STELLAR SYSTEMS AND NEBULÆ.
The study of double stars, commenced by Herschel, was taken up after his
death by several of the foremost astronomers, and has since been pursued
by quite a number of observers and computers. Herschel’s immediate
successor in the study of double stars was his son, who ranks only
second to his father as a student of stellar systems. Born at Slough on
March 7, 1792, John Frederick William Herschel passed his childhood
“within the shadow of the great telescope.” Although his early life was
spent with his father and aunt, astronomy does not appear to have taken
up his attention as a boy. Chemistry, however, always interested him,
and, as his aunt recorded, even while a child he was fond of making
experiments. He was educated at Hitcham, and afterwards at Eton. He was
delicate, however, so his mother removed him from school, and he was
trained at Slough by Mr Rogers, a Scottish mathematician. At the age of
seventeen Herschel entered the University of Cambridge, and Caroline
Herschel, who was exceedingly proud of him, recorded in her memoirs that
he gained all the first prizes without exception. He left the University
in 1813.
John Herschel did not turn his attention to astronomy until he had
attained the age of twenty-four. In a letter to a friend, September 10,
1816, he said, “I am going, under my father’s directions, to take up
star-gazing.” It was only reverence for his father that made him turn to
astronomy, and he gave up the science he loved most—chemistry. But his
unselfishness received its reward. In 1820 John Herschel constructed his
first reflector under his father’s guidance. Four years previously he
had begun to observe double stars, which had been for long studied by
his father, who discovered their revolutions. These observations were
continued from 1821 to 1823 at the Observatory of Sir _James South_
(1786-1867). John Herschel and South measured 380 of the elder
Herschel’s double stars. These investigations gained for Herschel and
South the Lalande Prize of the French Academy and the Gold Medal of the
Royal Astronomical Society.
When his mother died Sir John Herschel decided to sail to the Cape of
Good Hope to make an investigation of the stars of the southern
hemisphere, which until then had been much neglected. He was offered a
free passage in a ship of war, but declined. In November 1833 he left
England, taking with him his great telescopes. In two months he arrived
at Cape Town, and erected his astronomical instruments at Feldhausen, a
short distance off. In October 1835 he informed his aunt that he had
almost completed his survey of the southern hemisphere. During his
“sweeps” of the heavens he discovered 1202 double stars, and 1708 nebulæ
and star-clusters. In 1838 he returned to England, and devoted the
remainder of his life to the publication of his results, as well as to
other branches of science. He died at Collingwood, in Kent, on May 11,
1871, at the age of seventy-nine.
John Herschel’s favourite objects of study were double stars, of which
he discovered 3347 in the northern hemisphere, and 1202 in the southern.
He also computed several stellar orbits; but the first calculation of a
stellar orbit was made by the French astronomer _Felix Savary_
(1797-1841), who computed the orbit of ξ Ursæ Majoris, and found the
period to be about sixty years. Contemporary with John Herschel was his
great rival in double-star astronomy, Friedrich Georg Wilhelm Struve.
Born at Altona in 1793, Struve took his degree in 1811 at the Russian
University of Dorpat. In 1813 he became director of the Dorpat
Observatory, and was in 1839 promoted to Pulkowa, as director of the
great Observatory there, remaining at its head until within three years
of his death, on November 23, 1864. Struve’s first recorded observation
was on the double star Castor. In 1819 he commenced to measure the
position-angles of double stars, of which he published a catalogue of
795. In 1825 he commenced a review of the heavens down to fifteen
degrees south, and thus discovered 2200 previously unknown objects. The
results were published in Struve’s ‘Mensuræ Merometricæ,’ which appeared
in 1836, giving the places, distances, colours, position-angles, and
relative brilliance of 3112 double and multiple stars.
Struve’s successor in this branch of astronomy was his son, Otto Wilhelm
von Struve, born in 1819 at Dorpat, who became in 1837 assistant to his
father, and in 1861 succeeded him as director of the Pulkowa
Observatory. In 1890 he retired from this post, settling in Germany, at
Carlsruhe, where, on April 14, 1905, he died in his eighty-sixth year.
Otto Struve detected 500 double stars, among them γ Andromedæ,
discovered in 1842, and δ Equulei, discovered in 1852, within a period
of between five and eleven years.
Various other astronomers have devoted themselves to the observation of
double stars, among them _Ercole Dembowski_ (1815-1881), of Milan; _Karl
Hermann Struve_ (born 1854), son of Otto Struve; _William Doberck_ (born
1845); _William J. Hussey_ (born 1864), now director of the Detroit
Observatory; Camille Flammarion; N. C. Dunér; G. V. Schiaparelli;
_Thomas Jefferson Jackson See_ (born 1866). But the greatest living
_discoverer_ is _Sherburne Wesley Burnham_ (born 1838), now employed at
the Yerkes Observatory, in Wisconsin. Born in 1838 at Thetford, Vermont,
he commenced his career as a shorthand reporter, studying astronomy in
his leisure hours. With a small 6-inch refractor, mounted in a home-made
observatory, Burnham commenced in 1871 his discoveries of double stars,
which soon attracted the attention of noted astronomers, who permitted
him to use larger telescopes, with which he continued his researches.
His first official appointment was in 1888, when he became chief
assistant at the Lick Observatory, which position he resigned in 1892.
Some years later he became astronomer in the Yerkes Observatory.
Altogether he has discovered 1308 double stars, with telescopes ranging
from a 6-inch refractor to the gigantic 40-inch of the Yerkes
Observatory.
The computation of double-star orbits has been undertaken by various
astronomers, among them Mädler, Klinkerfues, Dunér, Flammarion,
Seeliger, See, Gore, Burnham, _Robert Grant Aitken_ (born 1864) of the
Lick Observatory, and _Giovanni Celoria_ (born 1842), who was, from 1866
to 1900, assistant in the Brera Observatory of Milan, and since 1900
director of that institution. On June 9, 1890, Gore presented to the
Royal Irish Academy a catalogue of computed binaries containing
reference to fifty-nine stars.
In 1844 Bessel discovered a remarkable irregularity in the proper motion
of Sirius. He ascribed this to the gravitational influence of some
obscure body, probably a large satellite. In 1857 Peters calculated an
orbit for the supposed satellite with a period of 50 years. In 1861 an
orbit was computed by _Truman Henry Safford_ (1836-1901), which
indicated the position of the satellite. Close to this position it was
accidentally discovered by _Alvan Clark_ (1832-1897), the famous
American optician. The period of the star seems to be about 50 years. In
1844 Bessel noticed irregularities in the proper motion of Procyon, and
put forward the idea of a disturbing satellite, as in the case of
Sirius. This was confirmed by Mädler, and in 1874 an orbit was computed
by Auwers, who found a period of 40 years. In 1896 the satellite was
found by Schaeberle with the 36-inch refractor of the Lick Observatory.
A period of 40 years was found by See, in agreement with the
hypothetical orbit.
In putting forward these theories as to invisible stellar satellites,
Bessel remarked that “light is no real property of mass,” and that the
existence of countless visible stars is nothing against the existence of
countless invisible and dark ones. In this he laid the foundation of the
branch of science termed by Mädler the “Astronomy of the invisible.” In
recent years the astronomy of the invisible has become a recognised
branch of astronomical research, through the application and
interpretation of Doppler’s principle in spectroscopic observations. In
the course of photographing the stellar spectra for the Draper
Catalogue, E. C. Pickering photographed the spectrum of Mizar (ζ Ursæ
Majoris) in 1887 and again in 1889. On some of these photographs the
line K was seen double, while on others it was seen under its normal
aspect. This doubling of the lines indicated that the star which we see
as single is in reality composed of two bodies in revolution round their
centre of gravity, so close together that even the largest telescopes
cannot divide them. Pickering assigned a period of 104 days, but in 1901
Vogel diminished this to 20 days. In the same year the star β Aurigæ was
similarly found to be double; and in 1890 Vogel, from photographs taken
at Potsdam, independently inaugurated the discovery of spectroscopic
binaries. In the spectrum of Spica he discovered the spectral lines to
be, not doubled, but periodically displaced, indicating the existence of
a dark or nearly dark companion, both stars revolving round their centre
of gravity. Spica was seen to belong to the same class as Algol, only
that in the case of Algol the plane of the satellite’s orbit passes
through the Earth and eclipses the star, while in the case of Spica the
orbit is inclined, and the star is constant in light.
The line of research commenced by Vogel and Pickering was soon followed
up by these investigators, as well as by Bélopolsky at Pulkowa, Campbell
at the Lick Observatory, Slipher at the Lowell Observatory, and by
_Edwin Brant Frost_ (born 1866), now director of the Yerkes Observatory,
and his assistant, _Walter Adams_. In 1894 Bélopolsky discovered the
duplicity of several variable stars, and in 1896 that of Castor, in
Gemini. Late in 1896 Campbell undertook a systematic investigation of
radial motions, and has since discovered about sixty spectroscopic
binaries,—among them, in 1899, the Pole Star, and in 1900 Capella. The
latter discovery was made independently by _Hugh Frank Newall_ (born
1857) at Cambridge, in England. It was found by Campbell that the
revolution of the stars round their centre of gravity is performed in
104 days; and it soon became apparent that, owing to the large size of
the orbit, the duplicity of Capella might be observed telescopically. At
Greenwich the star was seen to be elongated, but at the Lick Observatory
it was seen persistently single.
Campbell finds that of 285 stars observed by him, more than one in nine
is a spectroscopic binary. He concludes that at least one star in five
or six will be found to be spectroscopically double, and considers that
“the proven existence of so large a number of stellar systems, differing
so widely in structure from the Solar System, gives rise to a suspicion
at least that our system is not of the prevailing type of stellar
systems.”
The study of triple and multiple stars is of deep interest, but the
orbits of these objects cannot be said to be fully investigated by any
means. The first application of the problem of three bodies to stellar
astronomy was made by Seeliger in 1889. His researches, relating to the
famous star, ζ Cancri, disclosed the existence of three stars revolving
round a dark body, apparently the most massive in the system. The system
of ζ Cancri, at least, seems to be modelled on the Ptolemaic design.
In the study of star-clusters and nebulæ, as in the investigation of
double stars, Herschel’s successor was his son. His observations, both
in England and at the Cape of Good Hope, resulted in a large number of
new discoveries, and the results of his studies in this direction were
published in 1864 in his catalogue of all known clusters and nebulæ,
amounting to 5079. This catalogue was enlarged and revised in 1888 by
_John Louis Emil Dreyer_ (born 1852), a Danish astronomer, but director
of the Observatory at Armagh, in Ireland; and the same observer
published from 1888 to 1894 a supplementary list, bringing the number of
known clusters and nebulæ to about 10,000.
In the early part of his career, John Herschel held firmly to the views
of his father of the difference between star-clusters and nebulæ,
considering the latter to be composed of “shining fluid.” But he fell
off from this view with the resolution into stars of many irresolvable
nebulæ. In 1845 _William Parsons_, third _Earl of Rosse_ (1800-1867),
erected at Birr Castle, in Ireland, his great 6-foot reflector, which
still surpasses all other telescopes in point of size. With this
instrument Lord Rosse believed himself to have resolved the Crab nebula
in Taurus and the Nebula in Orion, which was also said to have been
resolved by Bond with the 15-inch refractor at Harvard; and in 1854
Olmsted declared the “resolution” of these nebulæ to be the signal for
the renunciation of Herschel’s nebular theory. Most astronomers fell in
with the view that all the nebulæ were distant clusters, which would
eventually be resolved into stars, although it is only right to state
that the Scottish astronomer, _John Pringle Nichol_ (1804-1859), and
some other investigators, held to the theory of Herschel.
The solution of the great problem was in 1864, when on August 29 of that
year Huggins turned his spectroscope on a bright planetary nebula in
Draco. To his amazement the spectrum was one of bright lines, proving
conclusively that the nebula was not a star-cluster, but a mass of
glowing gas,—hydrogen, and some other unknown substance, now named
“nebulium.” By 1868 Huggins had observed the spectra of seventy nebulæ.
Of these one-third proved to be gaseous, among them the great Orion
nebula which Lord Rosse was believed to have resolved into stars. In the
spectrum of the latter, the “chief nebular line” was at first ascribed
by Huggins to nitrogen, but this was a mistake. Later, it was believed
by Lockyer to coincide with the fluting of magnesium, but this was
disproved by Huggins in 1889-90, and by Keeler in 1890-91. The great
nebula in Andromeda and the great spiral in Canes Venatici were found by
Huggins to display a continuous spectrum, and a similar discovery was
made in regard to the cluster M 13 in Hercules, and other star-clusters.
In the case of the nebulæ, it is not believed that the continuous
spectrum is due to the existence of sun-like bodies, as a gas under
pressure would give a continuous spectrum.
The Orion nebula has been more thoroughly studied than any other object
of its class. The application of photography to spectroscopy has done
much to further the study of the lines in the nebular spectrum. In 1886
Copeland detected in the spectrum of the Orion nebula the yellow ray of
helium. On February 13, 1890, Scheiner announced an important discovery,
namely, the possession by both the nebula and the stars in Orion—with
the exception of Betelgeux—of a line, which appeared bright in the
nebular spectra and dark in the stellar. This line was identified by
Vogel, Lockyer, and others with that of helium.
Nebular photography was inaugurated in 1880 by Draper, who obtained a
remarkably good representation of the Orion nebula in that year. His
work in this direction, cut short by his death in 1882, was taken up by
Janssen at Meudon, and by Common in England, who obtained, in 1883,
several excellent photographs. Later photographs have shown the Orion
nebula to be much more extended than visual observations would lead one
to expect. A photograph secured in 1890 by W. H. Pickering revealed the
nebulous matter in Orion in its true form, that of a gigantic spiral,
starting from near Bellatrix, sweeping past κ Orionis and Rigel to η,
and joining with the great nebula surrounding θ; the entire
constellation being thus shown to be enwrapped in nebulous haze.
In 1885 nebular photography was commenced by _Isaac Roberts_
(1829-1904), the English amateur astronomer, who secured admirable
representations of clusters and nebulæ. He published, in 1893 and 1900,
two volumes of collected photographs of clusters and nebulæ. This
monumental work was thus referred to by Dr _William James Lockyer_: “Dr
Roberts has not only nobly enriched astronomical science, but has raised
a monument to himself which will last as long as astronomy has any
interest for mankind.”
Perhaps the most remarkable revelation made by photography in this
branch of research has been the discovery of the nebulæ in the Pleiades.
In 1859 Tempel observed at Florence an elliptical nebula south of the
star Merope. On November 16, 1885, the brothers Henry obtained at Paris
a photograph of the Pleiades, revealing the existence of a small spiral
nebula. This was confirmed by visual observations, and particularly by
the photographs of Roberts, which also showed the entire cluster to be
nebulous, and that “the nebulosity extends in streamers and fleecy
masses, till it seems almost to fill the spaces between the stars, and
to extend far beyond them.” In 1888 a further advance was made by the
brothers Henry, who found seven stars to be strung on a nebulous streak.
Since 1890 nebular photography has been pursued by Max Wolf in Germany,
and by E. E. Barnard and J. E. Keeler in America. Wolf’s photographs of
the constellation Cygnus brought out the close connection between the
stars and the extensively diffused nebulosities discovered by him. In
1901 Wolf discovered a “nebelhaufen” or cluster of nebulæ, and in 1902
published a catalogue of 1528 nebulæ round the pole of the Galaxy,
showing them to be systematically distributed. Keeler made his memorable
observations with the great 36-inch reflecting telescope, which was
constructed in England many years ago by Common. It afterwards passed
into the hands of Mr Crossley of Halifax, who presented it to the Lick
Observatory. With this great instrument Keeler commenced to take
photographs of the heavens. On one occasion he photographed a well-known
nebula, and on developing the plate was surprised to find seven new
nebulæ besides that which he had photographed. On another occasion he
exposed a plate to a nebula in Pegasus. He was amazed to find altogether
twenty-one nebulæ included in the photograph. To give another instance,
a plate directed to the constellation Andromeda contained no fewer than
thirty-two nebulous objects. This has given an enormous extension to our
knowledge of the nebulæ. But even this is not all. Keeler found on his
plates numerous points of light which seem to be also nebulæ, either too
small or too remote to appear as such. Apparently, however, they are not
stars. Keeler’s work convinced him that, on a modest estimate, there
must be at least _one hundred and twenty thousand_ new nebulæ within
reach of the Crossley reflector. Half of these, he announced, were
probably spiral. An idea of the vast importance of Keeler’s work may be
gained if we reflect that the observations of all the earlier
astronomers resulted in the discovery of six thousand nebulæ. The
investigations of Keeler, in all probability, were the means of adding
120,000 more.
Many observations have been made on nebulæ, for the purpose of
ascertaining their proper motions—but without success. Measurements were
made by D’Arrest in 1857 and by Burnham in 1891, but none of these
revealed any motion of the nebulæ across the line of sight. Even the new
spectroscopic method of determining motions in the line of sight, in the
hands of Huggins, failed in the case of the nebulæ. With the great Lick
refractor at his disposal, Keeler attacked the subject in 1890, and
measured the radial velocities of ten nebulæ. He found that the
well-known planetary nebula in Draco was moving towards the Solar System
at the rate of 40 miles a second; for the Orion nebula he found a motion
of recession of 11 miles a second; but probably this belongs chiefly to
the movement of the Solar System in the opposite direction.
Unfortunately Keeler did not live to carry on his investigations in
nebular astronomy. His early death brought to an abrupt end these
fruitful investigations. Appointed director of the Lick Observatory in
1898, he died suddenly at San Francisco on August 12, 1900, at the early
age of forty-two.