THE STARRY HEAVENS
The celestial vault, that, like a circling canopy of sapphire hue,
stretches overhead from horizon to horizon, resplendent by night with
myriad stars of different magnitudes and varied brilliancy, forming
clusterings and configurations of fantastic shape and beauty, arrests
the attention of the most casual observer. But to one who has studied
the heavens, and followed the efforts of human genius in unravelling the
mysteries associated with those bright orbs, the impression created on
his mind as he gazes upon them in the still hours of the night, when the
turmoil of life is hushed in repose, is one of wonder and longing to
know more of their being and the hidden causes which brought them forth.
Here, we have poetry written in letters of gold on the sable vestment of
night; music in the gliding motion of the spheres; and harmony in the
orbital sweep of sun, planet, and satellite.
Milton was not only familiar with 'the face of the sky,' as it is
popularly called, but also knew the structure of the celestial sphere,
and the great circles by which it is circumscribed. Two of those--the
colures--he alludes to in the following lines, when he describes the
manner in which Satan, to avoid detection, compassed the Earth, after
his discovery by Gabriel in Paradise, and his flight thence:--
The space of seven continued nights he rode
With darkness--thrice the equinoctial line
He circled, four times crossed the car of night
From pole to pole, traversing each colure.--ix. 63-66.
Aristarchus of Samos believed the stars were golden studs, that
illumined the crystal dome of heaven; but modern research has
transformed this conception of the ancient astronomer's into a universe
of blazing suns rushing through regions of illimitable space. In
Milton's time astronomers had arrived at no definite conclusion with
regard to the nature of the stars. They were known to be self-luminous
bodies, situated at a remote distance in space, but it had not been
ascertained with any degree of certainty that they were suns, resembling
in magnitude and brilliancy our Sun. Indeed, little was known of those
orbs until within the past hundred years, when the exploration of the
heavens by the aid of greatly increased telescopic power, was the means
of creating a new branch of astronomical science, called sidereal
astronomy.
We are indebted to Sir William Herschel, more than to any other
astronomer, for our knowledge of the stellar universe. It was he who
ascertained the vastness of its dimensions, and attempted to delineate
its structural configuration. He also explored the star depths, which
occupy the infinitude of space by which we are surrounded, and made
many wonderful discoveries, which testify to his ability as an observer,
and to his greatness as an astronomer.
William Herschel was born at Hanover, November 15, 1738. His father was
a musician in the band of the Hanoverian Guard, and trained his son in
his own profession. After four years of military service, young Herschel
arrived in England when nineteen years of age, and maintained himself by
giving lessons in music. We hear of him first at Leeds, where he
followed his profession, and instructed the band of the Durham Militia.
From Leeds he went to Halifax, and was appointed organist there; on the
expiration of twelve months he removed to Bath, and was elected to a
similar post at the Octagon Chapel in that city. Here, fortune smiled
upon him, and he became a busy and prosperous man. Besides attending to
his numerous private engagements, he organised concerts, oratorios, and
other public musical entertainments, which gained him much popularity
among the cultivated classes which frequented this fashionable resort.
Notwithstanding his numerous professional engagements, Herschel was able
to devote a portion of his time to acquiring knowledge on other
subjects. He became proficient in Italian and Greek, studied
mathematics, and read books on astronomy. In 1773 he borrowed a small
telescope, which he used for observational purposes, and was so
captivated with the appearances presented by the celestial bodies, that
he resolved to dedicate his life to acquiring 'a knowledge of the
construction of the heavens.' This resolution he nobly adhered to, and
became one of the most distinguished of astronomers. Like many other
astronomers, Herschel possessed the requisite skill which enabled him to
construct his own telescopes. Being desirous of possessing a more
powerful instrument, and not having the means to purchase one, he
commenced the manufacture of specula, the grinding and polishing of
which had to be done by hand, entailing the necessity of tedious labour
and the exercise of much patience. After repeated failures he at length
completed a 5-1/2-foot Gregorian reflector, and with this instrument
made his first survey of the heavens. Having perceived the desirability
of possessing a more powerful telescope, he equipped himself with a
reflector of twenty feet focal length, and it was with this instrument
that he made those wonderful discoveries which established his
reputation as a great astronomer.
On March 31, 1781, when examining the stars in the constellation Gemini,
Herschel observed a star which presented an appearance slightly
different to that of the other stars by which it was surrounded; it
looked larger, had a perceptible disc, and its light became fainter when
viewed with a higher magnifying power. After having carefully examined
this object, Herschel arrived at the conclusion that he had discovered a
comet. He communicated intelligence of his discovery to the Royal
Society, and, a notification of it having been sent to the Continental
observatories, this celestial visitor was subjected to a close scrutiny;
its progressive motion among the stars was carefully observed, and an
orbit was assigned to it. After it had been under observation for some
time, doubts were expressed as to its being a comet, these were
increased on further examination, and eventually it was discovered that
this interesting object was a new planet. This important discovery at
once raised Herschel to a position of eminence and distinction, and from
a star-gazing musician he became a famous astronomer. A new planet named
Uranus was added to our system, which completes a revolution round the
Sun in a little over eighty-four years, and at a distance of near 1,000
millions of miles beyond the orbit of Saturn. Herschel's name became a
household word. George III. invited him to Court in order that he might
obtain from his own lips an account of his discovery of the new planet;
and so favourable was the impression made by Herschel upon the King,
that he proposed to create him Royal Astronomer at Windsor, and bestow
upon him a salary of 200_l._ a year. Herschel decided to accept the
proffered appointment, and, with his sister Caroline, removed from Bath
to Datchet, near Windsor, in 1782, and from there to Slough in 1786. In
1788 he married the wealthy widow of a London merchant, by whom he had
one son, who worthily sustained his father's high reputation as an
astronomer. Herschel was created a Knight in 1816, and in 1821 was
elected first President of the Royal Astronomical Society. He died at
Slough on August 25, 1822, when in the eighty-fourth year of his age,
and was buried in Upton Churchyard.
It is inscribed on his tomb, that 'he burst the barriers of heaven;' the
lofty praise conveyed by this expression is not greater than what
Herschel merited when we consider with what unwearied assiduity and
patience he laboured to accomplish the results described in the words
which have been quoted. By a method called 'star-gauging' he
accomplished an entire survey of the heavens and examined minutely all
the stars in their groups and aggregations as they passed before his eye
in the field of the telescope. He sounded the depths of the Milky Way,
and explored the wondrous regions of that shining zone, peopled with
myriads of suns so closely aggregated in some of its tracts as to
suggest the appearance of a mosaic of stars. He resolved numerous nebulae
into clusters of stars, and penetrated with his great telescope depth
after depth of space crowded with 'island universes of stars,' beyond
which he was able to discern luminous haze and filmy streaks of light,
the evidence of the existence of other universes plunged in depths still
more profound, where space verges on infinity. In his exploration of the
starry heavens Herschel's labours were truly amazing. On four different
occasions he completed a survey of the firmament, and counted the stars
in several thousand gauge-fields; he discovered 2,400 nebulae, 800
double stars, and attempted to ascertain the approximate distances of
the stars by a comparison of their relative brightness.
It had long been surmised, though no actual proof was forthcoming, that
the law of gravitation by which the order and stability of our system
are maintained exercises its potent influence over other material bodies
existing in space, and that other systems, though differing in many
respects from that of ours, and presenting a more complex arrangement in
their structure, perform their motions subject to the guidance of this
universal law. The uncertainty with regard to the controlling influence
of gravity was removed by Herschel when he made his important discovery
of binary star systems. The components of a binary star are usually in
such close proximity that, to the naked eye, they appear as one star,
and sometimes, even with telescopic aid, it is impossible to distinguish
them individually; but when observed with sufficient magnifying power
they can be easily perceived as two lucid points. Double stars were for
a long time believed to be a purely optical phenomenon--an effect
created by two stars projected on the sphere so as to appear nearly in
the same line of vision, and, although apparently almost in contact,
situated at great distances apart. At one time Herschel entertained a
similar opinion with regard to those stars. In 1779 he undertook an
extensive exploration of the heavens with the object of discovering
double stars. As a result of his labours he presented to the Royal
Society in 1782 a list of 269 newly discovered double stars, and in
three years after he supplemented this list with another which contained
434 more new stars. He carefully measured the distances by which the
component stars were separated, and determined their position angles, in
order that he might be able to detect the existence of any sensible
parallax. On repeating his observations twenty years after, he
discovered that the relative positions of many of the stars had changed,
and in 1802 he made the important announcement of his discovery that the
components of many double stars form independent systems, held together
in a mutual bond of union and revolving round one common centre of
gravity.
The importance of this discovery, which we owe to Herschel's sagacity
and accuracy of observation, cannot be over-estimated; what was
previously conjecture and surmise, now became precise knowledge
established upon a sure and accurate basis. It was ascertained that the
law of gravity exerts its power in regulating and controlling the
motions of all celestial bodies within the range of telescopic vision,
and that the order and harmony which pervade our system are equally
present among other systems of suns and worlds distributed throughout
the regions of space. The spectacle of two or more suns revolving round
each other, forming systems of greater magnitude and importance than
that of ours, conveyed to the minds of astronomers a knowledge of the
mechanism of the heavens which had hitherto been unknown to them.
During the many years which Herschel devoted to the exploration of the
starry heavens, and when engaged night after night in examining and
enumerating the various groups and clusters of stars which passed before
his eye in the field of his powerful telescope, he did not fail to
remember the sublime object of his life, and to which he made all his
other investigations subordinate, viz., the delineation of the
structural configuration of the heavens, and the inclusion of all
aggregations, groups, clusters, and galaxies of stars which are
apparently scattered promiscuously throughout the regions of space into
one grand harmonious design of celestial architecture.
Having this object in view, he explored the wondrous zone of the Milky
Way, gauged its depths, measured its dimensions, and, in attempting to
unravel the intricacies of its structure, penetrated its recesses far
beyond the limit attained by any other observer. Acting on the
assumption that the stars are uniformly distributed throughout space,
Herschel, by his method of star-gauging, concluded that the sidereal
system consists of an irregular stratum of evenly distributed suns,
resembling in form a cloven flat disc, and that the apparent richness of
some regions as compared with that of others could be accounted for by
the position from which it was viewed by an observer. The stars would
appear least numerous where the visual line was shortest, and, as it
became lengthened, they would increase in number until, by crowding
behind each other as a greater depth of stratum was penetrated, they
would, when very remote, present the appearance of a luminous cloud or
zone of light. After further observation Herschel was compelled to
relinquish his theory of equal star distribution, and found, as he
approached the Galaxy, that the stars became much more numerous, and
that in the Milky Way itself there was evidence of the gravitation of
stars towards certain regions forming aggregations and clusters which
would ultimately lead to its breaking up into numerous separate sidereal
systems. As he extended his survey of the heavens and examined with
greater minuteness the stellar regions in the Galactic tract, he
discovered that by his method of star-gauging he was unable to define
the complexity of structure and variety of arrangement which came under
his observation; he also perceived that the star-depths are
unfathomable, and discerned that beyond the reach of his telescope there
existed systems and galaxies of stars situated at an appalling distance
in the abysmal depths of space. Though the magnitude of that portion of
the sidereal heavens which came under his observation was inconceivable
as regards its dimensions, Herschel was able to perceive that it formed
but a part--and most probably a small part--of the stellar universe, and
that without a more extended knowledge of this universe, which at
present is unattainable, it would be impossible to determine its
structural configuration or discover the relationships that exist among
the sidereal systems and Galactic concourses of stars distributed
throughout space. Herschel ultimately abandoned his star-gauging method
of observation and confined his attention to exploring the star depths
and investigating the laws and theories associated with the bodies
occupying those distant regions.
Since all the planets if viewed from the Sun would be seen to move
harmoniously and in regular order round that body, so there may be
somewhere in the universe a central point, or, as some persons imagine,
a great central sun, round which all the systems of stars perform their
majestic revolutions with the same beautiful regularity; having their
motions controlled by the same law of gravitation, and possessing the
same dynamical stability which characterises the mechanism of the solar
system.
The extent of the distance which intervenes between our system and the
fixed stars constituted a problem which exercised the minds of
astronomers from an early period until the middle of the present
century.
Tycho Brahe, who repudiated the Copernican theory, asserted as one of
his reasons against it that the distances by which the heavenly bodies
are separated from each other were greater than even the upholders of
this theory believed them to be. Although the distance of the Sun from
the Earth was unknown, Tycho was aware that the diameter of the Earth's
orbit must be measured by millions of miles, and yet there was no
perceptible motion or change of position of the stars when viewed from
any point of the vast circumference which she traverses. Consequently,
the Earth, if viewed from the neighbourhood of a star, would also appear
motionless, and the dimensions of her orbit would be reduced to that of
a point. This seemed incredible to Tycho, and he therefore concluded
that the Copernican theory was incorrect.
The conclusion that the stars are orbs resembling our Sun in magnitude
and brilliancy was one which, Tycho urged, should not be hastily
adopted; and yet, if it were conceded that the Earth is a body which
revolves round the Sun, it would be necessary to admit that the stars
are suns also. If the Earth's orbit, as seen from a star, were reduced
to a point, then the Sun, which occupies its centre, would be reduced to
a point of light also, and, when observed from a star of equal
brilliancy and magnitude, would have the same resemblance that the star
has when viewed from the Earth, which may be regarded as being in
proximity to the Sun. Tycho Brahe would not admit the accuracy of these
conclusions, which were too bewildering and overwhelming for his mental
conception.
But the investigations of later astronomers disclosed the fact that the
heavenly bodies are situated at distances more remote from each other
than had been previously imagined, and that the reasons which led Tycho
to reject the Copernican theory were based upon erroneous conclusions,
and could, with greater aptitude, be employed in its support. It was
ascertained that the distance of the Sun from the Earth, which at
different periods was surmised to be ten, twenty, and forty millions of
miles, was much greater than had been previously estimated. Later
calculations determined it to be not less than eighty millions of miles,
and, according to the most recent observations, the distance of the Sun
from the Earth is believed to be about ninety-three millions of miles.
Having once ascertained the distance between the Earth and the Sun,
astronomers were enabled to determine with greater facility the
distances of other heavenly bodies.
It was now known that the diameter of the Earth's orbit exceeded 183
millions of miles, and yet, with a base line of such enormous length,
and with instruments of the most perfect construction, astronomers were
only able to perceive the minutest appreciable alteration in the
positions of a few stars when observed from opposite points of the
terrestrial orbit.
It had long been the ambitious desire of astronomers to accomplish, if
possible, a measurement of the abyss which separates our system from the
nearest of the fixed stars. No imaginary measuring line had ever been
stretched across this region of space, nor had its unfathomed depths
ever been sounded by any effort of the human mind. The stars were known
to be inconceivably remote, but how far away no person could tell, nor
did there exist any guide by which an approximation of their distances
could be arrived at.
In attempting to calculate the distances of the stars, astronomers have
had recourse to a method called 'Parallax,' by which is meant the
apparent change of position of a heavenly body when viewed from two
different points of observation.
The annual parallax of a heavenly body is the angle subtended at that
body by the radius of the Earth's orbit.
The stars have no diurnal parallax, because, owing to their great
distance, the Earth's radius does not subtend any measurable angle, but
the radius of the Earth's orbit, which is immensely larger, does, in the
case of a few stars, subtend a very minute angle.
'This enormous base line of 183 millions of miles is barely sufficient,
in conjunction with the use of the most delicate and powerful
astronomical instruments, to exhibit the minutest measureable
displacement of two or three of the nearest stars.'--Proctor.
The efforts of early astronomers to detect any perceptible alteration in
the positions of the stars when observed from any point of the
circumference of the Earth's orbit were unsuccessful. Copernicus
ascribed the absence of any parallax to the immense distances of the
stars as compared with the dimensions of the terrestrial orbit. Tycho
Brahe, though possessing better appliances, and instruments of more
perfect construction, was unable to perceive any annual displacement of
the stars, and brought this forward as evidence against the Copernican
theory.
Galileo suggested a method of obtaining the parallax of the fixed stars,
by observing two stars of unequal magnitude apparently near to each
other, though really far apart. Those, when observed from different
points of the Earth's orbit, would appear to change their positions
relatively to each other. The smaller and more distant star would remain
unaltered, whilst the larger and nearer star would have changed its
position with respect to the other. By continuing to observe the larger
star during the time that the Earth accomplished a revolution of her
orbit, Galileo believed that its parallax might be successfully
determined. Though he did not himself put this method into practice, it
has been tried by others with successful results.
In 1669, Hooke made the first attempt to ascertain the parallax of a
fixed star, and selected for this purpose Gamma Draconis, a bright star
in the Head of the Dragon. This constellation passed near the zenith of
London at the time that he made his observations, and was favourably
situated, so as to avoid the effects of refraction. Hooke made four
observations in the months of July, August, and October, and believed
that he determined the parallax of the star; but it was afterwards
discovered that he was in error, and that the apparent displacement of
the star was mainly due to the aberration of light--a phenomenon which
was not discovered at that time.
A few years later, Picard, a French astronomer, attempted to find the
parallax of Alpha Lyrae, but was unsuccessful. In 1692-93, Roemer, a
Danish astronomer, observed irregularities in the declinations of the
stars which could neither be ascribed to parallax or refraction, and
which he imagined resulted from a changing position of the Earth's axis.
One of the principal causes which baffled astronomers in their
endeavours to determine the parallax of the fixed stars was a phenomenon
called the 'Aberration of Light,' which was discovered and explained by
Bradley in 1727. The peculiar effect of aberration was perceived by him
when endeavouring to obtain the parallax of Gamma Draconis.
Owing to the progressive transmission of light, conjointly with the
motion of the Earth in her orbit, there results an apparent slight
displacement of a star from its true position. The extent of the
displacement depends upon the ratio of the velocity of light as compared
with the speed of the Earth in her orbit, which is as 10,000 to 1. As a
consequence of this, each star describes a small ellipse in the course
of a year, the central point of which would indicate the place occupied
by the star if the Earth were at rest. The shifting position of the star
is very slight, and at the end of a year it returns to its former place.
Prior to the discovery of aberration, astronomers ascribed the apparent
displacement of the stars arising from this cause as being due to
parallax--a conclusion which led to erroneous results; but after
Bradley's discovery this source of error was avoided, and it was found
that the parallax of the stars had to be considerably reduced.
Bessel was the first astronomer who merited the high distinction of
having determined the first reliable stellar parallax, and by this
achievement he was enabled to fathom the profound abyss which separates
our solar system from the stars.
Frederick William Bessel was born in 1764 at Minden, in Westphalia. It
was his intention to pursue a mercantile career, and he commenced life
by becoming apprenticed to a firm of merchants at Bremen. Soon
afterwards he accompanied a trading expedition to China and the East
Indies, and while on this voyage picked up a good deal of information
with regard to many matters which came under his observation. He
acquired a knowledge of Spanish and English, and made himself acquainted
with the art of navigation. On his return home, Bessel endeavoured to
determine the longitude of Bremen. The only appliances which he made use
of were a sextant constructed by himself, and a common clock; and yet,
with those rude instruments, he successfully accomplished his object.
During the next two years he devoted all his spare time to the study of
mathematics and astronomy, and, having obtained possession of Harriot's
observations of the celebrated comet of 1607--known as Halley's
comet--Bessel, after much diligent application and careful calculation,
was enabled to deduce from them an orbit, which he assigned to that
remarkable body. This meritorious achievement was the means of procuring
for him a widely known reputation.
A vacancy for an assistant having occurred at Schroeter's Observatory at
Lilienthal, the post was offered to Bessel and accepted by him. Here he
remained for four years, and was afterwards appointed Director of the
new Prussian Observatory at Koenigsberg, where he pursued his
astronomical labours for a period of upwards of thirty years. Bessel
directed his energies chiefly to the study of stellar astronomy, and
made many observations in determining the number, the exact positions,
and proper motions of the stars. He was remarkable for the precision
with which he carried out his observations, and for the accuracy which
characterised all his calculations.
In 1837 Bessel, by the exercise of his consummate skill, endeavoured to
solve a problem which for many years baffled the efforts of the ablest
astronomers, viz., the determination of the parallax of the fixed stars.
This had been so frequently attempted, and without success, that the
results of any new observations were received with incredulity before
their value could be ascertained.
Bessel was ably assisted by Joseph Frauenhofer, an eminent optician of
Munich, who constructed a magnificent heliometer for the Observatory at
Koenigsberg, and in its design introduced a principle which admirably
adapted it for micrometrical measurement.
The star selected by Bessel is a binary known as 61 Cygni, the
components being of magnitudes 5.5 and 6 respectively. It has a large
proper motion, which led him to conclude that its parallax must be
considerable.
This star will always be an object of interest to astronomers, as it was
the first of the stellar multitude that revealed to Bessel the secret of
its distance.
Bessel commenced his observations in October 1837, and continued them
until March 1840. During this time he made 402 measurements, and, before
arriving at a conclusive result, carefully considered every imaginable
cause of error, and rigorously calculated any inaccuracies that might
arise therefrom. Finally, he determined the parallax of the star to be
0''.3483--a result equivalent to a distance about 600,000 times that of
the Earth from the Sun. In 1842-43 M. Peters, of the Pulkova
Observatory, arrived at an almost similar result, having obtained a
parallax of 0''.349; but by more recent observations the parallax of the
star has been increased to about half a second.
About the same time that Bessel was occupied with his observation of 61
Cygni, Professor Henderson, of Edinburgh, when in charge of the
Observatory at the Cape of Good Hope, directed his attention to Alpha
Centauri, one of the brightest stars in the Southern Hemisphere. During
1832-33 he made a series of observations of the star, with the object
of ascertaining its mean declination; and, having been informed
afterwards of its large proper motion, he resolved to make an endeavour
to determine its parallax. This he accomplished after his return to
Scotland, having been appointed Astronomer Royal in that country. By an
examination of the observations made by him at the Cape, he determined
the parallax of Alpha Centauri to be 1''.16, but later astronomers have
reduced it to 0''.75.
Professor Henderson's detection of the parallax of Alpha Centauri was
communicated to the Astronomical Society two months after Bessel
announced his determination of the parallax of 61 Cygni.
The parallax of 61 Cygni assigns to the star a distance of forty
billions of miles from the Earth, and that of Alpha Centauri--regarded
as the nearest star to our system--a distance of twenty-five billions of
miles.
It is utterly beyond the capacity of the human mind to form any adequate
conception of those vast distances, even when measured by the velocity
with which the ether of space is thrilled into light. Light, which
travels twelve millions of miles in a minute, requires 4-1/3 years to
cross the abyss which intervenes between Alpha Centauri and the Earth,
and from 61 Cygni the period required for light to reach our globe is
rather less than double that time.
The parallax of more than a dozen other stars has been determined, and
the light passage of a few of the best known is estimated as
follows:--Sirius, eight years; Procyon, twelve; Altair, sixteen;
Aldebaran, twenty-eight; Capella, thirty; Regulus, thirty-five; Polaris,
sixty-three; and Vega, ninety-six years.
It does not always follow that the brightest stars are those situated
nearest to our system, though in a general way this may be regarded as
correct. The diminishing magnitudes of the stars can be accounted for
mainly by their increased distances, rather than by any difference in
their intrinsic brilliancy. We should not err by inferring that the most
minute stars are also the most remote; the telescope revealing thousands
that are invisible to the naked eye. There are, however, exceptions to
this general rule, and there are many stars of small magnitude less
remote than those whose names have been enumerated, and whose light
passage testifies to their profound distances and surpassing magnitude
when compared with that of our Sun.
Sirius, 'the leader of the heavenly host,' is distant fifty billions of
miles. The orb shines with a brilliancy far surpassing that of the Sun,
and greatly exceeds him in mass and dimensions. Arcturus, the bright
star in Booetes, whose golden yellow light renders it such a conspicuous
object, is so far distant that its measurement gives no reliable
parallax; and if we may infer from what little we know of the stars,
Arcturus is believed to be the most magnificent and massive orb entering
into the structure of that portion of the sidereal system which comes
within our cognisance. Judging by its relative size and brightness,
this star is ten thousand times more luminous, and may exceed the Sun
one million times in volume.
Deneb, in the constellation of the Swan, though a first-magnitude star,
possesses no perceptible proper motion or parallax--a circumstance
indicative of amazing distance, and magnitude equalling, or surpassing,
Arcturus and Sirius.
Canopus, in the constellation Argo, in the Southern Hemisphere, the
brightest star in the heavens with the exception of Sirius, possesses no
sensible parallax; consequently, its distance is unknown, though it has
been estimated that its light passage cannot be less than sixty-five
years.
By establishing a mean value for the parallax of stars of different
magnitudes, it was believed that an approximation of their distances
could be obtained by calculating the time occupied in their light
passage. The light period for stars of the first magnitude has been
estimated at thirty-six and a half years; this applies to the brightest
stars, which are also regarded as the nearest. At the distance indicated
by this period, the Sun would shrink to the dimensions of a
seventh-magnitude star and become invisible to the naked eye; this of
itself affords sufficient proof that the great luminary of our system
cannot be regarded as one of the leading orbs of the firmament. Stars of
the second magnitude have a mean distance of fifty-eight light years,
those of the third magnitude ninety-two years, and so on. M. Peters
estimated that light from stars of the sixth magnitude, which are just
visible to the naked eye, requires a period of 138 years to accomplish
its journey hither; whilst light emitted from the smallest stars visible
in large telescopes does not reach the Earth until after the lapse of
thousands of years from the time of leaving its source.
The profound distances of the nearest stars by which we are surrounded
lead us to consider the isolated position of the solar system in space.
A pinnacle of rock, or forsaken raft floating in mid-ocean, is not more
distant from the shore than is the Sun from his nearest neighbours. The
inconceivable dimensions of the abyss by which the orb and his
attendants are surrounded in utter loneliness may be partially
comprehended when it is known that light, which travels from the Sun to
the Earth--a distance of ninety-three millions of miles--in eight
minutes, requires a period of four and a third years to reach us from
the nearest fixed star. A sphere having the Sun at its centre and this
nearest star at its circumference would have a diameter of upwards of
fifty billions of miles; the volume of the orb when compared with the
dimensions of this circular vacuity of space is as a small shot to a
globe 900 miles in diameter. It has been estimated by Father Secchi
that, if a comet when at aphelion were to arrive at a point midway
between the Sun and the nearest fixed star, it would require one hundred
million years in the accomplishment of its journey thither. And yet the
Sun is one of a group of stars which occupy a region of the heavens
adjacent to the Milky Way and surrounded by that zone; nor is his
isolation greater than that of those stars which are his companions, and
who, notwithstanding their profound distance, influence his movements by
their gravitational attraction, and in combination with the other stars
of the firmament control his destiny.
Ancient astronomers, for the purpose of description, have mapped out the
heavens into numerous irregular divisions called 'constellations.' They
are of various forms and sizes, according to the configuration of the
stars which occupy them, and have been named after different animals,
mythological heroes, and other objects which they appear to resemble. In
a few instances there does exist a similitude to the object after which
a constellation is called; this is evident in the case of Corona
Borealis (the Northern Crown), in which there can be seen a conspicuous
arrangement of stars resembling a coronet, and in the constellations of
the Dolphin and Scorpion, where the stars are so distributed that the
forms of those creatures can be readily recognised. There is some slight
resemblance to a bear in Ursa Major, and to a lion in Leo, and no great
effort of the mind is required to imagine a chair in Cassiopeia, and a
giant in Orion; but in the majority of instances it is difficult to
perceive any likeness of the object after which a constellation is
named, and in many cases there is no resemblance whatever.
The constellations are sixty-seven in number: excluding those of the
Zodiac, which have been already mentioned, the constellations of the
Northern Hemisphere number twenty-nine. The most important of these are
Ursa Major and Minor, Andromeda, Cassiopeia, Cepheus, Cygnus, Lyra,
Aquila, Auriga, Draco, Booetes, Hercules, Pegasus, and Corona Borealis.
To an observer of the nocturnal sky the stars appear to be very
unequally distributed over the celestial sphere. In some regions they
are few in number and of small magnitude, whilst in other parts of the
heavens, and especially in the vicinity of the Milky Way, they are
present in great numbers and form groups and aggregations of striking
appearance and conspicuous brilliancy. On taking a casual glance at the
midnight sky on a clear moonless night, one is struck with the apparent
countless multitude of the stars; yet this impression of their vast
number is deceptive, for not more than two thousand stars are usually
visible at one time.
Much, however, depends upon the keenness of vision of the observer, and
the transparency of the atmosphere. Argelander counted at Bonn more than
3,000 stars, and Hozeau, near the equator, where all the stars of the
sphere successively appear in view, enumerated 6,000 stars. This number
may be regarded as including all the stars in the heavens that are
visible to the naked eye. With the aid of an opera glass thousands of
stars can be seen that are imperceptible to ordinary vision.
Argelander, with a small telescope of 2-1/2 inches aperture, was able to
count 234,000 stars in the Northern Hemisphere. Large telescopes reveal
multitudes of stars utterly beyond the power of enumeration, nor do they
appear to diminish in number as depth after depth of space is penetrated
by powerful instruments. The star-population of the heavens has been
reckoned at 100,000,000, but this estimate is merely an assumption;
recent discoveries made by means of stellar photography indicate that
the stars exist in myriads. It is reasonable to believe that there is a
limit to the sidereal universe, but it is impossible to assign its
bounds or comprehend the apparently infinite extent of its dimensions.
Scintillation or twinkling of the stars is a property which
distinguishes them from the planets. It is due to a disturbed condition
of the atmosphere and is most apparent when a star is near the horizon;
at the zenith it almost entirely vanishes. Humboldt states that in the
clear air of Cumana, in South America, the stars do not twinkle after
they reach an elevation of 15 deg. above the horizon. The presence of
moisture in the atmosphere intensifies scintillation, and this is
usually regarded as a prognostication of rain. White stars twinkle more
than red ones. The occurrence of scintillation can be accounted for by
the fact that the stars are visible as single points of light which
twinkle as a whole, but in the case of the Sun, Moon, and planets, they
form discs from which many points of light are emitted; they,
therefore, do not scintillate as a whole, for the absence of rays of
light from one portion of their surface is compensated by those from
other parts of their discs, giving a mean average which creates a
steadiness of vision.
The stars are divided into separate classes called 'magnitudes,' by
which their relative apparent size and degree of brightness are
distinguished. The magnitude of a star does not indicate its mass or
dimensions, but its light-giving power, which depends partly upon its
size and distance, though mainly upon the intensity of its luminosity.
The most conspicuous are termed stars of the first magnitude; there are
ten of those in the Northern Hemisphere, and an equal number south of
the equator, but they are not all of the same brilliancy. Sirius
outshines every other star of the firmament, and Arcturus has no rival
in the northern heavens. The names of the first-magnitude stars north of
the equator are: Arcturus, Capella, Vega, Betelgeux, Procyon, Aldebaran,
Altair, Pollux, Regulus, and Deneb. The next class in order of
brightness are called second-magnitude stars; they are fifty or sixty in
number, the most important of which is the Pole Star. The stars diminish
in luminosity by successive gradations, and when they sink to the sixth
magnitude reach the utmost limit at which they appear visible to the
naked eye. In great telescopes this classification is carried so low as
to include stars of the eighteenth and twentieth magnitudes.
Entering into the structure of the stellar universe we have Single
Stars, Double Stars, Triple, Quadruple, and Multiple Stars, Temporary,
Periodical, and Variable Stars, Star-groups, Star-clusters, Galaxies,
and Nebulae.
SINGLE OR INSULATED STARS include all those orbs sufficiently isolated
in space so as not to be perceptibly influenced by the attraction of
other similar bodies. They are believed to constitute the centres of
planetary systems, and fulfil the purpose for which they were created by
dispensing light and heat to the worlds which circle around them.
The Sun is an example of this class of star, and constitutes the centre
of the system to which the Earth belongs. Reasoning from analogy, it
would be natural to conclude that there are other suns, numberless
beyond conception, the centres of systems of revolving worlds, and
although we are utterly unable to catch a glimpse of their planetary
attendants, even with the aid of the most powerful telescopes, yet they
have in a few instances been _felt_, and have afforded unmistakable
indications of their existence.
Since the Sun must be regarded as one of the stellar multitude that
people the regions of space, and whose surpassing splendour when
contrasted with that of other luminaries can be accounted for by his
proximity to us, it would be of interest to ascertain his relative
importance when compared with other celestial orbs which may be his
peers or his superiors in magnitude and brilliancy.
The Sun is one of a widely scattered group of stars situated in the
plane of the Milky Way and surrounded by that zone, and, as a star among
the stars, would be included in the constellation of the Centaur.
Although regarded as one of the leading orbs of the firmament, and of
supreme importance to us, astronomers are undecided whether to classify
the Sun with stars of greater magnitude and brightness, or assign him a
position among minor orbs of smaller size. Much uncertainty exists with
regard to star magnitudes. This arises from inability on the part of
astronomers to ascertain the distances of the vast majority of stars
visible to the naked eye, and also on account of inequality in their
intrinsic brilliancy. Among the stars there exists an indefinite range
of stellar magnitudes. There are many stars known whose dimensions have
been ascertained to greatly exceed those of the Sun, and there are
others of much smaller size. No approximation of the magnitude of
telescopic stars can be arrived at; many of them may rival Sirius,
Canopus, and Arcturus, in size and splendour, their apparent minuteness
being a consequence of their extreme remoteness. If the Sun were removed
a distance in space equal to that of many of the brightest stars, he
would in appearance be reduced to a minute point of light or become
altogether invisible; and there are other stars, situated at distances
still more remote, of which sufficient is known to justify us in
arriving at the conclusion that the Sun must be ranked among the minor
orbs of the firmament, and that many of the stars surpass him in
brilliancy and magnitude.
DOUBLE STARS.--To the unaided eye, these appear as single points of
light; but, when observed with a telescope of sufficient magnifying
power, their dual nature can be detected.
The first double star discovered was Mizar, the middle star of the three
in Ursa Major which form the tail of the bear. The components are of the
fourth and fifth magnitudes, of a brilliant white colour, and distant
fourteen seconds of arc.
In 1678, Cassini perceived stars which appeared as single points of
light when viewed with the naked eye, but when observed with the
telescope presented the appearance of being double.
The astronomer Bode, in 1781, published a list of eighty double stars,
and, in a few years after, Sir William Herschel discovered several
hundreds more of those objects. They are now known to exist in
thousands, Mr. Burnham, of the Lick Observatory, having, by his keen
perception of vision, contributed more than any other observer to swell
their number.
All double stars are not binaries; many of them are known as 'optical
doubles'--an impression created by two stars when almost in the same
line of vision, and, though apparently near, are situated at a great
distance apart and devoid of any physical relationship.
Binary stars consist of two suns which revolve round their common
centre of gravity, and form real dual systems.
The close proximity of the components of double stars impressed the
minds of some astronomers with the belief that a physical bond of union
existed between them. In the interval between 1718 and 1759, Bradley
detected a change of 30 deg. in the position angle of the two stars forming
Castor, and was very nearly discovering their physical connection.
In 1767, the Rev. John Michell wrote: 'It is highly probable in
particular, and next to a certainty in general, that such double stars
as appear to consist of two or more stars placed very near together do
really consist of stars placed near together and under the influence of
some general law.' Afterwards he says: 'It is not improbable that a few
years may inform us that some of the great number of double and triple
stars which have been observed by Mr. Herschel are systems of bodies
revolving about each other.' Christian Mayer, a German astronomer,
formed a list of stellar pairs, and announced, in 1776, the supposed
discovery of 'satellites' to many of the principal stars. His
observations were, however, not exact enough to lead to any useful
results, and the existence of his 'planet stars' was at that time
derided, and believed to find a place only in his imagination.
The conclusions arrived at by some astronomers with regard to double
stars were afterwards confirmed by Herschel, when, by his observation of
a change in the relative positions of many of their components, he was
able to announce that they form independent systems in mutual
revolution, and are controlled by the law of gravitation.
The number of binary stars in active revolution is known to exceed 500;
but, besides these, there are doubtless numerous other compound stars
which, on account of their extreme remoteness and the close proximity of
their components, are irresolvable into pairs by any optical appliances
which we possess.
The revolution of two suns in one sphere presents to our observation a
scheme of creative design entirely different to the single-star system
with which we are familiar--one of a higher and more complex order in
the ascending scale of celestial architecture. For, if we assume that
around each revolving sun there circles a retinue of planetary worlds,
it is obvious that a much more complicated arrangement must exist among
the orbs which enter into the formation of such a system than is found
among those which gravitate round our Sun.
The common centre of gravity of a binary system is situated on a line
between both stars, and distant from each in inverse proportion to their
respective masses. When the stars are of equal mass their orbits are of
equal dimensions, but when the mass of one star exceeds that of the
other, the orbit of the larger star is proportionately diminished as
compared with the circumference traversed by the smaller star. When
their orbits are circular--a rare occurrence--both stars pursue each
other in the same path, and invariably occupy it at diametrically
opposite points; nor is it possible for one star to approach the other
by the minutest interval of space in any duration of time, so long as
the synchronous harmony of their revolution remains undisturbed.
[Illustration: FIG. 3.--A Binary Star System--70 Ophiuchi
(_Drawn by Mr. J. E. Gore._)]
When a pair of suns move in an ellipse, their orbits intersect and are
of equal dimensions when the stars are of equal mass, their common
centre of gravity being then at a point equidistant from each.
Consequently, neither star can approach or recede from this point
without the other affecting a similar motion, they must be at periastron
and apastron together, and any acceleration or retardation of speed must
occur simultaneously with each. Stars of unequal magnitude always
maintain a proportionate distance from their common focus, and both
simultaneously occupy corresponding parts of their orbits.
The nature of the motions of those distant suns, and the form of the
orbits which they traverse, have been investigated by several eminent
astronomers, and although the subject is one of much difficulty, on
account of their extreme remoteness and the minute angles which have to
be dealt with, necessitating the carrying out of very refined
observations, yet a considerable amount of information has been obtained
with regard to the paths which they pursue in the accomplishment of
their revolutions round each other.
The orbits of about sixty stellar pairs have been computed, but only
with partial success. Some stars have shown themselves to be totally
regardless of theory and computation, and have shot ahead far beyond the
limits ascribed to them, whilst others, by the slowness of their
motions, have upset the calculations of astronomers as much in the
opposite direction. So that out of this number the orbits of not more
than half a dozen are satisfactorily known.
The dimensions of stellar orbits are of very varied extent. Some pairs
are apparently so close that the best optical means which we possess are
incapable of dividing them, whilst others revolve in wide and spacious
orbits.
The most marked peculiarity of the orbits of binary stars is their high
eccentricity; they are usually much more eccentric than are those of
the planets, and in some instances approach in form that of a comet.
The finest binary star in the northern heavens is Castor, the brighter
of the two leading stars in the constellation Gemini. The components are
of the second and third magnitudes, and over five seconds apart. They
are of a brilliant white colour, and form a beautiful object in the
telescope.
In 1719 Bradley determined the relative positions of those stars, and on
comparing the results obtained by him with recent measurements it was
found that they had altered to the extent of 125 deg.. Travelling at the
same rate of speed, they will require a period of about 420 years to
complete an entire circuit of their orbits. This pace, however, has not
been maintained, for, their periastron having occurred in 1750, they
travelled more rapidly in the last century than they are doing at
present, and, as their orbits are so eccentric that when at apastron the
stars are twice as remote from each other as at periastron, they will
for the next three and a half centuries continue to slacken their pace,
until they shall have reached the most remote points of their orbits,
when they will again begin to approach with an increasing velocity; so
that the time in which an entire revolution can be accomplished will not
be much less than 1,000 years.[8]
As the distance of Castor is unknown, it is impossible to compute the
combined mass of its components. They are very remote, their light
period being estimated at forty-four years. Castor is doubtless a more
massive orb than our Sun, and possesses a higher degree of luminosity.
Alpha Centauri, in the Southern Hemisphere, is the brightest binary, and
also the nearest known star in the heavens; its estimated distance being
twenty-five billions of miles. Both components equal stars of the first
magnitude, and are of a brilliant white colour. Since they were first
observed, in 1709, they have completed two revolutions, and are now
accomplishing a third. The eccentricity of their orbit approaches in
form that of Faye's comet, which travels round the Sun; consequently the
stars, when at apastron, are twice their periastron distance. Their
period of revolution is about eighty-eight years. The mean radius of
their orbit corresponds to a span of 1,000 millions of miles, so that
those orbs are sometimes as close to each other as Jupiter is to the
Sun, and never so far distant as Uranus.[9] Their combined mass is twice
that of the Sun, and the luminosity of each star is slightly greater.
The double star 61 Cygni--one of the nearest to our system--is believed
to be a binary the components of which move in an orbit of more spacious
dimensions than that of any other known revolving pair. Though they have
been under continuous observation since 1753, it is only within the last
few years that any orbital motion has been perceived. Some observers
are disinclined to admit the accuracy of this statement; whilst others
believe that the stars have executed a hyperbolic sweep round their
common centre of gravity and are now separating.
The radius of the orbit in which those bodies travel is sixty-five times
the distance of the Earth from the Sun; which means that they travel in
an orbit twice the width of that of the planet Neptune. It has been
estimated that they complete a revolution in about eight centuries. The
united mass of the system is about one-half that of the Sun, and in
point of luminosity they are much inferior to that orb.
The star 70 Ophiuchi (fig. 3) may be regarded as typical of a binary
system. The components are five seconds apart, and of the fourth and
sixth magnitudes. Their light period is stated to be twenty years, and
the combined mass of the system is nearly three times that of the Sun.
The pair travel in an orbit from fourteen to forty-two times the radius
of the Earth's orbit; so that when at apastron they are three times as
distant from each other as when at periastron. They complete a
revolution in eighty-eight years.
The accompanying diagram (fig. 4) is a delineation of the beautiful
orbits of the components of Gamma Virginis. These may be described as
elongated ellipses. Both stars being of equal mass, their orbits are of
equal dimensions, and their common centre of gravity at a point
equidistant from each. Any approach to, or recession from this point,
must occur simultaneously with each; they must always occupy
corresponding parts of their orbits, and be in apastron and at
periastron in the same period of time. The ellipse described by this
pair is the most eccentric of known binary orbits, and approaches in
form the path pursued by Encke's comet round the Sun. These orbs
complete a revolution in 180 years, and when in apastron are seventeen
times more remote from each other than when at periastron.
[Illustration: FIG. 4.--The Orbits of the Components of Gamma Virginis.]
From his observation of the motion of Sirius in 1844, Bessel was led to
believe that the brilliant orb was accompanied by another body, whose
gravitational attraction was responsible for the irregularities observed
in the path of the great dog-star when pursuing his journey through
space. The elements of this hypothetical body were afterwards computed
by Peters and Auwers, and its exact position assigned by Safford in
1861.
On January 31, 1862, Mr. Alvan Clarke, of Cambridgeport, Massachusetts,
when engaged in testing a recently constructed telescope of great power,
directed it on Sirius, and was enabled by good fortune to discover the
companion star at a distance of ten seconds from its primary. Since its
discovery, the star has pursued with such precision the theoretical path
previously assigned to it that astronomers have had no hesitation in
identifying it as the hypothetical body whose existence Bessel had
correctly surmised.
[Illustration: FIG. 5.--Apparent Orbit of the Companion of Sirius.
(_Drawn by Mr. Burnham._)]
The Sirian satellite is a yellow star of the eighth magnitude, and
shines with a feeble light when contrasted with the surpassing
brilliancy of its neighbour.
Astronomers were for some time in doubt as to whether the uneven motion
which characterised the path of Sirius could be ascribed to the
attraction of its obscure attendant, which presented such a marked
contrast to its primary, and several observers were inclined to believe
that the disturbing body still remained undiscovered. When, however, the
density of the lesser star became known, it was discovered that, weight
for weight, that of Sirius exceeded it only in the proportion of two to
one, though as a light-giver the great orb is believed to be 5,000 times
more luminous. The Sirian satellite revolves round its primary in about
fifty years, and at a distance twenty-eight times that of the Earth from
the Sun.
The surpassing brilliancy of Sirius as compared with that of the other
stars of the firmament has rendered it at all times an object of
interest to observers. The Egyptians worshipped the star as Sothis, and
it was believed to be the abode of the soul of Isis. The nations
inhabiting the region of the Nile commenced their year with the heliacal
rising of Sirius, and its appearance was regarded as a sure forerunner
of the rising of the great river, the fertilising flood of which was
attributed to the influence of this beautiful star. It is believed that
the Mazzaroth in Job is an allusion to this brilliant orb. Among the
Romans Sirius was regarded as a star of evil omen; its appearance above
the horizon after the summer solstice was believed to be associated with
pestilence and fevers, consequent upon the oppressive heat of the
season of the year. The _dies caniculares_, or dog-days, were reckoned
to begin twenty days before, and to continue for twenty days after, the
heliacal rising of Sirius, the dog-star. During those days a peculiar
influence was believed to exist which created diseases in men and
madness among dogs. Homer alludes to the star
'whose burning breath
Taints the red air with fevers, plagues and death.'
Sirius, which is in Canis Major (one of Orion's hunting dogs), is a far
more glorious orb than our Sun. According to recent photometric
measurements it emits seventy times the quantity of light, and is three
times more massive than the great luminary of our system. At the
distance of Sirius (fifty billions of miles) the Sun would shrink to the
dimensions of a third-magnitude star, and the light of seventy such
stars would be required to equal in appearance the brilliant radiance of
the great dog-star. The orb, with his retinue of attendant worlds--some
of which are reported as having been seen--is travelling through space
with a velocity of not less than 1,000 miles a minute.
An irregularity of motion resembling that of Sirius has been detected
with regard to Procyon, the lesser dog-star. But in this case the
companion star has not as yet been seen, though a careful search has
been made for it with the most powerful of telescopes. Should it be a
planetary body, illumined by its primary, its reflected light would not
appear visible to us, even if it were much less remote than it is.
We are able only to perceive the effulgence of brilliant suns scattered
throughout the regions of space; but besides those, there are doubtless
many faintly luminous orbs and opaque bodies of vast dimensions
occupying regions unknown to us, but by a knowledge of the existence of
which an enlarged conception is conveyed to our minds of the greatness
of the universe.
The most rapid of known revolving pairs is Delta Equulei. The components
are so close that only the finest instruments can separate them, and
this they cannot do at all times. They accomplish a revolution in eleven
and a half years. The slowest revolving pair is Zeta Aquarii. The motion
of the components is so tardy that to complete a circuit of their orbits
they require a period of about sixteen centuries. Other binary stars
have had different periods assigned to them; eleven pairs have been
computed to revolve round each other in less than fifty years, and
fifteen in less than 100 but more than fifty. There are other compound
stars whose motions appear to be much more leisurely than those just
mentioned, and although no orbital movement has, so far, been detected
among them, yet, so vast is the scale upon which the sidereal system is
constructed, that thousands of years must elapse before they can have
accomplished a revolution of their orbits.
The Pole Star is an optical double, but the components are of very
unequal magnitude. The Pole Star itself is of the second magnitude, but
its companion is only of the ninth, and on account of its minuteness is
regarded as a good test for telescopes of small aperture. Mizar, in the
constellation Ursa Major, is a beautiful double star. The components are
wide apart, and can be easily observed with a small instrument.
There is a remarkable star in the constellation of the Lyre (Epsilon
Lyrae), described as a double double. This object can just be
distinguished by a person with keen eyesight as consisting of two stars;
when observed with a telescope they appear widely separated, and each
star is seen to have a companion, the entire system forming two binary
pairs in active revolution. The pair which first cross the meridian
complete a revolution in about 2,000 years; the second pair have a more
rapid motion, and accomplish it in half that time. The two pairs are
believed to be physically connected, and revolve round their common
centre of gravity in a period of time not much under one million years.
Cor Caroli, in Canes Venatici, is a pleasing double star, the components
being of a pale white and lilac colour.
Albireo, in the constellation of the Swan, is one of the loveliest of
double stars. The larger component is of the third magnitude, and of a
golden yellow colour; the smaller of the sixth magnitude, and of a
sapphire blue.
Epsilon Booetis, known also as Mirac, and called by Admiral Smyth
'Pulcherrima,' on account of its surpassing beauty, is a delicate object
of charming appearance. The components of this lovely star are of the
third and seventh magnitudes: the primary orange, the secondary
sea-green.
The late Mr. R. A. Proctor, in describing a binary star system, writes
as follows: 'If we regard a pair of stars as forming a double sun, round
which--or, rather, round the common centre of which--other orbs revolve
as planets, we are struck by the difference between such a scheme and
our own solar system; but we find the difference yet more surprising
when we consider the possibility that in some such schemes each
component sun may have its own distinct system of dependent worlds. In
the former case the ordinary state of things would probably be such that
both suns would be above the horizon at the same time, and then,
probably, their distinctive peculiarities would only be recognisable
when one chanced to pass over the disc of the other, as our Moon passes
over the Sun's disc in eclipses. For short intervals of time, however,
at rising or setting, one or other would be visible alone; and the
phenomena of sunset and sunrise must therefore be very varied, and also
exquisitely beautiful, in worlds circling round such double suns. But
when each sun has a separate system, even more remarkable relations must
be presented. For each system of dependent worlds, besides its own
proper sun, must have another sun--less splendid, perhaps (because
farther off), but still brighter beyond comparison than our moon at the
full. And, according to the position of any planet of either system,
there will result for the time being either an interchange of suns,
instead of the change from night to day, or else double sunlight during
the day, and a corresponding intensified contrast between night and day.
Where the two suns are very unequal or very differently coloured, or
where the orbital path of each is very eccentric, so that they are
sometimes close together and at others far apart, the varieties in the
worlds circling round either, or around the common centre of both, must
be yet more remarkable. "It must be confessed," we may well say with Sir
John Herschel, "that we have here a strangely wide and novel field for
speculative excursions, and one which it is not easy to avoid
luxuriating in."'
Anyone who takes a cursory glance at the heavens on a clear night can
readily perceive that there exists considerable diversity of colour
among the stars. The contrast between some is pronounced and well
marked, whilst others exhibit refined gradations of hue.
The most numerous class of stars are those which are described as white
or colourless. They comprise about one-half of the stars visible to the
naked eye. Among the most conspicuous examples of this type are
Sirius--whose diamond blaze is sometimes mingled with an occasional
flash of blue and red--Altair, Spica, Castor, Regulus, Rigel, all the
stars of Ursa Major with the exception of one, and Vega--a glittering
gem of pale sapphire, almost colourless. The light emitted by stars of
this class gives a continuous spectrum, the predominating element being
hydrogen, having a very elevated temperature and under relatively high
pressure. The vapours of iron, sodium, magnesium, and other metals, are
indicated as existing in small quantities.
The second class of stars is that to which our Sun belongs. They are of
a yellow colour, and embrace two-thirds of the remaining stars. The most
prominent examples of this type are Arcturus, Capella, Aldebaran,
Procyon, and Pollux. Hydrogen does not predominate so much in these as
in the Sirian stars, and their spectra resemble closely the solar
spectrum, indicating that they are composed of elements similar to those
which exist in the Sun.
The star which bears the nearest resemblance to our Sun, both as regards
the colour of its light and physical structure, is Capella, the most
conspicuous star in the constellation Auriga, and one of the leading
brilliants in the Northern Hemisphere. Its spectrum presents all the
characteristics observed in the solar spectrum, and there exists an
almost identical similarity in their physical constitution, though
Capella is a much more magnificent orb than the Sun.
The third class of stars includes those which are of a ruddy hue, such
as Betelgeux in the right shoulder of Orion, Antares in Scorpio, and
Alpha Herculis. Their spectra present a banded or columnar appearance,
and there is greater absorption, especially of the blue rays of light.
It is believed that the temperature of stars of this colour is not so
elevated as that of those belonging to the other two orders, and that
this is a sufficient reason to account for the different appearance of
their spectra.
The aid of a good telescope is, however, necessary to enable us to
perceive the varied colours and tints of the sparkling gems with which
Nature has adorned her star-built edifice of the universe. Most of the
precious stones on Earth have their counterparts in the heavens,
presenting in a jewelled form contrasts of colour, pleasing harmonies,
and endless variety of shade. The diamond, sapphire, emerald, amethyst,
topaz, and ruby sparkle among crowds of stars of more sombre hue. Agate,
chalcedony, onyx, opal, beryl, lapis-lazuli, and aquamarine are
represented by the radiant sheen emanating from distant suns, displaying
an inexhaustible variety of colour, blended in tints of untold harmony.
It is among double stars that the richest and most varied colours
predominate. There are pairs of white, yellow, orange, and red stars;
yellow and blue, yellow and pale emerald, yellow and rose red, yellow
and fawn, green and gold, azure and crimson, golden and azure, orange
and emerald, orange and lilac, orange and purple, orange and green,
white and blue, white and lilac, lilac and dark purple, &c., &c. There
are companion stars revolving round their primaries, coloured olive,
lilac, russet, fawn, dun, buff, grey, and other shades indistinguishable
by any name.
Our knowledge of binary star systems brings us to what may be regarded
as the threshold of the fabric of the heavens. For it is known that
other systems exist into the construction of which numerous stars enter.
These form intricate and complex stellar arrangements, in which the
component stars are physically united and retained in their orbits by
their mutual attraction.