CHAPTER X.
THE LIGHT OF THE STARS.
“That a science of stellar chemistry should not only have become possible, but should already have made material advances, is assuredly one of the most amazing features in the swift progress of knowledge our age has witnessed.” So writes Miss Agnes Mary Clerke, the historian of modern astronomy. As long ago as 1823 Fraunhofer observed the spectra of the brighter stars, and gathered the first hint of the grouping of the stars into three classes. Then, after Fraunhofer’s death, the subject lay in abeyance for thirty-seven years. At length, in 1860, on Kirchhoff’s explanation of the Fraunhofer lines, the study of stellar spectra was inaugurated at Florence by Donati, who carefully fixed the positions of the more important lines. His instrumental means, however, were very limited, and his observations were not successful. In 1862 Rutherfurd, in New York, commenced the study of stellar spectra, but shortly afterwards turned his attention to astronomical photography. The actual founders of stellar spectroscopy were the eminent Italian observer, Angelo Secchi, and the illustrious Englishman, William Huggins.
Angelo Secchi was born in 1818 at Reggio, in the Emilia. Educated in the Collegio Romano, he was ordained priest in 1847, but his love of science, and particularly astronomy, dates from the beginning of his career. In 1849 he succeeded Di Vico as director of the Observatory of the Collegio Romano. This post he filled with conspicuous ability for a period of twenty-nine years, until his death on February 26, 1878. To Secchi is due the credit of the first spectroscopic survey of the heavens. He reviewed the spectra of 4000 stars, and classified them into four distinct groups, which are recognised to this day. The first type embraces over half of those which Secchi examined. This type is represented by Sirius, Vega, Altair, and other bluish-white stars, and is characterised by the intensity of the hydrogen lines. The second type embraces the yellow stars, such as Capella, Arcturus, Aldebaran, Pollux, and the Sun itself, and is known as the Solar type. The spectra of these stars closely resemble that of the Sun, and are distinguished by innumerable lines. Secchi’s third type, or red stars, represented by Betelgeux, Antares, and others, are characterised by strong absorption bands, and the spectra have been described as “fluted.” The third-type stars are comparatively scarce compared with the first and second, and the fourth is even less numerous. The fourth-type stars are also red with broad absorption lines. To Secchi’s four types a fifth was added in 1867 by Wolf and Rayet of Paris Observatory—namely, the gaseous stars. Secchi aimed at a comprehensive survey of the stellar spectra, and he accomplished much valuable work. He did not devote his time to analysing individual stars. This branch of study—analysis of spectra and the determination of the elements in the stars—was undertaken by his contemporary, William Huggins, one of the greatest astronomers whom England has ever produced.
Born in London in 1824, William Huggins commenced his astronomical researches at the age of twenty-eight. In 1856 he erected, at Tulse Hill, London, an observatory which he equipped at great expense. He commenced observations on the usual astronomical lines, taking times of transits and making drawings of the surfaces of the planets. But he soon tired of the routine of ordinary astronomical work, and on the publication of Kirchhoff’s explanation of the Fraunhofer lines in the solar spectrum, he commenced to investigate the spectra of the stars. Having constructed a suitable spectroscope, he commenced observations in 1862 in conjunction with his friend, William Allen Miller, Professor of Chemistry in London. He exhaustively investigated the two red stars, Betelgeux and Aldebaran, ascertaining the existence in the former star of sodium, iron, calcium, magnesium, and bismuth; and in the latter star the same elements, with the addition of tellurium, antimony, and mercury.
In 1863 Huggins made an attempt to photograph the spectra of the stars, and, indeed, obtained prints of Sirius and Capella, but no lines were visible in them. In 1874 Draper of New York obtained a photograph of the spectrum of Vega, showing four lines. Two years later Huggins again attacked the problem, and secured a photograph of the spectrum of Vega, showing seven strong lines. In 1879 he was enabled to communicate satisfactory results of his work to the Royal Society, and since then he has secured many admirable representations. In 1899 the monumental work, ‘An Atlas of Representative Stellar Spectra,’ the joint work of Sir William and Lady Huggins, was published.
In 1874 the German Government established at Potsdam the Astrophysical Observatory, for the spectroscopic study of the Sun and stars. A position on the staff was given to Hermann Carl Vogel, whose researches in astronomical spectroscopy rank with those of Secchi and Huggins. Born in Leipzig in 1842, he was from 1865 to 1869 employed in the Leipzig Observatory. Called to Bothkamp as director in 1870, he resigned his post in 1874 to accept a position on the staff at Potsdam Observatory. In 1882 he became director of that Institution, which position he still retains.
In 1874 Vogel revised Secchi’s classification of stellar spectra, and in 1895 he further improved on it. His classification improves rather than supersedes the previous work of Secchi; nevertheless, he approached the question from a different standpoint. Vogel concluded in 1874 that a rational scheme of stellar classification “can only be arrived at by proceeding from the standpoint that the phrase of development of the particular body is, in general, mirrored in its spectrum.” Vogel divides Secchi’s first type into three classes. In the first type, designated Ia,—represented by Sirius and Vega,—the metallic lines are “very faint and fine,” and the hydrogen lines conspicuous. In Ib no hydrogen lines are visible, while in Ic the hydrogen lines are bright. This class includes the gaseous stars. In 1895, after the recognition of helium in the stars by his assistant, Scheiner, Vogel separated the stars of class Ib from the first type altogether. These stars are sometimes designated as “Type O,” and sometimes as helium stars and Orion stars, as the majority of the stars in Orion are of that type. The solar type is divided into two classes, IIa being represented by the Sun, Capella, and other well-known stars, while IIb includes the Wolf-Rayet stars. Secchi’s third and fourth types are both classified by Vogel as of the third type. These red stars were specially studied from 1878 to 1884 by Dunér at Lund. His results were published in a descriptive catalogue which appeared at Stockholm in 1884. His researches related to the spectra of 352 stars, 297 of Secchi’s third type and 55 of his fourth. Dunér is perhaps the greatest authority on stars with banded spectra.
Vogel’s classification of spectra is generally adopted by astronomers, although others have been proposed by Lockyer and by Edward Charles Pickering (born 1846), director of the Harvard Observatory. Lockyer’s classification was designed to fit in with his “meteoritic hypothesis,” discussed in the chapter on Celestial Evolution. The stars were divided by Lockyer into seven groups, according to his views of their temperature, rising through gaseous stars, red stars of Secchi’s third type, and a division of solar stars to the Sirian type, and falling through a second division of the solar type to red stars of Secchi’s fourth type.
The first spectroscopic star-catalogue was published in 1883 by Vogel, assisted by Gustav Müller (born 1851), a son-in-law of Spörer. The catalogue contained details of 4051 stars to the seventh magnitude, and more than half of these proved to be of Secchi’s first type. Vogel’s work was completed in different latitudes by Dunér at Upsala, and by Nicolaus Thege von Konkoly (born 1842) at O’Gyalla in Hungary.
The famous ‘Draper Catalogue’ ranks as the greatest catalogue of stellar spectra. It was undertaken at Harvard Observatory by E. C. Pickering, in the form of a memorial to Henry Draper, the successful spectroscopist. Commenced in 1886, and published in 1890, it contains photographs of the spectra of no fewer than 10,351 stars, down to the eighth magnitude. Pickering subdivided Secchi’s types into various classes, the first or Sirian into four classes, the second into eight, while the third and fourth types each constitute a separate class. Pickering designated his classes by the capital letters of the alphabet.
Much useful work has been done also in the analysis of the various spectra. Julius Scheiner, now “chief observer” at Potsdam Astrophysical Observatory, has, since 1890, done much valuable work in this direction. Special attention was devoted to the spectrum of Capella, 490 lines in the spectrum of which were measured by Scheiner. In his own words, “he believes a complete proof of the absolute agreement between its spectrum and that of the Sun to be thereby furnished.” Other stars of the Sirian and solar classes were exhaustively studied by Scheiner.
The study of the exact brilliance of the stars was a branch of research long neglected, yet it is of much importance in astronomy, for it is only through exact measurement of stellar brilliance that stellar variation can be detected. Herschel commenced the study, which was continued by his son at the Cape, but it is only within the last twenty years that stellar photometry has become a recognised branch of astronomy; and the credit of this is due to the energy and zeal of the great American observer, Edward Charles Pickering.
Born in Boston in 1846, Edward Charles Pickering was appointed in 1865 Instructor of mathematics in the Lawrence Scientific School at Harvard, after a distinguished university career. In 1876 he succeeded Winlock as director of the Harvard Observatory, and in the following year he commenced his photometric studies. He invented an instrument named the meridian photometer, with the aid of which he succeeded in determining, in the years 1879 to 1882, the exact brilliance of 4260 stars to the sixth magnitude between the north celestial pole and thirty degrees of south declination. At a later date he devised a larger photometer, with which he made over one million observations. Pickering next extended his survey to the southern hemisphere, erecting the photometer on the slope of the Andes, where the Harvard auxiliary station at Arequipa is now located, and where 8000 determinations of stellar brilliance were made. Meanwhile Pritchard, at Oxford, published in 1885 his ‘Uranometria Nova Oxoniensis,’ with photometric determinations of the brilliance of 2784 stars from the pole to ten degrees of south declination. Both of these catalogues were epoch-making works, and testify to the enthusiasm and perseverance of the astronomers who designed them.
The study of stellar photometry glides into that of stellar variation. At the beginning of the nineteenth century the number of known variable stars was very small, as a glance at the list given in Brewster’s edition of Ferguson’s Astronomy (1811) will show. Some remarkable investigations were due to the English astronomer, John Goodricke (1764-1786), who rediscovered the variability of the star Algol, and accurately determined its period in 1782. Goodricke suggested that the regular variations in the light of Algol were due to the partial eclipse of its light by a dark satellite, a hypothesis now fully confirmed. Two years later, in 1784, Goodricke discovered other two variables, δ Cephei and β Lyræ. He died in 1786 at the age of twenty-one, and thus variable-star astronomy was deprived of its founder.
The foundation of variable-star astronomy as an exact branch of the science is due to Argelander. In the years 1837-1845, while residing at Bonn during the erection of the observatory, of which he had been made director, he erected a temporary observatory, and there he carefully determined the magnitudes of all stars visible in Central Europe. From this he was led to the discussion of stellar variation, to which subject he continued to give much attention. He was the first to describe a method of observing variable stars scientifically and accurately,—a method consisting in estimating in “steps” or “grades” the difference in brilliance between the variable, or suspected variable, and other stars which are selected for comparison, and which are of various degrees of brilliance, so that they may be available for comparison with the variable throughout its fluctuations. Argelander’s “steps” are tenths of a magnitude, and Gore describes the method of observation as follows: “If we call a and b the comparison stars, and v the variable, a being brighter than b, and if v is judged to be midway in brightness between a and b, we write a5v5b. If v is slightly nearer to b, we write a6v4b. We may also write a3v7b, or a7v3b, the sum of the steps being always ten.”
This method, described in 1844, led to many discoveries at Bonn in the following twenty years by Argelander and his assistants Schmidt and Schönfeld. At this time Eduard Heis (1806-1877), at Münster, who also ranks as one of the founders of meteoric astronomy, while engaged on the construction of his great atlas, attentively determined the change of magnitude of stars visible to the naked eye; and by means of the naked eye, the opera-glass, and a small telescope, he amassed a large number of observations. At the same time two English observers, Hind and Pogson, were making remarkable discoveries which greatly increased the number of known variables. Among Hind’s discoveries were S Cancri of the Algol type; while Schmidt discovered another of the same class, δ Libræ, and also the famous ζ Geminorum. While director of the Observatory of Mannheim, an institution equipped with very antiquated instruments, Schönfeld devoted himself to the study of variable stars, and increased the number of known variables considerably. In the southern hemisphere Gould, in South America, did for the observation of variable stars what Argelander did in the northern.
In 1874 a very important, although not so obvious, service to variable-star astronomy was rendered by the Danish observer, Hans Carl Fredrik Christian Schjellerup (1827-1887). He translated from Arabic into French the works of the Persian astronomer of a thousand years ago, Al-Sufi, and thus rendered his observations available to modern astronomers. Al-Sufi was a most accurate observer, and, by comparing his catalogue with those of modern observers, it can be found whether stars have changed in brilliance during the past thousand years.
The study of variable stars has been pursued in recent years by many astronomers, both by means of photography and by the visual method. The most important names in the visual discovery of variables are Gustav Müller and Paul Friedrich Ferdinand Kempf (born 1856) of Potsdam; Alexander William Roberts of Lovedale, South Africa; Seth Carlo Chandler of Boston; Nils Christopher Dunér at Upsala; and John Ellard Gore (born 1845) in Dublin.
The researches of J. E. Gore are a brilliant example of how much may be done for astronomy by means of very moderate instruments. Born in 1845 at Athlone, in Connaught, he went to India in 1868 as engineer on the Sirhind Canal in the Punjab. While in India he erected his small telescopes on brick pillars, and took observations, many of them of stellar brilliance. In 1879 he returned to Ireland, and since then has devoted himself to astronomy with zeal and enthusiasm. His discoveries and investigations of variables have been made by means of the binocular. On December 13, 1885, he discovered a remarkable star in Orion, which was at first considered to be temporary, and called “Nova Orionis,” but was afterwards found to be a long-period variable star.
Recently photography has come much to the front in the discovery of variable stars. Pickering at Harvard, and Wolf at Heidelberg, have particularly distinguished themselves in this branch, and the number of known variables is now very large, as every year brings fresh discoveries, mostly by aid of photography. Many of these newly-discovered variables are in star-clusters and nebulæ.
Pickering proposed in 1880 the following classification of variable stars, which has been adopted all over the scientific world: Class I., temporary star; Class II., stars undergoing in several months large variations, such as Mira Ceti and U Orionis; Class III., irregular variables, such as Betelgeux and α Herculis; Class IV., short-period variables, such as δ Cephei, ζ Geminorum, and β Lyræ; Class V., “Algol variables,” which undergo variations lasting but a few hours. It is doubtful whether new stars should be included in a classification of variables, although in one case, at least, a new star was found to be a long-period variable. To these a sixth class may now be added. This class, the detection of which is mainly due to the profound investigations of Gore, is composed of what have been termed “secular variables,” which undergo slow fluctuations in periods of many years, and sometimes of centuries. This Class includes δ Ursæ Majoris, Al-Fard, λ Draconis, θ Serpentis, ε Pegasi, 83 Ursæ Majoris, ζ Piscis Australis, β Leonis, α Ophiuchi, η Crateris, and others. The secular variations of some of these stars have been detected by Gore himself during the past thirty years, while in other cases he has detected them by comparison of the most important star-catalogues, from Hipparchus and Al-Sufi down to our own time. In some cases the star in question seems to be slowly gaining in brilliance, in others slowly diminishing.
Thanks to the application of the spectroscope, much is now known of the cause of the light changes in variable stars. Goodricke’s theory of the variations of Algol was theoretically confirmed by the researches of E. C. Pickering in 1880. In 1889 Vogel proved beyond a doubt that the variation in the light of Algol is due to the partial eclipse of its light by a dark satellite. It was obvious to Vogel that, as both Algol and its companion are in revolution round their common centre of gravity, the motion of Algol in the line of sight might be detected by the spectroscopic method of observation. Vogel found that before each eclipse Algol was retreating from our system, while on recovering it gave signs of rapid approach, proving conclusively that both the star and its dark satellite were in revolution round their centre of gravity,—Algol suffering partial eclipse only because the plane of the orbit lies in our line of sight. Algol, therefore, is not inherently a variable star, but merely a binary. Following up his researches, Vogel, assuming that the bright and dark stars are of equal density, arrived at the conclusion that Algol is a globe about one and a half million miles in diameter, the satellite equalling the size of the Sun, and the centres of the stars being separated by about 3,230,000 miles. Thus, variable stars of the Algol type are not variable in the true sense of the word. Even the most irregular of the Algol variables have been explained. Perhaps the most irregular was Y Cygni, discovered by Chandler in 1886. It was soon found, however, that the variations recurred with great irregularity: in less than two years the phases differed by as much as seven hours from the predicted times. At length the subject was taken up by Dunér at Upsala. A series of observations made with the 14-inch refractor at Upsala in 1891 and 1892 convinced him in the latter year that two eclipses take place in the course of one revolution: one star occults the other. Dunér showed that the intervals between minima were thus—1 day 9 hours; 1 day 15 hours; 1 day 9 hours, and so on. Thus, the first, third, fifth, and seventh sets of minima obeyed a different law from the second, fourth, sixth, and eighth. Dunér proved that two stars revolve round their centre of gravity in less than three days, alternately occulting each other, while the ellipticity of the orbit explains the irregularity of the light changes. In April 1900 Dunér gave his final conclusions as follows: “The variable star Y Cygni consists of two stars of equal size and equal brightness, which move about their common centre of gravity in an elliptical orbit, whose major axis is eight times the radius of the stars.” He also stated the exact period of revolution and the eccentricity of the orbit.
In the case of the short-period variables, such as β Lyræ, δ Cephei, ζ Geminorum, and η Aquilæ, the variations do not seem to be due to eclipse. It was discovered by Professor Pickering that β Lyræ is a spectroscopic binary, but Vogel and Keeler showed that the supposed orbit is incompatible with the eclipse theory. Vogel says: “I am convinced that β Lyræ represents a binary or multiple system, the fundamental revolutions of which in 12 days 22 hours in some way control the light change.” The eclipse theory, however, is still maintained by Bélopolsky, who has framed a hypothesis according to which the chief minimum of the star’s light corresponds with the obscuration of the lesser star, the lesser minimum with that of the primary, implying that the primary is much less luminous in proportion to its light than its satellite,—a state of affairs which Miss Clerke concludes to be improbable.
The variable stars, δ Cephei and η Aquilæ, were both found in 1894 by Bélopolsky to be binaries; but as the times of minimum light do not correspond with those of eclipses in the hypothetical orbits, he concludes that the variations cannot be explained on the eclipsing satellite theory. Miss Clerke is inclined to the theory that the increase of luminosity in short-period variables is due to tidal action, so that while the revolutions of the stars control their variability, they are inherently unstable in light. A large number of these stars are known, and it is a remarkable fact that the majority of these variables lie on or near the Galaxy, so that their variations have probably something to do with their vicinity.
We now come to the long-period variables of which Mira Ceti, χ Cygni, and U Orionis are examples. Although varying in regular periods, generally of about a year, they are subject to remarkable irregularities, so that an exact period cannot be assigned even to Mira Ceti, of which the maxima are at times retarded and at others accelerated with no apparent law. The spectroscopic investigations of Campbell in 1898 have shown that Mira Ceti is a solitary star, while bright lines of hydrogen appear in its spectrum at maximum, showing that the variations are due to periodical conflagrations in its atmospheres. In many other long-period variables bright lines have been observed.
A remarkable fact regarding these stars is the amount of their light change. Mira Ceti, for instance, varies from the first to the ninth magnitude, and U Orionis from the sixth to the twelfth. As M. Flammarion says, “the longer the period the greater the variation.” Another remarkable fact is that their light curves show a curious resemblance to the curves of the solar spots, only on a vastly greater scale, which indicates that, relatively, these long-period variables are much older than our Sun, the small variations in the light of which are imperceptible. “Here, if anywhere,” says Miss Clerke, “will be found the secret of stellar variability.”
To the irregular variables no period can be assigned. Betelgeux, in Orion, the variation of which was noted by Sir John Herschel in 1840, is a typically irregular variable. But the most extraordinary of all variables is η Argus, in the southern hemisphere, which is probably a connecting link between variable and temporary stars. The traveller Burchell, from 1811 to 1815, observed the star as of the second magnitude, but in 1827 he noted it to be of the first magnitude. In the following year it fell to the second magnitude. In 1834 Sir John Herschel noted the star to be between the first and second magnitude, and in 1838 it rose to the first, being equal to α Centauri. After a decline, it became in 1843 equal to Canopus, and not much inferior to Sirius. Then it began to fade, and in 1868 it was only of the sixth magnitude. In 1899 Innes estimated it as 7·71. Rudolf Wolf suggested a period of 46 years, and Loomis 67 years; but astronomers generally agree with Schönfeld that the star has no regular period.
The first temporary star of the nineteenth century was discovered by Hind, in London, April 28, 1848. It was of the fifth magnitude at maximum, and soon after began to fade, falling to the tenth magnitude. In 1860 a new star appeared in the cluster Messier 80 in Scorpio, and was discovered by Auwers at Königsberg. It reached only the seventh magnitude.
On the night of May 12, 1866, a new star of the second magnitude blazed out in the constellation Corona Borealis. It was first observed at Tuam, in Ireland, by the Irish astronomer, John Birmingham. Four hours earlier Schmidt had been observing that part of the heavens, and it was not then visible. Birmingham at once communicated the discovery to Huggins, at Tulse Hill, who had commenced his spectroscopic observations. On May 16 Huggins observed its spectrum. In the words of Miss Clerke, “The star showed what was described as a double spectrum. To the dusky flutings of Secchi’s third type, four brilliant rays were added. The chief of these agreed in position with lines of hydrogen; so that the immediate cause of the outburst was plainly perceived to have been the eruption, or ignition, of vast masses of that subtle kind of matter.” Nine days after the appearance of the new star it was invisible to the naked eye, and afterwards fell to the tenth magnitude. In 1856 Schönfeld had observed it at Bonn as a telescopic star, so that it was not a “new star” in the true sense of the word.
The next temporary star observed was discovered by Schmidt, at Athens, November 24, 1876. It was of the third magnitude, situated in the constellation Cygnus. On December 2 its spectrum was examined at Paris by Alfred Cornu (1841-1902), and some days later at Potsdam by Vogel and Lohse. It was closely similar to that of the new star of 1866, bright lines of hydrogen and other elements standing out in front of an “absorption” spectrum. By the end of 1876 the star was of the seventh magnitude. On September 2, 1877, Nova Cygni was observed at Dunecht, and its spectrum was found to have been transformed into that of a planetary nebula. Three years later, however, the ordinary stellar spectrum reappeared.
A new star appeared in the centre of the great nebula in Andromeda in August 1885. The first announcement of the discovery was by Karl Ernst Albrecht Hartwig (born 1851), who observed the new star on August 31; but it had been previously seen by several other observers. On September 1 it was of the seventh magnitude, and by March of the following year had fallen to the sixteenth. Observed by Vogel, Young, and Hasselberg, the new star gave a continuous spectrum, but Huggins and Copeland succeeded in discerning bright lines. Hall, at Washington, undertook a series of measures to detect the parallax of Nova Andromedæ, but his efforts were unsuccessful.
The discovery of the next temporary star was announced February 1, 1892, by the Rev. Thomas D. Anderson, a Scottish amateur astronomer, in a post-card to the Astronomer-Royal of Scotland. The star was situated in the constellation Auriga. An examination of photographs, taken at Harvard Observatory, showed that the new star had appeared December 10, 1891, and had risen to a maximum of the fourth magnitude ten days later. On a photograph taken by Max Wolf on December 8 the new star was not visible. After Anderson’s visual discovery, the spectrum of the new star was studied by Copeland, Huggins, Lockyer, Vogel, Campbell, and others. Bright hydrogen lines were visible in the spectrum, which appeared to be actually double, giving support to the theory that the outburst was the result of a collision between two dark bodies; and this was confirmed by the measurements of radial motion by the Potsdam astronomers.
After March 9, 1892, the new star steadily faded, falling to the sixteenth magnitude on April 26. But on August 17 Edward Singelton Holden (born 1846), director of the Lick Observatory, and his assistants, Schaeberle and Campbell, found it of the tenth magnitude. On August 19 Barnard found it transformed into a planetary nebula: while various spectroscopic observations of the revived Nova revealed the nebular lines. By the end of 1894 the new star had faded to the eleventh magnitude, and early in 1901 was observed as a minute nebula.
After 1892 several new stars appeared, and were detected on photographic plates by Mrs Fleming (born 1857), of Harvard Observatory. The first of these, in the southern constellation Norma, was discovered in 1893 by its peculiar spectrum on a Draper spectrographic plate taken at Harvard. But the new star rose only to the seventh magnitude. Other new stars were discovered in Carina (Argo) in 1895, in Centaurus in 1895, in Sagittarius in 1898, and in Aquila in 1900. Nova Sagittarii was, at its brightest, fully equal to Nova Aurigæ, and was plainly visible to the naked eye, but was never observed visually.
A temporary star, appropriately designated “the new star of the new century,” blazed out in Perseus on the night of February 21, 1901. It was discovered independently by several observers: on February 21, by Borisiak, a student at Kiev, in Russia; on the following morning, by Anderson in Edinburgh; and on the next evening, by Gore at Dublin, Nordvig in Denmark, Grimmler at Erlangen, and other observers. When first seen by Anderson, it was equal to Algol, of the second magnitude. A photograph by Williams at Brighton showed that it must have been fainter than the twelfth magnitude on February 20. On the evening of February 23 the star was brighter than Capella, and was then the brightest star in the northern hemisphere. On February 25 it fell to the first magnitude; on March 1 to the second, and on March 6 to the third. During the spring and summer the light fluctuated considerably, but in September and October faded to the 6·7 magnitude. In March 1902 it was of the eighth magnitude, and in July 1903 of the twelfth.
The spectrum of Nova Persei was found by Pickering to be of the Orion type on February 22 and 23. On February 24 the spectrum had become one of the bright and dark lines, and the hydrogen lines indicated a velocity of 700 to 1000 miles a second. Measures of the sodium and calcium lines, by Campbell and others, indicated a velocity of only three miles a second, so that the displacements of the hydrogen lines may have been due to an outburst of hydrogen in the star. The spectrum was carefully studied during the spring and summer by Pickering, Lockyer, Huggins, Vogel, and others. On June 25 Pickering reported that the spectrum was slowly changing into that of a gaseous nebula. In August and September 1901 the nebular spectrum became more apparent.
In August 1901 Wolf at Heidelberg discovered a faint trace of nebula near the nova. On September 20 this nebula was photographed by George Ritchey at the Yerkes Observatory, and was seen to be of a spiral form. This was confirmed by Perrine, who also found, from plates taken in November, that the nebula was moving at the rate of eleven minutes of arc a year. This extraordinary velocity was exceedingly puzzling to astronomers, and at length Kapteyn suggested that the nebula shone only by reflected light from the new star, and that the apparent motion was an illusion caused by the flare of the explosion travelling out from the nova.
On March 16, 1903, Herbert Hall Turner (born 1861), Professor of Astronomy at Oxford, discovered a new star of the seventh magnitude in the constellation Gemini, from an examination of photographic plates. Photographs taken at Harvard showed that on March 1 it must have been fainter than the twelfth magnitude, while five days later it was of the fifth. In August 1903 Pickering found its spectrum nebular. In August 1905 another small nova was found by Mrs Fleming on the Harvard photographs, situated in Aquila.
Many theories have been advanced to account for temporary stars. Flammarion has shown that a body surrounded by a hydrogen atmosphere, on grazing a dark body enveloped in oxygen, would produce a tremendous explosion. In 1892 Huggins suggested that the outburst of Nova Aurigæ was due to the near approach of two bodies with large velocities, disturbances of a tidal nature resulting and producing enormous outbursts. Vogel suggested that the new star was due to the encounter of a dark star with a worn-out system of planets; while Lockyer believes all new stars to be due to the collision of swarms of meteors. Perhaps the most probable theory is that of Seeliger, which attributes these outbursts to the movement of a dark body through nebulous matter, which is extensively diffused throughout space. This theory explains the changes in the spectra as well as the revivals of brightness which characterised Nova Aurigæ and the fluctuations of Nova Persei. In a paper read to the Royal Society of Edinburgh in November 1904, the German astronomer, Jacobus Halm, of the Royal Observatory, Edinburgh, extended and developed Seeliger’s theory, showing also that the necessary consequence of such an encounter as the theory assumes is the formation of an atmosphere of incandescent gases, followed by that of a revolving ring of nebulous matter. In the hands of Halm, therefore, Seeliger’s theory leads to the nebular hypothesis as advanced by Laplace and Herschel.
CHAPTER XI.
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.