A Century's Progress in Astronomy :: CHAPTER XI. STELLAR SYSTEMS AND NEBULAE.

CHAPTER XI. STELLAR SYSTEMS AND NEBULAE.

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 ? 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 ? AndromedÆ, discovered in 1842, and d 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 (? 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 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, ? Cancri, disclosed the existence of three stars revolving round a dark body, apparently the most massive in the system. The system of ? 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 ? Orionis and Rigel to ?, and joining with the great nebula surrounding ?; 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.

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