregularly variable; 4. Stars varying in short periods; 5. Stars of
the type of Algol, which are liable to temporary diminutions of lustre.
On the preceding page is a list of the most noteworthy variable stars.
_List of Variable Stars._
+——————————————+——————————————————-+——————————————+——————————————————-+
| Name of Star.| Position, 1890. | Mags. | Period. |
| +————————-+————————-+ | |
| | R.A. | Dec. | | |
+——————————————+——————————————————-+——————————————+——————————————————-+
| | h m | ° ′ | | |
| μ Cephei | 0 52.5 | +81 17 | 7.2 9.4 | 2^d 11^h 50^m |
| ο Ceti | 2 13.8 | - 3 29 | 2 0 | 331-1/3 days |
| β Persei | 3 1.0 | +40 32 | 2.2 3.7 | 2^d 20^h 49^m |
| λ Tauri | 3 54·6 | +12 11 | 3.4 4.2 | 3^d 22^h 52^m |
| U Orionis | 5 49·3 | +20 9 | 6 12½ | |
| ζ Geminorum | 6 8.2 | +22 32 | 3.2 4·2 | 135-151 days |
| ζ Geminorum | 6 57.6 | +20 44 | 3.7 4·5 | 10^d 3^h 43^m |
| L_{2} Puppis | 7 10·2 | -44 28 | 3.5 6.3 | 136 days |
| R Canis Maj. | 7 14.5 | -16 11 | 6.2 6.8 | 1^d 3^h 16^m |
| U Geminorum | 7 48.6 | +22 18 | 9 14 | 71-126 days |
| S Cancri | 8 37.7 | +19 26 | 8.2 11·7 | 9^d 11^h 38^m |
| ζ Argûs | 10 40.8 | -59 6 | 1 6 | 46 or 67 yrs.? |
| R Hydræ | 13 23.7 | -22 43 | 4 10 | 436 days |
| δ Libræ | 14 55.1 | -8 5 | 4.9 6·1 | 2^d 7^h 51^m |
| U Coronæ | 15 13.7 | +32 3 | 7.6 8·8 | 3^d 10^h 51^m |
| α Herculis | 17 9.6 | +14 31 | 3.1 3.9 | 88^d 12^h (irreg.)|
| U Ophiuchi | 17 11.0 | + 1 20 | 6 6.7 | 0^d 20^h 8^m |
| β Lyræ | 18 46.0 | +33 14 | 3.5 4.5 | 12^d 21^h 47^m |
| χ Cygni | 19 46·3 | +32 38 | 4-6.5 13 | 406 days |
| ζ Aquilæ | 19 46.9 | + 0 44 | 3.6 4.7 | 7^d 4^h 14^m |
| Y Cygni | 20 47.7 | +34 15 | 7.1 7.9 | 1^d 11^h 57^m |
| μ Cephei | 21 40.1 | +58 16 | 3.6 4·8 | 432 days? |
| δ Cephei | 22 25.1 | +57 51 | 3.7 4.8 | 5^d 8^h 48^m |
+——————————————+——————————————————-+——————————————+——————————————————-+
+——————————————+——————————————————-+
| Name of Star.| Observer. |
| | |
+——————————————+——————————————————-+
| | |
| μ Cephei | Ceraski, 1880. |
| ο Ceti | Fabricius, 1596. |
| β Persei | Montanari, 1669. |
| λ Tauri | Baxendell, 1848. |
| U Orionis | Gore, 1885. |
| ζ Geminorum | Schmidt, 1865. |
| ζ Geminorum | Schmidt, 1847. |
| L_{2} Puppis | Gould, 1872. |
| R Canis Maj. | Sawyer, 1887. |
| U Geminorum | Hind, 1855. |
| S Cancri | Hind, 1848. |
| ζ Argûs | Burchell, 1827. |
| R Hydræ | Maraldi, 1704. |
| δ Libræ | Schmidt, 1859. |
| U Coronæ | Winnecke, 1869. |
| α Herculis | W. Herschel, 1795.|
| U Ophiuchi | Sawyer, 1881. |
| β Lyræ | Goodricke, 1784. |
| χ Cygni | Kirch, 1686. |
| ζ Aquilæ | Pigott, 1784. |
| Y Cygni | Chandler, 1886. |
| μ Cephei | Hind, 1848. |
| δ Cephei | Goodricke, 1784. |
+——————————————+——————————————————-+
_New or Temporary Stars._—These stars (sometimes classed with variable
stars) furnish us with rare instances of vast physical changes
occurring among sidereal objects, usually so steadfast and endurable.
The alternating lustre of certain variable stars represents phenomena
of regular recurrence, and is probably to be explained by simple
causes; but the sudden outbursts and rapid decline of temporary stars
are facts of a more startling character, and need a more exceptional
explanation. The first of these objects recorded in history appears
to have been seen in Scorpio 134 years before the Christian era, and
it suggested to Hipparchus of Rhodes the idea of forming a catalogue
of stars, so that in future ages observers might have the means of
recognizing new stars or any other changes in the configuration of the
heavens. Hipparchus completed his catalogue in 128 B.C.; it contained
1025 stars, and forms one of the most valuable memorials we possess of
the labours of the ancient astronomers. Another temporary star is said
to have appeared in 130 A.D., but this and several other objects of
presumably similar character noticed in later years may just possibly
have been comets, and considerable doubt hangs over the descriptions.
It will therefore be safest to confine our remarks to more modern and
better attested instances of these phenomena[53]:—
1572, November 11.—The famous star of Tycho Brahe. He thus described
his first view of it:—“One evening when I was considering, as
usual, the celestial vault, the aspect of which is so familiar to
me, I perceived with indescribable astonishment a bright star of
extraordinary magnitude near the zenith in the constellation of
Cassiopeia.” He adds:—“The new star was destitute of a tail, or of
any appearance of nebulosity; it resembled the other stars in all
respects, only that it twinkled even more than stars of the first
magnitude. In brightness it surpassed Sirius, α Lyræ, or Jupiter. It
could be compared in this respect only to Venus when she is nearest
the earth (when a fourth part of her illuminated surface is turned
towards us). Persons who were gifted with good sight could distinguish
the star in the daytime, even at noon, when the sky was clear.” This
brilliant NOVA began to fade early in Dec. 1572, and in April and May
1573 it resembled a star of the 2nd mag., in July and Aug. one of the
3rd mag., and in Oct. and Nov. one of the 4th mag. In March 1574 the
star completely disappeared (to the naked eye), after a visibility of
about 17 months. It exhibited some curious variations of colour. It
was white when most brilliant; it then became yellow, and afterwards
red, so that its hue in the early part of 1573 was similar to that
of Mars. But in May it again became white, and continued so until it
ceased to be visible. The position of this star (for 1890) is R.A.
0^h 18^m 41^s, Dec. +63° 32′·2. It was supposed to be a reapparition
of the brilliant stars which shone between Cepheus and Cassiopeia in
945 and 1264, and to have possibly been associated with the “Star of
Bethlehem;” but there is no reliable evidence on which this view can
be supported, as the alleged “stars” of 945 and 1264 were undoubtedly
comets, misdescribed in old records. Cornelius Gemma is reputed to have
seen the celebrated star of 1572 a few days before Tycho Brahe, viz.,
on November 8, 1572.
1604, October 10.—Discovered by Brunowski, who announced it to
Kepler. It was brighter than a star of the 1st mag., also than Mars,
Jupiter, or Saturn, which were not far distant at the time. It did
not begin to fade immediately; for a month after its discovery it was
still brighter than Jupiter, and of a white lustre. At the middle of
November it surpassed Antares, but was inferior to Arcturus. In April
1605 it had fallen to the 3rd mag., and went on decreasing until in
October it could scarcely be seen with the naked eye owing to the
twilight resulting from its proximity to the Sun. In March 1606 it was
invisible. The position of this object was about midway between ξ and
58 Ophiuchi, or at R.A. 17^h 24^m, Dec.-21° 207′ (1890).
1670, June 20.—Discovered by the Carthusian Monk Anthelme in R.A. 19^h
43^m 3^s, Dec. +27° 3′ (1890), a few degrees east of β Cygni. It was
of the 3rd mag., and continued in view, with constantly fluctuating
brightness, for nearly two years. At the end of March 1672 it was
6th mag., and has never reappeared. Hind found a small, hazy, and
ill-defined star in the same place, but this is probably not the same
as Anthelme’s star of 1670.
1848, April 28.—During the long interval of 178 years separating 1670
from 1848 not a single new star appears to have revealed itself.
Observers had multiplied, astronomical instruments had been much
improved, star-catalogues were plentiful, and yet the sidereal heavens
gave no intimation of a stellar outburst. No better proof than this
could be afforded as to the great rarity of temporary stars. At length,
in the spring of 1848, the spell was broken, and Mr. Hind announced
that a new star of a reddish-yellow colour had appeared in Ophiuchus,
R.A. 16^h 53^m 20^s, Dec.-12° 43′ (1890). He expressed himself as
certain that no star brighter than the 9th mag. had been there previous
to April 5. After rising to the 4th mag. it soon faded, and in 1851
could only be observed in large instruments. In 1875 it was still
visible as a very minute star.
1860, May 21.—M. Auwers, of Konigsberg, noticed a star of the 7th mag.
near the centre of the bright resolvable nebula (M. 80), lying between
α and β Scorpii, R.A. 16^h 10^m 29^s, Dec.-22° 42′ (1890). On May 18
the star was not there, and it disappeared altogether in three weeks.
It was independently seen by Pogson on May 28, and to him it seemed
that “the nebula had been _replaced_ by a star, so entirely were its
dim rays overpowered by the concentrated blaze in their midst.”
1866, May 12.—Discovered by Birmingham at Tuam. It was of the 2nd mag.,
and situated in Corona, R.A. 15^h 54^m 54^s, Dec. +26° 14′ (1890).
The outburst must have been very sudden, as Schmidt, at Athens, was
observing this region three hours before the new star was detected, and
is certain it was then fainter than the 4th mag. The star was found
to be identical with one on Argelander’s charts estimated as 9½ mag.
It faded from the 2nd to the 6th mag. by May 20, and was thereafter
invisible to the naked eye.
1876, Nov. 24.—A yellow star of the 3rd mag. was seen by the ever
vigilant Schmidt at Athens near ρ Cygni, and where no such star existed
on Nov. 20. The position of the object was R.A. 21^h 37^m 23^s, Dec.
+42° 20′ (1890). It soon grew fainter, so that on Dec. 13 it was of the
6th mag. and devoid of colour. In the spectroscope it presented much
the same lines as Birmingham’s star of May 1866. In addition to the
continuous spectrum it showed bright lines of hydrogen.
1885, August 31.—Dr. Hartwig announced the appearance of a star-like
nucleus in the great nebula (M. 31) of Andromeda, R.A. 0^h 36^m 43^s,
Dec. +40° 40′ (1890). Other observers soon corroborated the discovery.
The star appears to have been first seen on Aug. 19; it was not visible
on the preceding night. On Sept. 1 its mag. was 6·5, on Sept. 2, 7·3,
on Sept. 3, 7·2, Sept. 4, 8·0, Sept. 18, 9·2, &c. On Feb. 7, 1886,
it had dwindled down to the 16th mag., according to an estimate made
by Prof. Hall with the great Washington refractor. The spectrum was
continuous, and Proctor and Gore considered “that the evidence of the
spectroscope showed that the new star was situated _in_ the nebula.”
The phenomena presented by the temporary stars alluded to are so
different to those of ordinary variables that it is very questionable
whether they ought to be classed together. Our knowledge of the former
would no doubt progress more rapidly were they specially looked for
and more instances discovered. Those who have acquired a familiar
acquaintance with the naked-eye stars should examine them as often as
possible with this end in view. Some of these objects lose light so
quickly that unless they are caught near the maximum they are likely to
escape altogether, and this shows the necessity of being constantly on
the alert for their appearance. I have frequently, while watching for
meteors, reviewed the different constellations in the hope of picking
up a new object, but have never succeeded in doing so.
_Star Colours_ form another interesting department of sidereal
astronomy. It is obviously desirable to record the hues presented,
not only by double stars and binary systems, but by isolated stars
also, as changes of tint have been strongly suspected. Cicero, Seneca,
Ptolemy, and others speak of Sirius as a red star, whereas it is
now an intense white; and if we rely on ancient descriptions similar
changes appear to have affected some other prominent stars. But the
old records cannot be implicitly trusted, owing to the errors of
transcribers and translators; and Mr. Lynn (‘Observatory’, vol. ix. p.
104) quotes facts tending to disprove the idea that Sirius was formerly
a red star. In the majority of cases double stars are of the same
colour, but there are many pairs in which the complementary colours
are very decided. Chambers remarks that the brighter star is usually
of a ruddy or orange hue, and the smaller one blue or green. “Single
stars of a fiery red or deep orange are not uncommon, but isolated
blue or green stars are very rare. Amongst conspicuous stars β Libræ
(green) appears to be the only instance.” As an example of fiery-red
stars Antares may be mentioned; Aldebaran is deep reddish orange, and
Betelgeuse reddish orange. Amongst the more prominent stars Capella,
Rigel, and Procyon may be mentioned as showing a bluish tinge, Altair
and Vega are greenish, Arcturus is yellow, while Sirius, Deneb,
Polaris, Fomalhaut, and Regulus are white. Mr. Birmingham published a
catalogue of “The Red Stars” in the ‘Transactions of the Royal Irish
Academy’, for August 1877, and Mr. Chambers has a _working-catalogue_
of 719 such objects in the ‘Monthly Notices,’ vol. xlvii. pp. 348-387.
The region of Cygnus appears to be especially prolific in red stars,
and many of these objects are variable. In a paper read at a recent
meeting of the Astronomical Society of the Pacific Mr. Pierson stated
that in binary systems where the stars are of equal magnitude the
colours are invariably the same, while those differing in magnitude
differ also in colour and the larger star is always nearer the red end
of the spectrum than its secondary. In the estimation of star-colours
reflecting-telescopes are very reliable owing to their perfect
achromatism.
_Groups of Stars._—Great dissimilarity is apparent in the clustering of
stars. The heavens furnish us with all gradations—from the loose, open
groups like that in Coma Berenices, in the Pleiades, or in Præsepe, to
the compact globular clusters, in which some thousands of stars are so
densely congregated that considerable optical power is required to
disintegrate them. Some, it is true, yield more easily than others.
The great cluster (Messier 13) in Hercules readily displays the swarms
of stars of which it is composed; but others are so difficult that it
is only in the largest instruments they are resolved into star-dust.
Further references to these wonderful objects will be made in the next
chapter, and some of the principal examples described; our purpose
here is to allude to a few of the more scattered groups, and to some
noteworthy instances of multiple stars.
_Coma Berenices._ A naked-eye cluster, consisting of many stars,
chiefly from the 5th to 6th mags. A telescope adds a number of smaller
stars. Nebulæ may be often swept up hereabouts, as it is not far north
of the rich nebulous region of Virgo.
_The Pleiades._ Six stars are usually distinguished by the naked eye,
and a seventh is occasionally remarked. Möstlin (the instructor of
Kepler) counted fourteen, Miss Airy has drawn twelve, and Carrington,
like Möstlin, saw fourteen. In 1877 I distinctly made out fourteen
stars in this group. The telescope reveals a considerable number
of small stars and Tempel’s large nebula near Merope. Kepler saw
thirty-two stars with a telescope, and Hooke seventy-eight; but Wolf,
at Paris, after three years of unremitting labour with a 4-foot
reflector, catalogued 671 stars in the group. A photograph, however,
with a 12-inch refractor showed 1421 stars; and a more recent negative
includes no less than 2326. There is an interesting little triangle
close to the brightest star, Alcyone; and several of the leading stars
are involved in nebulosity, discovered by means of photography.
_Præsepe._ A fine group of small stars, divisible by the unaided eye
on a clear night. Chambers says the components are not visible without
a telescope; while Webb notes that the group is just resolvable by the
naked eye. Thirty-six stars were glimpsed with Galilei’s telescope;
but modern instruments show many more. Marth, using Lassell’s 4-foot
reflector at Malta, discovered several faint nebulæ and nebulous stars
in this cluster.
χ _Persei._ Perceptible to the eye as a patch of hazy material lying
between the constellations Cassiopeia and Persei. In a telescope it
forms a double cluster, and is one of the richest and most beautiful
objects that the sky affords. The tyro who first beholds it is
astonished at the marvellous profusion of stars. It can be fairly well
seen in a good field-glass, but its chief beauties only come out in
a telescope, and the larger the aperture the more striking will they
appear. It is on groups of this character that the advantage of large
instruments is fully realized. The power should be very low, so that
the whole of the two clusters may be seen in the field. An eyepiece of
40, field 65′, on my 10-inch reflector, presents this object in its
most imposing form.
κ _Crucis_. Sir J. Herschel’s observations at the Cape have made this
object familiar to northern observers. It is composed of more than 100
stars, from the 7th mag. downwards; and some of the brighter ones are
highly coloured, so that the general effect is greatly enhanced and
fully justifies Herschel’s statement that the group may be likened to
“a superb piece of fancy jewelry.”
ζ _Ursæ Majoris_ (_Mizar_). This group is interesting both as a
naked-eye and as a telescopic object. There is a 5th mag. star, named
Alcor, about 11½′ distant from Mizar, and the former was considered
a good test-object for unaided vision by the Arabian astronomers.
But the star has probably brightened; for it can now be easily seen,
and certainly offers no criterion of good vision. Mizar is a fine
telescopic double, the companion being 4th mag. and distant 14½″. Any
small telescope will show it, and there is another 8th mag. star very
near.
σ _Orionis_. This appears as a double-quadruple star, with several
others in the same field. A 3-inch will reveal most of them, though
some of the fainter stars in the group will be beyond its reach.
θ _Orionis_. In the midst of the great nebula of Orion there is a
tolerably conspicuous quadruple star, the components of which form
a trapezium. This is visible in a 2-inch refractor. In 1826 Struve
discovered a fifth star, and in 1830 Sir J. Herschel found a sixth;
these were both situated a little outside the trapezium. All these
stars have been seen in a 3-inch telescope. The great 36-inch
equatoreal at Mount Hamilton has added several others; one was
detected by Alvan G. Clark (the maker of the object-glass) and another
by Barnard. These were excessively minute, and placed within the
trapezium. Barnard[54] has also glimpsed an extremely minute double
star exterior to the trapezium, and forming a triangle with the stars A
and C on the following diagram:—
[Illustration: Fig. 63.
The Trapezium in Orion, as seen with the 36-inch refractor.]
Several observers, including Huggins, Salter, and others, had
previously drawn faint stars in the interior of the trapezium; but
these could not be seen by Hall and Burnham in the large refractors at
Washington and Chicago, and were thus proved to have no real existence.
The new stars observed in the 36-inch telescope are only just within
the limits of its capacity, and therefore cannot be identical with
stars alleged to have been previously seen in small instruments.
The fifth and sixth stars in the trapezium have been supposed to be
variable, and not without reason; possibly the others are equally
liable to change, but this is only conjecture. Sir J. Herschel says
that to perceive the fifth and sixth stars “is one of the severest
tests that can be applied to a telescope” (‘Outlines,’ 11th edit. p.
610); yet Burnham saw them both readily in a 6-inch a few minutes
before sunrise on Mount Hamilton in September 1879.
β and ε Lyræ also form multiple groups, which will well repay
observation either with large or small telescopes.
_Further Observations._—Anyone who attempts to indicate with tolerable
fulness the methods and requirements of observation in the stellar
department of astronomy will find a heavy task lies before him; and
it is one to which he will be unable to do justice in a small space,
owing to the variety of matters to be referred to and the necessity
of being particular in regard to each one. In what follows I shall
merely make very brief allusions, as it is hoped the description
already given of past work will be a sufficient guide for the future.
Moreover, those who take up a special branch of inquiry will hardly
rest content with the meagre information usually conveyed in a general
work on astronomy, but will consult those authorities who deal more
exclusively with that branch. Double and binary stars may be said to
form one department, variable and temporary stars another, the colours
of stars a third, while many others may be signified—such as the
determination of star-magnitudes, positions, grouping, and distances;
also the proper motions and number of stars, besides photographic and
spectroscopic work,—each and all of which comprise a field of useful
and extensive inquiry. The amateur will of course choose his own sphere
of labour, consistently with his inclination and the character of his
appliances. In connection with double stars, valuable work yet remains
to be done, though the Herschels and the Struves gathered in the
bulk of the harvest and Burnham has gleaned much that was left. With
regard to bright stars, it may be assumed that very few, if any, close
companions, visible in moderately small glasses, now await discovery,
unless, indeed, in cases where the star forms part of a binary system
of long period, and the epoch of periastron has fallen in recent years.
But with telescopic stars there must be many interesting doubles, some
of them binaries, still unknown. These should be swept up and submitted
to measurement. A great desideratum in this branch is a new general
catalogue of double stars; for such a work would greatly facilitate
reference, and save the trouble of searching through different lists
in order to identify an object. Burnham has given some practical hints
on double-star work in the ‘Sidereal Messenger,’ and his remarks are
reproduced in that excellent work ‘Astronomy for Amateurs.’
As to variable stars, some of these permit of naked-eye estimation,
others need a field-glass, and there are some which require to be
followed in a good telescope. The observer who enters this department
may either desire to find new objects or to obtain further data with
regard to old ones. If the former, he cannot do better than watch
some of the suspected variables in Gore’s Catalogue of 736 objects,
published by the Royal Irish Academy. Whether suspected or known
variables are put under surveillance, the plan of comparison will be
the same. Several stars near the variable in position, and nearly equal
in light, should be compared with it, and the differences in lustre,
in tenths of a magnitude, recorded as frequently as possible. The
extent and period of the variation will become manifest by a discussion
of the results. The comparison-stars should of course be constant
in light, and, if naked-eye stars, they may be selected from the
_Uranometria Nova Oxoniensis_ or ‘Harvard Photometry.’ If telescopic
stars are required, then recourse must be had to comprehensive charts
such as Argelander’s _Durchmusterung_, which includes stars up to 9½
mag. Variable stars of the Algol type are especially likely to escape
recognition, as they retain a normal brilliancy except during the few
hours near the time of a minimum.
As to star-colours, it must be admitted that our knowledge is in
an unsatisfactory condition. The results of past observation show
discordances which are difficult to account for. When, however, all the
circumstances are considered, we need feel no surprise at this want of
unanimity. In certain cases it is probable that actual and periodical
changes occur in the colours of stars, though absolute proof is still
required. Atmospheric variations unquestionably affect the tints of
stars, and some alterations depend upon altitude, for a celestial
object seen through the dense lower air-strata near the horizon will
hardly preserve the same apparent hues when on the meridian. Telescopes
are also liable to induce false impressions of colour, and especially
by the employment of different eyepieces not equally achromatic. And
the observer’s judgment is sometimes at fault through physiological
influences, or he may have a systematic preference for certain hues
which little impress another observer. Those engaged in this branch
feel the want of a reliable and ready means of comparison, and several
have been tried; but there are objections to their use, and it seems
that the best objects are furnished by the stars themselves. Let the
observer study the colours of well-known stars, and familiarize his
eye with the distinctions in various cases (also with the differences
due to meteorological effects &c.); he will then gradually acquire
confidence, and may use these objects as standards. The difficulty will
be that they cannot be directly compared, in the same field, with other
stars; but relative differences may be noted by turning the telescope
from one object to the other. This will be better than forming
estimates on the basis of an artificial method, which will sure to be
troublesome to arrange, and probably erroneous in practice. In some
stars the colour is so curious as to be attributed with difficulty, and
with regard to faint stars colour-estimates are often unreliable; so
that it is not desirable to go below the 9th mag. unless a very large
instrument is employed.
The necessity of being constantly on the look-out for temporary
stars has been already mentioned. There is also the need for further
observations of such of these objects as still exist. They are,
however, very minute, and the observer will have to be careful as
to their identity. Though no great revival in brilliancy is to be
expected, these objects exhibited some singular fluctuations during
their decline, and it is important to keep them under view as long as
possible.
Many other departments of sidereal work are best left to the
professional astronomer. The derivation of accurate star-places, proper
motions, distances, &c. requires instruments of great refinement and
trained hands to use them. Researches such as these do not come within
the scope of ordinary amateurs. But a vast field is open to them in
respect to double and variable stars; and the physical relations of
many of the former greatly intensify the interest in this branch, and
make it necessary to secure frequent observations.
FOOTNOTES:
[47] There are several forms of this instrument: for particulars of
construction and use the reader is referred to Thornthwaite’s ‘Hints on
Telescopes,’ and Chambers’s ‘Astronomy,’ 4th ed. vol. ii.
[48] Mr. George Knott, of Cuckfield, mentions that the radius of the
first bright diffraction-ring of a stellar image, for a 7-1/3-inch
aperture, is 1″·01, and for one of 2 inches 3″·70 (‘Observatory,’
vol. vi. p. 19; see also vol. i. pp. 107 and 145). Mr. Dawes is quoted
as giving 1″·25 for a 7-inch, 1″·61 for a 5½-inch, and 3″·57 for a
2·4-inch. These figures exceed the theoretical values, if the latter
are adopted from Sir G. B. Airy’s ‘Undulatory Theory of Optics,’ where,
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).