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
I_a_,—represented by Sirius and Vega,—the metallic lines are “very faint
and fine,” and the hydrogen lines conspicuous. In I_b_ no hydrogen lines
are visible, while in I_c_ 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
I_b_ 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, II_a_ being represented by the Sun,
Capella, and other well-known stars, while II_b_ 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 _a_5_v_5_b_. If _v_ is slightly nearer to
_b_, we write _a_6_v_4_b_. We may also write _a_3_v_7_b_, or
_a_7_v_3_b_, 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.