THE STARS.
The most remarkable progress in astronomy during the past century has
been in the department of sidereal science, or the study of the Suns of
space, observed for their own sakes, and not merely for the purpose of
determining the positions of the Sun and Moon, and to assist navigation.
Thanks to Herschel, the nineteenth century witnessed the steady
development of stellar astronomy, combined with many important
discoveries and investigations.
The one pre-Herschelian problem in sidereal astronomy was the distance
of the stars. Owing to its bearing on the Copernican theory, the problem
was attacked by the astronomers of the seventeenth and eighteenth
centuries. Herschel made numerous attempts to detect the parallax of the
brighter stars, but failed. Meanwhile there had been many illusions.
Piazzi believed that his instruments—which in reality were worn out and
unfit for use—had revealed parallaxes in Sirius, Aldebaran, Procyon, and
Vega; Calandrelli, another Italian, and _John Brinkley_ (1763-1835),
Astronomer-Royal of Ireland, were similarly deluded; and in 1821 it was
shown by _Friedrich Georg Wilhelm Struve_ (1793-1864), the great German
astronomer, that no instruments then in use could possibly be successful
in measuring the stellar parallax. A few years later, however,
Fraunhofer brought the refractor to a degree of perfection surpassing
all previous efforts. In 1829 he mounted for the observatory at
Königsberg a heliometer, the object-glass of which was divided in two,
and capable of very accurate measurements. This heliometer eventually
revealed the parallax of the stars in the able hands of Friedrich
Wilhelm Bessel.
Friedrich Wilhelm Bessel was born at Minden, on the Weser, south-west of
Hanover, on July 22, 1784. His father was an obscure Government
official, unable to provide a university education for his son. Bessel’s
love of figures, together with an aversion to Latin, led him to pursue a
commercial career. At the age of fourteen, therefore, he entered as an
apprenticed clerk the business of Kuhlenkamp & Sons, in Bremen. He was
not content, however, to remain in that humble position. His great
ambition was to become supercargo on one of the trading expeditions sent
to China; and so he learned English, Spanish, and geography. But he
never became a supercargo. In order to be fully equipped for such a
position, he determined to learn how to take observations at sea, and
his acquaintance with observation aroused a desire to study astronomy.
He constructed for himself a sextant, and by means of this, along with a
common clock, he determined the longitude of Bremen.
Such enthusiasm could not be long without its reward. For several years
Bessel remained a clerk, and the hours devoted to study were those
spared from sleep. He studied the works of Bode, Von Zach, Lalande, and
Laplace, and in two years was able to compute the orbits of comets by
means of mathematics. From some observations of Halley’s comet at its
appearance in 1607, Bessel calculated its orbit, and forwarded the
calculation to Olbers, then the greatest authority on cometary
astronomy. Olbers was delighted at this work, and he sent the results to
Von Zach, who published them. The self-taught young astronomer had
accomplished a piece of work which fifteen years before had taxed the
skill and patience of the French Academy of Sciences.
In 1805, Harding, Schröter’s assistant at Lilienthal, resigned his
position for a more promising one at Göttingen. Olbers procured for
Bessel the offer of the vacant post, which the latter accepted. At
Lilienthal Bessel received his training as a practical astronomer. He
remained in Schröter’s observatory until 1809. Although only twenty-five
years of age, he had become so well known in Germany that in that year
he was appointed Professor of Astronomy in the University of Königsberg,
and was chosen to superintend the erection of the new observatory there.
Within a few years a clerk in a commercial office had worked his way
from obscurity to fame.
In 1813 the Königsberg Observatory was completed, and here Bessel worked
for thirty-three years, until his death, on March 17, 1846. It was only
about ten years before his death that he commenced his search for the
stellar parallax, with the aid of Fraunhofer’s magnificent heliometer.
He determined to make a series of measures on a small double star of the
fifth magnitude in the constellation Cygnus, named 61 Cygni, the large
proper motion of which led him to suspect its proximity to the Solar
System. From August 1837 to September 1838 he made observations on 61
Cygni, and he found that there was an annual displacement which could
only be attributed to parallax. In order to have no mistake, he made
another year’s observations, which confirmed the results he arrived at
previously, and all doubt was removed by a third series. The resulting
parallax was 0·3483″, corresponding to a distance of 600,000 times the
Earth’s distance from the Sun. This was confirmed some years later by C.
A. F. Peters at Pulkowa, and still later by Otto Struve, who estimated
the distance at forty billions of miles. Meanwhile, F. G. W. Struve,
working at Pulkowa, found a parallax of 0·2613″ for Vega, but this was
afterwards found to be considerably in error. Accordingly, Struve does
not rank with Bessel as a successful measurer of star-distance. But
independently of Bessel, another accurate measure had been made by
_Thomas Henderson_, the great Scottish astronomer.
Born in Dundee in 1798, Thomas Henderson was the youngest of five
children of a hard-working tradesman. After education in his native town
he went to Edinburgh, where he worked for years as an advocate’s clerk,
pursuing studies in astronomy as a recreation from his boyhood. In 1831
he had become so well known, that he received the appointment of
Astronomer-Royal at the new observatory at the Cape of Good Hope. But
the climate of South Africa did not suit his health, and after a year he
returned to Scotland. In 1834 he became Professor of Astronomy in the
University of Edinburgh, and Astronomer-Royal of Scotland, which
position he held till his death on November 23, 1844, at the early age
of forty-six.
During a year’s work at the Cape, Henderson undertook a series of
observations on the bright southern star, α Centauri, with a view to
determining its parallax. These observations were made in 1832 and 1833,
but were not reduced until Henderson’s return to Scotland. At length, on
January 3, 1839, he announced to the Royal Astronomical Society that he
had succeeded in measuring the parallax of α Centauri, which he
determined as about one second of arc, corresponding to a distance of
about twenty billions of miles. This result was confirmed by the
observations of _Thomas Maclear_ (1794-1879), his successor at the Cape,
and by those of later observers, notably Sir David Gill, who has reduced
the parallax to 0·75″.
Other determinations of stellar parallax, some genuine and others
illusory, were made soon after these successful observations. C. A. F.
Peters and Otto Struve at Pulkowa were among the most famous
parallax-hunters in the middle of the century. One of the most
successful searchers after parallax was the German astronomer _Friedrich
Brünnow_ (1821-1891), who was employed from 1865 to 1874 as
Astronomer-Royal of Ireland. He determined the parallax of Vega as
0·13″, and this was confirmed in 1886 by Hall at Washington: while he
measured the parallax of the star Groombridge 1830, which turned out to
be 0·09″. He resigned his post in 1874, and his successor at Dublin
Observatory proved to be his successor also in this branch of astronomy.
_Robert Stawell Ball_, born in Dublin in 1840, was astronomer to Lord
Rosse in 1865 and 1866, and became in 1874 Astronomer-Royal of Ireland
in succession to Brünnow, a position which he filled until his
appointment in 1892 as Professor of Astronomy at Cambridge, and director
of the observatory there. During his term of office in Dublin he
undertook, in 1881, a “sweeping search” for large parallaxes, thereby
disproving certain ideas as to the proximity to the Earth of red and
temporary stars; while he also determined the parallax of the star 1618
Groombridge.
But the greatest extension of our knowledge of stellar distances, in
recent years, is due to a Scottish astronomer, who has maintained the
reputation of Scotland, and also of the Cape Observatory, in this line
of research. Born in Aberdeen in 1843, _David Gill_ directed Lord
Lindsay’s private observatory at Dunecht, in Aberdeenshire, from 1876 to
1879. In the latter year he succeeded _Edward James Stone_ (1831-1897)
as Astronomer-Royal at the Cape, a position which he has since filled
with conspicuous ability. From 1881 he has been engaged in the hunt for
parallax. In conjunction with _William Lewis Elkin_ (born 1855), now
director of Yale College Observatory, he determined the parallaxes of
nine stars with the aid of Lord Lindsay’s heliometer. In 1887, with a
larger instrument, he resumed the search, while Elkin worked in
co-operation with him, but at Yale Observatory, where he undertook the
measurement of the parallaxes of northern stars. He fixed in 1888 an
average parallax for first-magnitude stars, which was determined at
0·089″, corresponding to a journey for light of thirty-six years.
Most of the successful determinations of parallax have been made by the
“relative” method—that is, the determination of the displacement of a
star in reference to another star, assumed to be situated at an
immeasurable distance. The method of absolute parallax, on the other
hand,—the star’s displacement in right ascension and declination,—has
been seldom used, owing to the laborious reduction which has to be gone
through before the result can be reached. In 1885, however, a series of
observations were undertaken at Leyden by _Jacobus Cornelius Kapteyn_
(born 1851), who determined by the absolute method the parallaxes of
fifteen northern stars.
The first application of photography to the problem was due to the zeal
and energy of _Charles Pritchard_ (1808-1893), Professor of Astronomy at
Oxford, who determined by this method the parallax of 61 Cygni, which he
announced in 1886 to be 0·438″, in agreement with Ball’s determination.
He also determined the average parallax of second-magnitude stars, which
came out as 0·056″. Since the time of Pritchard’s observations various
other more or less satisfactory determinations of parallax have been
made. Few of the parallax determinations are probably very accurate, and
none exact; but an idea of the difficulty of the measurement may be
gathered from the remark of an American writer, Mr G. P. Serviss, that
the displacement “is about equal to the apparent distance between the
heads of two pins, placed an inch apart, and viewed from a distance of a
hundred and eighty miles.”
Closely allied to the question of parallax is the determination of the
exact positions of the stars and the formation of star-catalogues. In
this branch, too, much is due to the genius of Bessel. The observations
of Bradley at Greenwich from 1750 to 1762 were reduced by Bessel into
the form of a catalogue, which was published in 1818, with the title of
‘Fundamenta Astronomiæ.’ During the years 1821 to 1823 Bessel took
75,011 observations, by which he brought up the number of accurately
known stars to 50,000. At the same time notable catalogues had been
constructed, particularly by the English astronomer, _Francis Baily_
(1774-1844), and by _Giovanni Santini_ (1786-1877), director of the
observatory at Padua; but Bessel’s successor in this branch of research
was _Friedrich Wilhelm August Argelander_ (1799-1875). In 1821 he became
assistant to Bessel at Königsberg, in 1823 director of the Observatory
at Abo, in Finland, and in 1837 of that at Bonn. Here he commenced in
1852 the great ‘Bonn Durchmusterung,’ a catalogue and atlas of 324,198
stars visible in the northern hemisphere. The great catalogue was
published in 1863. After Argelander’s death it was extended so as to
include 133,659 stars in the southern hemisphere, by his assistant
_Eduard Schönfeld_ (1828-1891), who succeeded him in 1875 as director of
Bonn Observatory, where he died in 1891. Meanwhile a greater undertaking
was commenced in 1865 by the Astronomische Gesellschaft. This was the
co-operation of thirteen observatories in Europe and America for the
exact determination of the places of 100,000 of Argelander’s stars.
In the southern hemisphere, working at Cordova in Argentina, was the
great American astronomer, _Gould_, whose ‘Uranometria Argentina,’
published in 1879, gives the magnitudes of 8198 stars, and whose
Argentine General Catalogue, containing reference of 32,448 stars, was
published in 1886. The late Radcliffe observer, Stone, published a
useful catalogue in 1880 from his observations at the Cape.
The application of photography to the work of star-charting dates from
1882, when Gill photographed the comet of 1882, and was struck with the
distinctness of the stars on the background. For some time he had
contemplated the extension of the ‘Durchmusterung,’ from the point where
Schönfeld left it, to the southern pole, and the idea struck him to
utilise photography for the purpose. In 1885, accordingly, Gill
commenced work, and in four years all the photographs were taken. The
reduction of the observations into the form of a catalogue was
spontaneously undertaken by the great Dutch astronomer, Kapteyn, who was
occupied with the work for fourteen years, until in 1900 the great
catalogue, known as the ‘Cape Photographic Durchmusterung,’ was
completed. Half a million stars are represented on the plates taken at
the Cape.
By the time the ‘Durchmusterung’ was completed, a greater undertaking
was in progress. Paul and Prosper Henry, astronomers at the Paris
Observatory, when engaged in continuing Chacornac’s ecliptic charts,
applied photography to their work, and found it very successful.
Accordingly Gill’s proposal, on June 4, 1886, of an International
Congress of Astronomers, to undertake a photographic survey of the
heavens, was enthusiastically received by the French astronomers. The
Congress met at Paris in 1887, under the presidentship of _Amédée
Mouchez_ (1821-1892), director of the Paris Observatory, fifty-six
astronomers of all nations being present. The Congress resolved to
construct a Photographic Chart, and a Catalogue, the former containing
twenty million stars, the latter a million and a quarter. Meetings were
held in Paris in 1891, 1893, 1896, and 1900 to superintend the progress
of the work, which is now (1906) well advanced towards completion.
A unique star catalogue is in course of preparation by the Scottish
astronomer, _William Peck_ (born 1862), astronomer to the City of
Edinburgh since 1889. Mr Peck’s catalogue is accompanied by a series of
charts. His star-magnitudes are those of all famous catalogues reduced
to a standard scale. This catalogue, the result of more than fifteen
years’ work, will be an important addition to the many valuable works of
the kind already in existence, and will further increase the already
great reputation of Scotsmen in practical astronomy.
The determination of the proper motions of the stars is another
important branch of practical astronomy in which much progress has been
made since the time of Herschel. Stars with much larger proper motions
than those of the first magnitude have been discovered. For many years
the small sixth-magnitude star in Ursa Major, 1830 Groombridge, was
supposed to be the swiftest of the stars, and was named by Newcomb the
“runaway star.” But in 1897, on examining the plates of the ‘Cape
Durchmusterung,’ Kapteyn discovered a still swifter star of the eighth
magnitude, situated in the southern constellation, Pictor. The rate of
its motion is over eight seconds of arc yearly; and an idea of the vast
distance of the stars may be obtained by the statement that it would
take 200 years for the star—known as Gould’s Cordova Zones, V Hour
243—to move over a space equal to the moon’s diameter. Important
observations have been made on the stellar motions, and on their bearing
on the structure of the Universe, by various astronomers, including J.
C. Kapteyn and _Ludwig Struve_ (born 1858), son of Otto Struve; but
these must be reserved for a later chapter.
Richard Anthony Proctor, born at Chelsea, in London, in 1837, graduated
at Cambridge in 1860. For the next twenty-eight years he earned his
living by publishing many volumes on astronomy, popular and technical,
fifty-seven having appeared at the time of his death, which took place
at New York on September 12, 1888. Notwithstanding the vast amount of
work bestowed on his books, his original investigations were permanent
contributions to astronomical science. In 1870 he undertook to chart the
directions and amounts of 1600 proper motions. While engaged on this
work, it occurred to him that it would be “desirable and useful to
search for subordinate laws of motion.” He found, from the laborious
process of charting, that five of the seven stars of the Plough had a
motion in common—that is to say, were moving in the same direction at
the same rate. This phenomenon was termed by Proctor “star-drift.” He
also recognised other instances of star-drift in other portions of the
heavens.
The subject was soon afterwards taken up by the French astronomer,
Camille Flammarion. Born in 1842 at Montigny-le-Roi, in Haute Marne,
Flammarion was appointed assistant to Le Verrier in 1858, but gave up
his post in 1862. Employed successively at the Bureau des Longitudes,
and as editor of scientific papers, he founded in 1882 his private
observatory at Juvisy-sur-Orge, where he has since continued his
investigations.
Following up Proctor’s discovery of star-drift, Flammarion drew charts
of proper motions. He demonstrated the “common proper motion” of Regulus
and an eighth-magnitude star, Lalande 19,749, from a comparison of his
measures in 1877 with those of Christian Mayer a century previously;
while he discovered many other instances. His reflections on these
motions, as given in his ‘Popular Astronomy,’ are worthy of
reproduction: “Such are the stupendous motions which carry every sun,
every system, every world, all life, and all destiny in all directions
of the infinite immensity, through the boundless, bottomless abyss; in a
void for ever open, ever yawning, ever black, and ever unfathomable;
during an eternity, without days, without years, without centuries, or
measures. Such is the aspect, grand, splendid, and sublime, of the
universe which flies through space before the dazzled and stupefied gaze
of the terrestrial astronomer, born to-day to die to-morrow, on a
globule lost in the infinite night.”
Measures of proper motion only enable us to determine the motion of
stars across the line of sight. They do not tell us whether the star is
advancing or receding. Here, however, the spectroscope comes to our aid
by means of Doppler’s principle, described in the chapter on the Sun. It
occurred to Huggins that, by observing the displacement of the lines in
the spectra of the stars, he could determine their motion in the line of
sight. His first results were announced in 1868. In the case of Sirius,
the displacement of the line marked F was believed to indicate a
velocity of recession of 29 miles a second. Some time later Huggins
announced that Betelgeux, Rigel, Castor, and Regulus were retreating,
while Arcturus, Pollux, Vega, and Deneb were approaching. Soon after
this successful work the subject was taken up by Maunder at Greenwich
and by Vogel at Bothkamp; but the delicacy of the measurements prevented
satisfactory results from being reached through visual observations, and
accordingly the measurements were very discordant.
In 1887 H. C. Vogel, working at Potsdam Astrophysical Observatory,
applied photography to the measurement of radial motion. Assisted by
_Julius Scheiner_ (born 1858), he determined the radial motions of
fifty-one bright stars by photographing the stellar spectra and
measuring the photographs. Vogel found 10 miles a second to be the
average velocity of stars in the line of sight, the tendency of the eye
being to exaggerate the displacements. The swiftest of the stars
measured by Vogel proved to be Aldebaran, with a velocity of recession
of 30 miles a second. Since 1892 the subject has been pursued by Vogel
himself with the new 30-inch refractor at Potsdam, by Campbell at the
Lick Observatory, Bélopolsky at Pulkowa, and other observers. Towards
the end of 1896 Campbell undertook, with the 36-inch Lick refractor, a
series of measures on radial motion, and many important discoveries were
made. These, however, must be reserved for the chapter dealing with
double stars.
Herschel’s great discovery, from the apparent motions of the stars, of
the movement of the Solar System was not accepted by the next generation
of astronomers. Bessel declared in 1818 that there was absolutely no
evidence to show that the Sun was moving towards Hercules. Even Sir John
Herschel rejected his father’s views, although some confirmatory results
had been reached by Gauss. At length, in 1837, Argelander, in a
memorable paper, based on his observations at Abo, in Finland, attacked
the problem, and demonstrated, from a discussion of the motions of 390
stars, quite independently of Herschel’s work, that the Solar System was
moving towards Hercules. This was confirmed in 1841 by Otto Struve, in
1847 by _Thomas Galloway_, and in 1859 and 1863 by Airy and _Edwin
Dunkin_ (1821-1898), assistant at Greenwich Observatory.
Meanwhile, in 1886, _Arthur Auwers_, permanent Secretary of the Berlin
Academy of Sciences, completed the re-reduction of Bradley’s
observations at Greenwich, and brought out 300 reliable proper motions,
which were utilised by Ludwig Struve, whose investigation removed the
solar apex from Hercules to the neighbouring constellation Lyra: this
slight change was confirmed by _Oscar Stumpe_, of Bonn, and _Lewis_
_Boss_ (born 1847), director of the Observatory at Albany, New York. An
investigation by Newcomb fully confirmed the previous results. In 1900,
1901, and 1902 Kapteyn made three distinct investigations on the solar
motion, and still further confirmed the previous investigations.
These investigations are fully confirmed by the application to the
question of Doppler’s principle of measuring radial motion. The
spectroscopic researches of Campbell at the Lick Observatory place the
solar apex very near the position assigned to it by Newcomb and Kapteyn.
Campbell finds the solar velocity to be about 12 miles a second, and
Kapteyn thinks a velocity of about 11 miles a second is “the most
probable value that can at present be adopted.”