appearance of the object which suddenly blazed out in Cassiopeia’s Chair
in November, 1572. It was called the “Pilgrim Star,” and was observed by
the famous astronomer, Tycho Brahé, who has left us a very elaborate
account of its appearance, position, etc. Although usually spoken of as
Tycho Brahé’s star, it seems to have been really discovered by Cornelius
Gemma on the evening of November 9. That its appearance was very sudden
may be inferred from Cornelius Gemma’s statement, that it was not
visible on the preceding night in a clear sky. Tycho Brahé’s attention
was first attracted to it on November 11. His description of the new
star is as follows—as quoted by Humboldt:[121]—“On my return to the
Danish islands from my travels in Germany, I resided for some time with
my uncle, Steno Bille, in the old and pleasantly situated monastery of
Herritzwadt, and here I made it a practice not to leave my chemical
laboratory until the evening. Raising my eyes, as usual, during one of
my walks, to the well-known vault of heaven, I observed with
indescribable astonishment, near the zenith in Cassiopeia, a radiant
fixed star of a magnitude never before seen. In my amazement, I doubted
the evidence of my senses. However, to convince myself that it was no
illusion, and to have the testimony of others, I summoned my assistants
from the laboratory, and inquired of them, and of all the country people
that passed by, if they also observed the star that had thus suddenly
burst forth. I subsequently heard that in Germany, waggoners and other
common people first called the attention of astronomers to this great
phenomenon in the heavens—a circumstance which, as in the case of
non-predicted comets, furnished fresh occasion for the usual raillery at
the expense of the learned. This new star I found to be without a tail,
not surrounded by any nebula, and perfectly like all other fixed stars,
with the exception that it scintillated more strongly than stars of the
first magnitude. Its brightness was greater than that of Sirius, α Lyræ,
or Jupiter. For splendour, it was only comparable to Venus when nearest
to the earth (that is, when only a quarter of her disc is illuminated).
Those gifted with keen sight could, when the air was clear, discern the
new star in the day-time, and even at noon. At night, when the sky was
overcast, so that all other stars were hidden, it was often visible
through the clouds, if they were not very dense (_nubes non admodum
densas_). Its distances from the nearest stars of Cassiopeia, which
throughout the whole of the following year I measured with great care,
convinced me of its perfect immobility. Already, in December, 1572, its
brilliancy began to diminish, and the star gradually resembled Jupiter,
but by January, 1573, it had become less bright than that planet.
Successive photometric estimates gave the following results: for
February and March, equality with stars of the first magnitude
(_stellarum affixarum primi honoris_—for Tycho Brahé seems to have
disliked Manilius’ expression of _stellæ fixæ_); for April and May, with
stars of the second magnitude; for July and August, with those of the
third; for October and November, those of the fourth magnitude. Towards
the month of November, the new star was not brighter than the eleventh
in the lower part of Cassiopeia’s Chair. The transition to the fifth and
sixth magnitude took place between December, 1573, and February, 1574.
In the following month the new star disappeared, and, after having shone
seventeen months, was no longer discernible to the naked eye.” (The
telescope was not invented until thirty-seven years afterwards.)
Humboldt adds:—“At its first appearance, as long as it had the
brilliancy of Venus and Jupiter, it was for two months white, and then
passed through yellow into red. In the spring of 1573, Tycho Brahé
compared it to Mars; afterwards he thought it nearly resembled
Betelgeuse, the star in the right shoulder of Orion. The colour for the
most part was like the red tint of Aldebaran. In the spring of 1573, and
especially in May, its white colour returned (_albedinam quandam
sublividam induebat, qualis Saturni stellæ subesse videtur_). So it
remained in January, 1574; being, up to the time of its entire
disappearance in the month of March, 1574, of the fifth magnitude, and
white, but of a duller whiteness, and exhibiting a remarkably strong
scintillation in proportion to its faintness.”
[Illustration:
FIG. 8.—_The Temporary Star of 1572._
(From “Planetary and Stellar Studies.”)
]
According to a sketch of the position given in Tycho Brahé’s work,
referred to above, the star was situated a little to the north of Kappa
Cassiopeiæ, the faintest star in the Chair. This position is confirmed
by Argelander’s examination of Tycho Brahé’s observations: The spot is a
rather blank one to the naked eye, and even with an opera-glass, only a
few faint stars are visible. Quite close to the place fixed by
Argelander, d’Arrest observed in 1865 a star of the eleventh magnitude,
which seems to have escaped Argelander’s notice. Hind and Plummer
observed this small star in 1873, and thought they could detect
fluctuations in its light to the extent of about one magnitude. Espin
has also observed it, and the region has been photographed by Dr.
Roberts. Some have thought that Tycho Brahé’s star might possibly be
identical with the Star of Bethlehem, and this idea has been supported
by Cardanus, Chladni, and Klinkerfues, but Lynn and Sadler have shown
that the theory is quite untenable, and it has now been rejected by all
astronomers.
Ma-tuan-lin speaks of a star in 1578 “as large as the sun”(!) but does
not state its position.
The star known as P (34) Cygni is sometimes spoken of as a “Nova,” or
new star; but it is still visible to the naked eye as a star of the
fifth magnitude. It was observed of the third magnitude by Jansen in
1600 and by Kepler in 1602. After the year 1619, it appears to have
diminished in brightness, and is said to have vanished in 1621; but it
may merely have become too faint to be seen with the naked eye. It was
again observed of the third magnitude by Dominique Cassini in 1655, and
it afterwards disappeared. It was again seen by Hevelius in November,
1665. In 1667, 1682, and 1715, it is recorded as of the sixth magnitude,
and there is no further record of any marked increase in its light. A
period of about 18 years was assumed by Pigott; but this is now
disproved, and it seems probable that the star is a variable of
irregular period and fitful variability, and not, properly speaking, a
temporary star. Its present colour is yellow, and bright lines have been
seen in its spectrum.
Another remarkable object of the temporary class was observed by Kepler
in 1604 in Ophiuchus, and is described by him in his work, “De Stella
Nova in pede Serpentarii.” He and his assistants were observing the
planets Mars, Jupiter, and Saturn, which were then near each other in
this region of the heavens, a few degrees to the south-east of the star
Eta Ophiuchi, and on the evening of October 10, Brunowski, a pupil of
Kepler’s, noticed that a new and very brilliant star was added to the
group[122]. When first seen, it was white, and exceeded in brightness
Mars and Jupiter, but seems not to have quite equalled Venus in
brilliancy. It slowly diminished, and in January, 1605, it was brighter
than Antares but less than Arcturus. At the end of March, 1605, it had
faded to the third magnitude. Its proximity to the sun then prevented
further observations for several months. In March, 1605, it had
disappeared to the naked eye. It was also observed by Galileo and by
David Fabricius, whose observations place it about midway between the
fifth magnitude star Xi and 58 Ophiuchi. Its exact position, however,
does not seem to be known with such accuracy as that of Tycho Brahé’s
star, nor is there any known star very close to the spot indicated by
Schönfeld from an examination of Fabricius’ observations. It seems
possible that Kepler’s star may have been seen previously by Ptolemy,
for in his catalogue he gives a star of the fourth magnitude close to
the position of Kepler’s star; but there is some doubt about the exact
position indicated by Ptolemy. The Chinese annals mention a “ball-like”
star as having appeared near Pi Scorpii on September 30, 1604, and
remaining visible until March, 1606, which may possibly be identical
with Kepler’s star.
A new star of the third magnitude was observed near Beta Cygni by the
Carthusian monk Anthelmus in 1670. It remained visible for about two
years, and is said to have increased and diminished several times before
its final disappearance. Schönfeld computed its exact position from
observations made by Hevelius and Picard. Quite close to the spot
indicated, a star of the eleventh magnitude has been observed at the
Greenwich Observatory, and fluctuations of light were suspected in this
small star by Hind and others. Hind says that, to his eye, “there is a
hazy, ill-defined appearance about it which is not perceptible in other
stars in the same field of view. Mr. Talmage received the same
impression; and I may add that Mr. Baxendell, who has examined it with
Mr. Worthington’s reflector, observed that no adjustment of focus would
bring the star up to a sharp focus.” This hazy appearance is very
suggestive, as it indicates that the “Nova” may possibly have faded into
a small planetary nebula, as in the case of the new star in Cygnus,
observed by Schmidt in 1876, and the new star in Auriga, found by Dr.
Anderson in 1892. Near the position of Anthelm’s new star is a known
variable star, S Vulpeculæ, discovered by Hind in 1861, which might be
suspected to be identical with Anthelm’s star; but Hind has shown that
the variable has no proper motion which would account for the difference
of position since 1670, and he concludes that, “from the fixity of its
position during eight years, it may be inferred that the variable is
distinct from Anthelm’s.” It has been supposed that the star 11
Vulpeculæ in Flamsteed’s catalogue is identical with Anthelm’s star; but
Baily could not find any evidence to show that Flamsteed’s star ever
really existed, and he says: “Under the presumption, however, that it
may be a variable and not a _lost_ star, I have preserved its recorded
position with a view of inducing astronomers to look out for it from
time to time.”
On the evening of April 28, 1848, Hind, observing at Mr. Bishop’s
private observatory, in Regent’s Park, London, noticed a new star of
about the fifth magnitude, between Zeta and Eta Ophiuchi. Its colour was
reddish-yellow, and it seems to have subsequently increased in
brightness to nearly the fourth magnitude, but it soon faded to the
tenth or eleventh magnitude. This curious object has become very faint
in recent years. In 1866, it was of the twelfth magnitude, and in 1874
and 1875, not above the thirteenth.
On May 28, 1860, Pogson discovered a new star in the globular cluster,
80 Messier, which lies between Antares and Beta Scorpii. When first
noticed, it was about the seventh magnitude, and its brightness was
sufficient to obscure the cluster. In other words, the cluster was
apparently replaced by a star. On June 10, the star had nearly
disappeared, and the cluster again shone with great brilliancy, and with
a condensed centre. The observations of Auwers and Luther confirm those
of Pogson. Pogson states that he examined the cluster on May 9, but
noticed nothing peculiar; and, according to Schönfeld, the cluster
presented its usual appearance on May 18, when examined at the
Königsberg Observatory. The apparition of the temporary star was,
therefore, probably sudden, as in the case of other “new” stars. The
phenomenon was possibly caused by a collision between two of the stars
composing the cluster, which is, at least, apparently very condensed.
A very remarkable star, sometimes called the “Blaze Star,” suddenly
appeared in Corona Borealis, in May, 1866. It was first seen by the late
Mr. Birmingham, at Tuam, Ireland, about midnight, on the evening of May
12, when it was of the second magnitude, and equal to Alphecca, “the gem
of the coronet.” Its appearance must have been very sudden, for Schmidt,
the Director of the Athens Observatory, stated that he was observing the
constellation on the same evening, about 2½ hours previous to
Birmingham’s discovery, and observed nothing unusual. He was certain
that no star, of even the fifth magnitude, could possibly have escaped
his notice. On the following night it was seen by several observers in
different parts of the world. M. Faye, the French astronomer, in his
work—“L’Origine du Monde”—attributes the discovery to M. Courbebaisse, a
French engineer, and does not mention Mr. Birmingham! He says M.
Courbebaisse first saw it on the evening of May 13. This may be true; he
was not the only observer who saw it on that evening; but it was,
undoubtedly, _first_ seen by Mr. Birmingham on the _preceding_ night,
and to Mr. Birmingham alone is certainly due the credit of the
discovery. The star rapidly diminished in brightness, and on May 24 of
the same year, had faded to 8½ magnitude. It afterwards increased to
about 7·8 magnitude, but soon diminished again. Soon after its discovery
it was found that the star was not really a new one, as it had been
previously observed at Bonn by Schönfeld, in May, 1855, and March, 1856,
while making the observations for Argelander’s _Durchmusterung_, in
which it appears as No. 2765, in degree 26. On both occasions it was
rated as 9½ magnitude, and no suspicion of variable light seems to have
arisen. When viewed with the naked eye at the time of its greatest
brilliancy, it was remarked by some observers that it twinkled decidedly
more than other stars in the vicinity, and that this peculiarity made it
very difficult to form a correct estimation of its relative brilliancy
During the years 1866 to 1876, fluctuations in its light were observed
by Schmidt, and he deduced a probable period of about 94 days, with a
variation from the seventh to the ninth magnitude. This conclusion was
confirmed by Schönfeld, and the star would therefore seem to be an
irregular variable, and not a true temporary star.
A very remarkable and interesting variable star was discovered by
Schmidt at Athens, near Rho Cygni, on the evening of November 24, 1876,
when it was about the third magnitude, and somewhat brighter than Eta
Pegasi. Schmidt stated that he had observed the vicinity on several
occasions between November 1 and 20, and was certain that no star of
even the fifth magnitude could possibly have escaped his notice, so that
the star probably blazed out very suddenly, as most of these
extraordinary objects have done. Between November 20 and 24, the sky was
overcast, so the exact time of its appearance is unknown. The star would
seem to be quite new, as there is no star in any of the catalogues in
the position of the “Nova,” the nearest being one of the ninth
magnitude, which occurs in the Bonn observations. The new star rapidly
faded, and on November 30 had descended to the fifth magnitude. On the
night of its discovery it was remarked that its brightness was such as
to render its near neighbour, 75 Cygni (a sixth magnitude star),
invisible; while on December 14 and 15, 75 Cygni, in its turn, nearly
obliterated the light of the stranger. In the 48 hours following the
night of November 27, the star diminished in light to the extent of
nearly 1½ magnitude! It afterwards faded very regularly to August, 1877,
and showed no oscillations of brightness as have been observed in other
temporary stars. On the evening of its discovery, Schmidt considered the
star to be of a strong golden-yellow, and that it afterwards remained of
a deep golden-yellow, but at no time was it as ruddy as 75 Cygni. I
could see no trace of colour in the star with a 3-inch telescope in the
Punjab on January 12, 1877, but it had then faded to the eighth
magnitude. On February 7, 1877, I estimated it ninth magnitude. A few
days after its discovery, it was examined with the spectroscope, and its
spectrum showed bright lines similar to the “Blaze Star” in Corona,
which appeared in May, 1866. One of the bright lines was thought to be
identical with the line numbered 1474 by Kirchoff, visible in the
spectrum of the solar Corona during total eclipses of the sun. The other
bright lines were identified by M. Cornu of the Paris Observatory with
some of the lines of hydrogen, sodium, and magnesium. In September,
1877, the star was examined with a 15-inch refractor by Lord Lindsay
(now Lord Crawford), who found “the light coming from it almost entirely
monochromatic, that is, of only one colour, the star appearing exactly
the same as when looked at without the spectroscope, the direct prism
having no effect on it,” and he considers that “there is little doubt
that the star has changed into a planetary nebula of small angular
diameter!” On September 3, the star’s magnitude was 10½; “faint blue,
near another star of same size rather red.” Lord Crawford remarks that
no observer, discovering the object in its present state, would, after
viewing it through a prism, hesitate to pronounce as to its nebulous
character,[123] but no disc was detected with powers ranging up to 1000
diameters. Ward found the star only sixteenth magnitude in October,
1881, and it was estimated fifteenth magnitude at Mr. Wigglesworth’s
Observatory in September, 1885. At Lord Crawford’s Observatory the exact
position of the star, with reference to above fifty closely adjacent
stars, was carefully determined with the micrometer. The vicinity was
photographed by Dr. Roberts on September 27, 1891, with an exposure of
two hours, and “the _Nova_ appears as a star of about the thirteenth
magnitude.” Observations in 1894 and 1895, made its magnitude about
14·8, with an apparently continuous spectrum.[124]
In August, 1885, a star of about the seventh magnitude made its
appearance close to the nucleus of the Great Nebula in Andromeda
(Messier 31), a remarkable nebula, which will be described in the next
chapter. The new star was independently discovered by several observers
towards the end of August. It was not visible to Tempel at the Florence
Observatory on August 15 and 16, but is said to have been seen by M.
Ludovic Gully on August 17. It was, however, certainly seen by Mr. I. W.
Ward at Belfast on August 19, at 11 P.M., when he estimated it 9½
magnitude, and it was independently detected by the Baroness Podmaniczky
on August 22, by M. Lajoye on August 30, by Dr. Hartwig, at Dorpat, on
August 31, and by Mr. G. T. Davis, at Theale, near Reading, on September
1. On September 3, the star was estimated 7½ magnitude by Lord Crawford
and Dr. Copeland, and its spectrum was found to be “fairly continuous.”
On September 4, Mr. Maunder, at the Greenwich Observatory, found the
spectrum “of precisely the same character as that of the nebula, _i.e._,
it was perfectly continuous, no lines, either bright or dark, being
visible, and the red end was wanting.” Dr. Huggins, however, on
September 9, thought he could see a few bright lines in its spectrum, a
continuous spectrum being visible from the line D to F. The star
gradually faded away. On December 10, 1885, it was estimated of the
fourteenth magnitude at the Radcliffe Observatory, Oxford, and on
February 7, 1886, it was rated only sixteenth magnitude with the 26-inch
refractor of the Washington Observatory. A series of measures by
Professor Hall, from September 29, 1885, to February 9, 1886, showed “no
certain indications of any parallax,” so that the star and the nebula,
in which it probably lies, are evidently situated at a vast distance
from the earth. Seeliger has investigated the decrease in the light of
the star on the hypothesis that it was a cooling body, which had been
suddenly raised to an intense heat by the shock of a collision, and
finds a fair agreement between theory and observation. Auwers points out
the similarity between this outburst and the new star of 1860, in the
cluster 80 Messier (already described), and thinks it probable that both
phenomena were caused by physical changes in the nebulæ in which they
occurred. Proctor considered that the evidence of the spectroscope shows
that the new star was situated _in_ the nebula, and in this opinion I
fully concur.
Several temporary stars have been detected in recent years by Mrs.
Fleming, from an examination of photographs of stellar spectra, taken at
the Harvard Observatory, for the Draper Memorial. Plates of the
constellation Perseus show the existence of a star in 1887, the spectrum
of which shows the bright lines of hydrogen, and it was on this account
assumed to be a long period variable. During the following eight years,
however, 81 photographs of the same region show no trace of the star,
and it has been frequently looked for with a telescope, but without
success. It would, therefore, seem probable that the star was a
temporary one. Its magnitude was about the ninth.
A remarkable and very interesting temporary star was discovered in 1892
in the constellation Auriga. On February 1, of that year, an anonymous
post-card was received by Dr. Copeland at the Royal Observatory,
Edinburgh, with the following announcement:
“Nova in Auriga. In Milky Way, about two degrees south of χ Aurigæ,
preceding 26 Aurigæ. Fifth magnitude, slightly brighter than χ.”
Such an announcement evidently required immediate attention, and on that
evening, Dr. Copeland and his assistants looked for the new star, and
easily found it with an opera-glass at 6 hours 8 minutes. They estimated
it of the sixth magnitude, and equal to 26 Aurigæ. It was of a yellow
colour. When examined with a prism placed before the eye-piece of a
24-inch reflector, its spectrum was seen to resemble the “Blaze Star” of
1866 in Corona. “The C line was intensely bright, a yellow line about D
fairly visible; four bright lines, or bands, were conspicuous in the
green; and, lastly, a bright line in the violet (probably Hγ) was easily
seen.” Notice of the discovery was at once telegraphed to Greenwich and
Keil Observatories, and the star was photographed at Greenwich on the
same night. It is not in the Bonn star charts, which show stars to
nearly the tenth magnitude. In _Nature_ of February 18, 1892, a letter
appeared, signed Thomas D. Anderson, in which the writer stated that the
post-card was sent by him, and he gives the following details respecting
the discovery:
“Prof. Copeland has suggested to me that as I am the writer of the
anonymous post-card mentioned by you a fortnight ago (p. 325), I should
tell your readers what I know about the Nova.
“It was visible as a star of the fifth magnitude certainly for two or
three days, very probably even for a week, before Prof. Copeland
received my post-card. I am almost certain that at two o’clock on the
morning of Sunday, the 24th ult., I saw a fifth magnitude star making a
very large obtuse angle with β Tauri and χ Aurigæ, and I am positive
that I saw it, at least, twice subsequently during that week.
Unfortunately, I mistook it on each occasion for 26 Aurigæ, merely
remarking to myself that 26 was a much brighter star than I used to
think it. It was only on the morning of Sunday, the 31st ult., that I
satisfied myself that it was a strange body. On each occasion of my
seeing it, it was slightly brighter than χ. How long before the 24th
ult. it was visible to the naked eye I cannot tell, as it was many
months since I had looked minutely at that region of the heavens.
“You might also allow me to state, for the benefit of your readers, that
my case is one that can afford encouragement to even the humblest of
amateurs. My knowledge of the technicalities of astronomy is,
unfortunately, of the most meagre description; and all the means at my
disposal on the morning of the 31st ult., when I made sure that a
strange body was present in the sky, were Klein’s ‘Star Atlas’ and a
small pocket-telescope, which magnifies ten times.”
Soon after the discovery of the new star, an examination was made by
Professor Pickering of photographs taken of the region at Harvard
Observatory, previous to Dr. Anderson’s discovery. It was found that on
eighteen photographs taken between the dates November 3, 1885, and
November 2, 1891, there is no trace of the new star; but in those taken
from December 16, 1891, to January 31, 1892, a star of the fifth
magnitude is shown in the position of the new star. “In another series
of plates taken with the transit photometer, no record of the new star
up to December 1, 1891, was obtained, although χ Aurigæ (magnitude 5·0)
was always visible, but the plates taken on the nights of December 10,
1891, and ending January 20, 1892, indicated clearly the position of the
new star.” Professor Pickering says: “It appears that the star was
fainter than the eleventh magnitude on November 2, 1891, than the sixth
magnitude on December 1, and that it was increasing rapidly on December
10. A graphical construction indicates that it had probably attained the
seventh magnitude within a day or two of December 2, and the sixth
magnitude on December 7. The brightness increased rapidly until December
18, attaining its maximum about December 20, when its magnitude was 4·4.
It then began to decrease slowly, with slight fluctuations, until
January 20, when it was slightly below the fifth magnitude. All these
changes took place before its discovery, so that it escaped observation
nearly two months. During half of this time it was probably brighter
than the fifth magnitude.”
It would seem from the above remarks that the star did not—like some
other temporary stars—attain its full brilliancy at once, but increased
gradually in brightness. After the decrease of light in January, 1892,
it seems to have again risen to another maximum, for photographs taken
at the Greenwich Observatory after its discovery show that the star rose
to a magnitude of 3·5 (photographic) on February 3, and then began to
fade again slowly during February, but rapidly during the month of
March. Owing to cloudy weather in the west of Ireland, I could not
observe the new star until February 14. The following are my
observations, made with a binocular field-glass, the comparison stars
being Chi Aurigæ, 26 Aurigæ, and D M + 30°, 898:—February 14, 4·55
magnitude; February 15, 5·56; February 16, 5·84; February 18, 5·51;
February 21, 5·56; February 24, 5·66; February 28, 5·44; March 1, 5·68;
March 5, 5·66; March 10, 7·3; March 11, 7¾; March 16, 8½, or fainter;
March 18, 9 magnitude, or less, “only _very_ faint stars seem near the
place of the Nova; clear sky, no moon.” The general accuracy of the
above observations were confirmed by the photographic estimates of the
star’s light made at Greenwich,[125] and also by Schaeberle’s
observations of its brightness.
After March 18, the light of the star steadily and rapidly decreased,
and on April 1, it had faded to nearly the fifteenth magnitude, and
afterwards to about the sixteenth. In August, 1892, it brightened again,
as it was found by Corder of about the ninth magnitude on August 21. Dr.
J. Holetschek of the Vienna Observatory observed it from August 24 to
September 2, 1892, and estimated it about 9½ magnitude. In October,
1892, most observers rated it between 10 and 10½ magnitude. Observations
by Mr. C. E. Peck, “from October 3, 1893, to May 4, 1894, only vary from
10·1 to 11·0 magnitude, and observations up to the end of 1894 give the
same results.”[126] In 1895 Professor Barnard found that it “is still
visible as a small star, and has not changed in physical appearance
since the autumn of 1892. It remains perfectly fixed with reference to
the comparison stars.”[127]
Examined with the spectroscope soon after its discovery, many bright
lines were seen in its spectrum, and it was found that “the bright lines
in the spectrum of the new star were accompanied by dark ones on their
more refrangible sides,” that is, the dark lines were on the blue side
of the bright ones. This suggested the idea that the outburst was
probably due to a collision between two bodies, one of which, having a
spectrum of dark lines, was rushing towards the earth, and the other,
with a bright-line spectrum, was receding. Lockyer supposed the outburst
to be due to a collision between two swarms of meteorites. Dr. Huggins
advanced the view that the phenomenon was due to the near approach of
two gaseous bodies. “But,” he says, “a casual near approach of two
bodies of great size would be a greatly less improbable event than an
actual collision. The phenomena of the new star scarcely permits us to
suppose even a partial collision, though, if the bodies were diffused
enough, or the approach close enough, there may have been, possibly,
some interpenetration and mingling, of the rare gases near the
boundaries.” But Maunder and Seeliger consider this hypothesis to be
untenable. Mr. Monck suggested that a star or swarm of meteorites
rushing through a gaseous nebula might explain the phenomena. Seeliger
advocates a similar theory. Maunder also favours a collision theory.
A photograph of the spectrum taken by Maunder on February 22, 1892 (when
the photographic magnitude was 4·78, and visual magnitude about 5·7),
showed a displacement of the dark lines, which implied a relative motion
of the two supposed colliding bodies of about 820 miles a second! Vogel
found that the bright lines showed a double maxima, and he thought that
these were due to “two different bodies moving with different
velocities, so that the spectrum of the Nova consists of, at least,
three spectra superposed. The measurement of the photograph gives the
body showing the dark line spectrum as approaching the earth with a
speed of nearly 420 miles per second, one of the two bright line bodies
as approaching with a speed of 22 miles, whilst the other is receding
with a speed of 300 miles a second.”[128]
At the time of its increase of brightness, in August, 1892, Professor
Barnard, observing it with the great 36-inch Lick telescope, says, the
“Nova appeared as a small, bright nebula, with a star-like nucleus of
the tenth magnitude. The nebulosity was pretty bright and dense, and was
3″ in diameter. Surrounding this was a fainter glow, perhaps half a
minute in diameter.” At this time, Professor Campbell of the Lick
Observatory found that its spectrum showed the characteristic nebular
lines. This observation was confirmed by Dr. Copeland on August 25 and
26, and by Herr Gothard, who photographed the spectra of a number of
nebulæ, and compared them with his photograph of the spectrum of the new
star. He says, “Each new photograph increased the probability, which may
be considered as a proved fact, that the _spectrum not only resembles,
but that the aspect and position of the lines show it to be identical
with the spectra of the planetary nebula_. In other words, the new star
has changed into a planetary nebula.”
A nebulous spectrum was also found by Espin. From observations of the
spectrum in November, 1894, Professor Campbell finds that “the spectrum
is not only nebular, but it is approaching the average type of nebular
spectrum,” and he adds, “We may say that only five ‘new stars’ have been
discovered since the application of the spectroscope to astronomical
investigations, and that three of these had substantially identical
spectroscopic histories.” Espin found the star distinctly nebulous on
December 9, 1895, and its magnitude about 10½.
Another new star was discovered by Mrs. Fleming by the photographic
method in the southern constellation, Norma, in the year 1893. When at
its brightest, it seems to have been about the seventh magnitude. It was
situated in the Milky Way, a little to the east of the pair of stars
known as Gamma one and Gamma two Normæ. Its spectrum was similar to that
of the new star in Auriga, when it first appeared, and, like that
object, the spectrum has now, according to Professor Campbell, “become
distinctly nebular.”
Another temporary star of about the eighth magnitude was also discovered
by Mrs. Fleming in 1895, in that portion of the southern constellation
Argo, known as Carina. It was in or close to the Milky Way—like so many
of these new stars—between the variable star Eta Argûs and the star
Lambda Centauri, near the Southern Cross, and close to a star of
magnitude 5½. The photographic plates on which the discovery was made
were taken at the Arequipa Station, in Peru. An examination of 62
photographs of the region showed no trace of the star on May 17, 1889,
and March 5, 1895, although stars so faint as the fourteenth magnitude
are visible on some of the plates. On nine plates, however, taken
between April 8, 1895, and July 1, 1895, the star is visible, and during
this interval the brightness diminished from the eighth to the eleventh
magnitude. The spectrum showed the bright lines of hydrogen “accompanied
by dark lines of slightly shorter wave-length,” and in all its
“essential features” was “apparently identical” with the spectra of the
temporary stars in Auriga and Norma.
With reference to this outburst, and the similarity of the star’s
spectrum to that of the new star in Auriga, Professor William H.
Pickering points out “the improbability of two successive collisions
between stars, occurring nearly in the line of sight, in both cases a
bright and a dark line star being involved, and in each case the
bright-line star being the one to recede from us. The same remark
applies to the theory of a collision of a star and a nebula. As a
substitute I offered an explosion hypothesis, in which a dark sun
suddenly gave out in all directions large quantities of hydrogen in an
incandescent state. This would, of course, merely produce a spectrum
with bright lines. But if the expulsion of hydrogen continued, the outer
layers of gas would cool, producing absorption lines in the spectrum of
the approaching hydrogen, but still leaving the spectrum lines of the
receding hydrogen bright. Finally, when the expulsion ceased, we should
find a heated spherical mass of gas, similar to a planetary nebula. It
was shown that the velocities which were observed in the cases of these
two _novæ_ were less than fifty per cent. greater than had been observed
in our own sun. The discovery of this third _nova_, with a spectrum
identical with that of the two others, increases many times the
improbability of the collision theories, and thereby strengthens the
explosion hypothesis. If this latter is correct, we must look upon the
phenomena presented by a _nova_ not as indicating the birth of a new
star, but rather as a cataclysm testifying to the death and final
disrupture of an old one.”[129]
Another apparently new star was detected by Mrs. Fleming in 1895, in the
constellation Centaurus. It was situated about three degrees north-west
of the double star 3 Centauri, and when at its brightest, seems to have
been about the seventh magnitude. Mrs. Fleming’s attention was first
directed to it by its peculiar spectrum, as shown on a photographic
plate taken at Arequipa in July, 1895. No trace of the star is visible
on 55 plates taken from May 21, 1889, to June 14, 1895, but on plates
taken on July 8 and 10, 1895, it appears of about the seventh magnitude.
A photograph taken on December 16, 1895, shows it as a star of about the
eleventh magnitude. On that date, and on December 19, it was seen about
the same magnitude by Mr. O. C. Wendell, with a 15-inch telescope. The
spectrum at first resembled that of the nebula 30 Doradus, and was
unlike the spectra of the temporary stars in Auriga, Norma, and Carina.
When it had faded to the eleventh magnitude, its spectrum seemed to be
monochromatic, and very similar to that of a neighbouring nebula, N G C
5253, so that, like the new stars in Cygnus, Auriga, and Norma, “it
appears to have changed into a gaseous nebula.”
It is a remarkable fact that the great majority of the temporary stars
appeared in or near the Milky Way. The chief exceptions to this rule
are:—the star of 76 B. C., in the Plough, the star recorded by
Hepidannus in Aries, 1012, A.D., and the “Blaze Star” of 1866 in Corona
Borealis.
CHAPTER VI.
CLUSTERS AND NEBULÆ.
Clusters of stars and nebulæ are frequently classed together in one
group. But this is incorrect. The term nebulæ should be restricted to
those objects which the spectroscope shows to consist of gaseous matter,
while the term cluster should be applied to those groups of stars in
which the components are individually visible as distinct star-like
points. There may be, of course, intermediate forms, like the Great
Nebula in Andromeda, which, although not resolvable into stars with
powerful telescopes, the spectroscope shows to be not gaseous. We will
begin with clusters of stars, many of which can be seen with telescopes
of moderate power, and some, like the Pleiades, even with the naked eye.
The Pleiades form perhaps the most remarkable group of stars in the
heavens, and are probably familiar to most people, even to those whose
knowledge of the constellations is limited to a few of the brighter
stars. The cluster is a very remarkable and brilliant one, and forms a
striking object in a clear sky. There is no other group visible to the
naked eye in either hemisphere similar to it in the brightness and
closeness of the component stars. It seems to have attracted the
attention of observers since the earliest ages. Job says: “Can’st thou
bind the sweet influences of Pleiades, or loose the bands of Orion?”
Hesiod, writing nearly 1,000 years B.C., speaks of the Pleiades in words
thus translated by Cooke:—
“There is a time when forty days they lie,
And forty nights conceal’d from human eye;
But in the course of the revolving year,
When the swain sharps the scythe, again appear.”
This passage refers to the disappearance of the group in the sun’s rays
in summer, and their reappearance in the evening sky in the east at
harvest time. Hesiod also speaks of them as the seven sisters, and in
Cicero’s “Aratus,” they are represented as female heads, bearing the
names Merope, Alcyone, Celæno, Electra, Taygeta, Asterope, and Maia,
names by which they are still known to astronomers. The origin of the
name Pleiades is somewhat doubtful. Some think that it is derived from
the Greek word _pleia_, to sail. Others from the words _pleios_, full, a
name perhaps suggested by the appearance of the cluster. Although seven
stars are mentioned by Hipparchus and Aratus, Homer only speaks of six,
and this is the number now visible to average eyesight. A larger number
has, however, been seen with the naked eye by those gifted with
exceptionally keen eyesight. Möstlin, Kepler’s tutor, is said to have
seen fourteen, and he actually measured and recorded the position of
eleven, with wonderful accuracy, without the aid of a telescope! In
recent years, Miss Airy, daughter of the late astronomer-royal, has seen
twelve, and Carrington and Denning fourteen. But to most eyes probably
six only are visible with any certainty. There is a tradition that,
although seven stars were originally visible, one disappeared at the
taking of Troy. Professor Pickering has recently discovered that the
spectrum of Pleione, which forms a wide pair with Atlas, bears a
striking resemblance to that of P Cygni, the so-called “temporary star”
of 1600. This similarity of spectra suggests the idea that Pleione may
possibly—like the star in Cygnus—be subject to occasional fluctuations
of light, which might perhaps account for its visibility to the naked
eye in ancient times.
The grouping of even six stars visible to the naked eye in so small a
space is very remarkable. Considering the total number of stars visible
without optical aid, Mitchell—writing in 1767—calculated by the
mathematical theory of probability that the chances are 500,000 to one
against the close arrangement of six stars in the Pleiades being merely
the result of accident. He therefore concludes “that this distribution
was the result of design, or that there is reason or cause for such an
assemblage.”
Although to a casual observer the component stars may appear of merely
equal magnitude, there is considerable difference in their relative
brilliancy. Measures with a photometer show that Alcyone—the brightest
of the group—is of the third magnitude, Maia, Electra, and Atlas of the
fourth, Merope about 4⅓, Taygeta 4½, Celæno about 5⅓, and Asterope about
the sixth. Pleione is about 5½, according to the photometric measures
made at Oxford, but it lies so close to Atlas that to most eyes the two
will probably appear as one star. About thirty more range from the sixth
to the ninth magnitude, and this is about the number visible with an
opera-glass. Galileo counted thirty-six stars with his small telescopes,
but with modern instruments the number is largely increased. Some years
since, M. Wolf, the distinguished French astronomer, published a chart
of the Pleiades, showing about 500 stars made from his own observations.
Photography has further added to the number of stars visible in this
interesting group. On a photograph taken at the Paris Observatory in
1887, with an exposure of three hours, no less than 2,326 stars can be
distinctly counted on a space of about three square degrees. The fainter
stars on this photograph are supposed to be of the seventeenth
magnitude. Now, as Alcyone, the brightest star of the group, is of the
third magnitude, we have a difference of fourteen magnitudes between the
brightest and the faintest. This implies that Alcyone is 398,100 times
brighter than the faintest stars visible on the photographic plate. If
we could conclude that the fainter stars really belonged to the cluster,
they would be at practically the same distance from the earth, and the
great difference of brightness would be very remarkable, and would
suggest that Alcyone is a vastly larger body than the smallest stars of
the group. The difference of brilliancy given above would indicate that
the diameter of Alcyone is 631 times greater than that of the faintest
stars revealed by photography. This is of course on the assumption that
all the stars of the cluster are, surface for surface, of the same
intrinsic brilliancy, and that this apparent brightness to the eye
depends simply on their diameter. As spheres vary in volume as the cubes
of their diameters, we have the volume of Alcyone equal to the cube of
631, or over 250 million times the volume of the faintest stars of the
group. This startling result was very difficult to explain, for either
we must assume that Alcyone is an enormously vast body, or else that the
faint stars of the group are exceedingly small. If we take the diameter
of Alcyone as 1,400,000 miles, then the diameter of the faintest stars
in the group would be only 2,200 miles, or about the size of our moon,
and it seems highly improbable, if not impossible, that such small
bodies should shine with inherent light of their own. They would indeed
be “miniature suns.” On the other hand, if we assume that the faintest
stars are of about the same size as the planet Jupiter, or about 87,000
miles, the diameter of Alcyone would be nearly 55 millions of miles, a
result which is also highly improbable. The difficulty has, I think,
been satisfactorily cleared up by some photographs recently taken by
Professor Barnard at the Lick Observatory. A photograph taken with a
lens of six inches aperture, and 31 inches focal length, and an exposure
of 10 hours 15 minutes, shows that the sky surrounding the Pleiades is,
on all sides, as thickly studded with small stars as the cluster itself.
It seems clear, therefore, that the faint stars in the Pleiades are
merely some of the “hosts of heaven” which happen to lie in that
direction, and have probably no connexion with the cluster, which is
merely projected on a starry background of faint and distant stars.
The brilliancy of the Pleiades cluster would naturally suggest a
comparative proximity to the earth. Attempts to determine their distance
have, however, hitherto proved unsuccessful. This would indicate that
the distance is very great, and would, of course, lead to the conclusion
that the group is of vast dimensions. An effort has been made to
determine the distance indirectly by a consideration of the “proper
motion” of the principal stars. Professor Newcomb finds a proper motion
for Alcyone of about 5·8 seconds of arc per century. This motion is in a
direction nearly opposite to that of the sun’s motion in space, and may
possibly be due to that cause. If we assume that this apparent motion of
Alcyone is wholly due to the effect of the sun’s real motion at the rate
of, say, fourteen miles a second, the distance of Alcyone would
correspond to a “light journey” of about 267 years! Our sun, placed at
this vast distance, would, I find, be reduced in brilliancy to a star of
about the ninth magnitude, or six magnitudes fainter than Alcyone. This
would imply that Alcyone is about 250 times brighter than the sun! As,
however, the spectrum of Alcyone is of the first or Sirian type, it
cannot properly be compared with the sun.
There are six other small stars in the Pleiades having proper motions
similar in amount and direction to that of Alcyone. As the other bright
stars of the group have much smaller motions, it has been suggested that
the seven stars with comparatively large, proper motions do not really
belong to the group, but are only optically associated with it. This
would imply that the real cluster lies much farther from us than
Alcyone, and the comparative brilliancy of some of its component stars
would still denote enormous size.
In the year 1859, the well-known astronomer, Tempel, announced his
discovery of a faint nebulosity extending in a southerly direction from
Merope, the nearest bright star to Alcyone. This interesting discovery
was practically confirmed by other astronomers; but from its visibility
to some observers with small telescopes, and the failure of others to
detect it with much larger instruments, the variability of its light was
strongly suspected. The question remained in doubt for many years, but
has now been finally set at rest by photography, which shows not only a
mass of nebulous light surrounding Merope, but other nebulous spots
involving Alcyone, Maia, and Electra. Indeed, a photograph taken by Dr.
Roberts in 1889 shows that all the brighter stars of the group are more
or less surrounded by nebulosity. The nebula surrounding Maia is of a
somewhat spiral form, and its existence was not even suspected until it
was revealed by photography. It was afterwards seen with the great
30-inch refractor of the Pulkowa Observatory. Had, however, its
existence been unknown, it would probably have escaped detection, even
with this large telescope, as it is one thing to see a faint object
known to exist and another to discover it independently. Maia is
surrounded by several faint stars of the twelfth to the fourteenth
magnitude; and the Russian observers believe that one of these is
variable in light, as it was seen distinctly on February 5, 1886, when
its magnitude was carefully determined with reference to the
neighbouring stars; but on February 24 of the same year, it could not be
seen with a telescope of 15 inches aperture. Some of the other stars in
the group seem to be connected by nebulous rays with the principal
nebulous centres, and in looking at this wonderful Paris chart it seems
impossible to avoid the conclusion that the stars and nebulous masses
are actually mixed up together, and not merely placed accidentally in
the same direction. Indeed, Professor Barnard’s photograph referred to
above shows the whole group involved in dense nebulosity.
Other well-known clusters or groups of stars are the Hyades, marked by
the bright, reddish star, Aldebaran, the Præsepe, or Beehive, in Cancer,
and Comæ Berenices, but these are larger and more scattered.
[Illustration:
FIG. 9.—_The Double Star Cluster in Perseus._
(From “Scenery of the Heavens.”)
]
Of other irregular clusters, somewhat similar to the Pleiades, but not
so bright, may be mentioned the double cluster in Perseus, which is
visible to the naked eye on a clear night as a hazy spot of light in the
midst of the Milky Way. Admiral Smyth says they form “one of the most
brilliant telescopic objects in the heavens.” They may be seen with a
binocular field-glass, but, of course, a good telescope is necessary to
see them well. They have been beautifully photographed at the Paris
Observatory, the photograph showing no trace of nebulosity. They have
also been photographed by Dr. Roberts, who says, “The photograph
presents to the eye the stars in the two clusters, and in the
surrounding parts of the sky, with a completeness and accuracy of detail
never before seen. The stars are shown in their true relative positions
and magnitudes to about the sixteenth, and among them are many apparent
double, triple, and multiple stars. They also appear to be arranged in
clusters, curves, festoons, and patterns that are suggestive of some
physical connexion existing between the groups; but it is premature to
assert that these appearances are not due to perspective effect by the
eye arranging numerous close points of light into various patterns.
Similar photographs to this, taken at intervals of several years between
them, will determine the reality, or otherwise, of these remarkable
groupings of the stars.”
[Illustration:
FIG. 10.—_Star Cluster in Gemini._
(From “Scenery of the Heavens.”)
]
A little north of the star Eta Geminorum is a pretty cluster of small
stars known as 35 Messier, which is just visible to the naked eye. The
component stars may be well seen with a telescope of moderate power.
This cluster has been also photographed at the Paris Observatory, and
shows a well-marked clustering tendency in the component stars. Admiral
Smyth says: “It presents a gorgeous field of stars from the ninth to the
sixteenth magnitude, but with the centre of the mass less rich than the
rest. From the small stars being inclined to form curves of three or
four, and often with a large one at the root of the curve, it somewhat
reminds one of the bursting of a sky rocket.”
[Illustration:
FIG. 11.—_37 Messier._
(From “Worlds of Space.”)
]
About ten degrees to the north of the cluster just described is another
fine cluster known as 37 Messier. The accompanying photograph will show
its telescopic appearance.
In the Southern Hemisphere there is a magnificent cluster of small stars
surrounding the star Kappa Crucis, a reddish star of the seventh
magnitude. It was thus described by Sir John Herschel: “A most vivid and
beautiful cluster of 50 to 100 stars. Among the larger there are one or
two evidently greenish. South of the red star is one, 13 minutes, also
red, and near it one, 12 minutes, bluish ... though neither a large nor
a rich one, is yet an extremely brilliant and beautiful object when
viewed through an instrument of sufficient aperture to show distinctly
the very different colours of its constituent stars, which gives it the
effect of a superb piece of fancy jewellery.” He gives the positions of
110 stars, from the seventh to the sixteenth magnitude. It lies near the
northern edge of the well-known “coal sack,” and Dr. Gould says of it:
“The exquisitely beautiful cluster, κ _Crucis_, contains a large number
of stars of various tints and hues, contrasting wonderfully with each
other, when viewed with a telescope of large aperture.” Mr. Russell’s
drawing of this cluster, made at Sydney (N.S.W.) in 1872, shows several
changes in the relative positions of the stars as laid down by Sir John
Herschel, probably the result of proper motion.
About 2½° north of the star M Velorum, Sir John Herschel describes “an
enormous cluster, of a degree and a half in diameter, very rich in stars
of all magnitudes, from 8 minutes downwards, a sort of telescopic
Præsepe.”
Another fine cluster is that known as 11 Messier. It lies a little to
the west of the star Lambda Aquilæ, and is just visible to the naked eye
on a clear night. It consists of stars of about the eleventh magnitude,
and Admiral Smyth compared it to a “flight of wild ducks.” It has been
beautifully photographed by Dr. Roberts, who says: “The negative shows
the stars individually, though the print, owing to their closeness, does
not separate them.... It is entirely free from nebulosity.”
There are many other similar objects in both hemispheres too numerous to
mention here, but those described are interesting objects of their
class.
[Illustration:
FIG. 12.—_Star Cluster in Hercules._
(From “Scenery of the Heavens.”)
]
We now come to the “globular clusters.” This term has been applied to
those clusters of stars which evidently occupy a space of more or less
spherical form. Some of these “balls of stars,” as they have been
called, are truly wonderful, and are among the most interesting objects
visible in the sidereal heavens. Good specimens of the class are,
however, rather rare objects, and there are not many in the Northern
Hemisphere. The most remarkable, perhaps, is that called “the Hercules
cluster,” but known to astronomers as 13 Messier, it being No. 13 in the
first catalogue of remarkable “nebulæ” formed by Messier, the famous
discoverer of comets. It was discovered by Halley in 1714. This
wonderful object lies between the stars Zeta and Eta in Hercules, nearer
to the latter star. It may be seen with a binocular or good opera-glass
as a hazy star of the sixth magnitude. Messier was certain that it
contained no stars; but when examined with a good telescope it is at
once resolved into a multitude of small stars, which can be individually
seen, and even counted, with large telescopes. According to Admiral
Smyth, “No plate can give a fitting representation of this magnificent
cluster. It is indeed truly glorious, and enlarges on the eye by
studious gazing.” And Dr. Nichol says: “Perhaps no one ever saw it for
the first time through a telescope without uttering a shout of wonder.”
The number of stars included in the cluster was estimated by Sir William
Herschel at 14,000; but the real number is probably much smaller. Were
the number so great as Herschel supposed, I find that the cluster would
form a much brighter object than it does. Assuming the average magnitude
of the component stars at 12½, I find that an aggregation of 14,000
stars would shine as a star of about the second magnitude. But the
cluster is only as bright as a star of about the sixth magnitude, and,
with this magnitude, I find that the total number would be about 400.
Examining it with his giant telescope, Lord Rosse observed three dark
rifts radiating from the centre. These were afterwards seen by Buffham
with a 9-inch reflector, and also by Webb. They were also observed at
Ann Arbor Observatory (U.S.A.), in April, 1887, by Professor Harrington
and Mr. Schaeberle, using telescopes of six and twelve inches aperture.
It has been well photographed at the Paris Observatory, and also by Dr.
Roberts and Mr. Wilson. In some of these photographs the dark rifts are
perceptible to some extent, but owing to the over exposure of the
central portion of the cluster, they are not so distinct as in drawings
made at the telescope. Dr. Huggins, examining it with the spectroscope,
finds that the spectrum is not gaseous; but spectroscopic evidence is
not necessary to prove that the cluster consists of small stars, as
these are distinctly seen as points of light with telescopes of moderate
power, and with the great Lick telescope the component stars are visible
even in the central portion of the cluster. Its globular shape is
evident at a glance, and we cannot doubt that the stars composing it
form a gigantic system, probably isolated in space. Many people might
think that this cluster was a mass of double and multiple stars; but
this is not so. The components, close as they are, are too far apart to
constitute true double stars. Mr. Burnham, the famous double star
observer, finds _one_ close double star near the centre, and notes the
remarkable absence of close double stars in bright and apparently
compressed clusters.
In the same constellation, Hercules, between the stars Eta and Iota, but
nearer the latter, will be found another object of the globular class,
but not so bright or so easily resolvable into stars as the cluster
described above. It is known as 92 Messier. Buffham, with a 9-inch
mirror, thought the component stars brighter and more compressed than in
13 Messier. Sir William Herschel found it seven or eight minutes of arc
in diameter. The brighter components are easily visible in telescopes of
moderate power, but even Lord Rosse’s giant telescope failed to resolve
the central blaze. This object was photographed by Dr. Roberts in May,
1891, with a 20-inch reflecting telescope, and an exposure of one hour.
He says: “The photograph shows the cluster to be involved in dense
nebulosity, which, on the negative, almost prevents the stars being seen
through it, and on the print quite obscures the stars. The stars in
this, as in all other globular clusters, are arranged in various
patterns, and many of them appear to be nebulous.”
About three degrees north preceding the star 9 Boötis, is another fine
globular cluster, known as 3 Messier. Smyth describes it as “a brilliant
and beautiful globular congregation of not less than 1,000 stars,
between the southern Hound and the knee of Boötis; it blazes splendidly
towards the centre, and has outliers.... This mass is one of those balls
of compact and wedged stars, whose laws of aggregation it is so
impossible to assign.” The idea of the component stars being “compact
and wedged” is, however, a mistake, as I have shown elsewhere.[130] Sir
John Herschel described it as a remarkable object, exceedingly bright
and very large, with stars of the eleventh magnitude. Buffham found it
resolved even in the centre with a 9-inch mirror. It was photographed by
Dr. Roberts in May, 1891, with an exposure of two hours, and the
photograph confirms the general descriptions given of the cluster,
though “the print fails to show the stars that on the negative crowd the
space covered by the dense nebulosity.” Dr. Roberts remarks that
“nebulosity seems invariably to be present in globular clusters.” From
photographs of this cluster, taken at Arequipa in Peru, Professor Bailey
finds 87 stars of the cluster to be variable in light, the variability
amounting in some cases to two magnitudes, with usually short periods.
Another fine globular cluster is that known as 5 Messier. It lies
closely north of the fifth magnitude star, 5 Serpentis. It was
discovered by Kirch in 1702, and was observed in 1764 by Messier, who
found he could see it with a telescope of one foot in length, but could
not resolve it into stars. Smyth says: “This superb object is a noble
mass, refreshing to the senses after searching for faint objects, with
outliers in all directions, and a bright central blaze, which even
exceeds 3 Messier in concentration.” Sir William Herschel, with his
40-foot telescope, could count about 200 stars, but could not
distinguish the stars near the central blaze. Sir John Herschel
describes it as an excessively compressed cluster of a globular form,
with stars from the eleventh to the fifteenth magnitude, condensed into
a blaze at the centre. Lord Rosse found it more than seven or eight
minutes of arc in diameter, with a nebulous appearance in the centre.
This cluster was photographed by Dr. Roberts in April, 1892. “The
photograph shows the stars to about the fifteenth magnitude, and the
cluster is involved in dense nebulosity about the centre. The nebulosity
hides the stars even on the negative.” With reference to this latter
remark, however, Dr. Common says[131] that, in photographs of this
cluster taken with a larger instrument, “the stars are quite distinct,
though the exposure was much longer, a result that might fairly be
expected.” From photographs of this cluster taken at Arequipa, Peru, by
Professor Bailey, he finds that the cluster contains about 750 stars, of
which 46 are variable in light, or about 6 per cent. of the whole. This
is remarkable, for, of the stars visible to the naked eye, less than 1
per cent. are variable, so far as is at present known. A further
examination of the photographs made by Miss Leland shows that the
periods of these variables are in general very short, not exceeding a
few hours.[132] One star, situated about eight minutes of arc from the
centre of the cluster, has a probable period of 11 hours, 7 minutes, 52
seconds, and varies from about magnitudes 13·50 to 14·73. The star
remains at the minimum light for about half the period, and the maximum
brightness is of comparatively short duration. The rate of increase is
more rapid than the decrease—as in most short period variables—but in
other respects the character of the light fluctuations does not seem to
be similar to that of any other known variable star.
Another fine object of this class is that known as 15 Messier in
Pegasus, discovered by Maraldi in 1745. Sir John Herschel describes it
as a remarkable globular cluster, very bright and large, and blazing in
the centre. Webb found it a glorious object with a nine and one-third
inch mirror. It was photographed by Dr. Roberts in November, 1890, with
an exposure of two hours. He says: “The photograph confirms the general
descriptions, and the negative shows, separately, the stars of which the
cluster is composed distinctly through the nebulosity in the centre.
Many of the stars have a nebulous appearance, and they are arranged in
curves, lines, and patterns of various forms, with lanes or spaces
between them.”
We may also mention the globular cluster known as 2 Messier, which is
situated about five degrees north of the star Beta Aquarii. It was
discovered by Maraldi in 1746 while looking for Cheseaux’s comet. Sir
William Herschel, with his forty-foot telescope, could “actually see and
distinguish the stars even in the central blaze.” Sir John Herschel
compared it to a mass of luminous sand, and estimated the stars to be of
the fifteenth magnitude. It is about five or six minutes of arc in
diameter, and Smyth says: “This magnificent ball of stars condenses to
the centre, and presents so fine a spherical figure that imagination
cannot but figure the inconceivable brilliancy of the visible heavens to
its animated myriads.” Taking Sir John Herschel’s estimate of the
component stars at fifteenth magnitude, and the total light of the
cluster at sixth magnitude, I find that the total number of stars it
contains would be about 4,000.
[Illustration:
FIG. 13.—_The Star Cluster, Omega Centauri._
(From “Worlds of Space.”)
]
In the Southern Hemisphere there are some magnificent examples of
globular clusters, and indeed, this hemisphere seems to be richer in
these objects than the northern sky. Among these southern clusters is
the truly marvellous object known as Omega Centauri. Its apparent size
is very large—about two-thirds of the moon’s diameter—and it is
distinctly visible to the naked eye as a hazy star of the fourth
magnitude, and I have often so seen it in the Punjab sky. Sir John
Herschel, observing it with a large telescope at the Cape of Good Hope,
describes it as “beyond all comparison, the richest and largest object
of its kind in the heavens. The stars are literally innumerable.... All
clearly resolved into stars of two sizes, _viz._, 13 and 15; the larger
lying in lines and ridges over the smaller.... The larger form rings
like lace-work on it. One of these rings, 1½″ diameter, is so marked as
to give the appearance of comparative darkness, like a hole in the
centre.... On further attention, the hole is double, or an oval space
crossed by a bridge of stars.... Altogether, this object is truly
astonishing.” This wonderful object has recently been photographed by
Dr. Gill, at the Royal Observatory, Cape of Good Hope, and also at
Arequipa, Peru, with a telescope of thirteen inches aperture. On the
latter photograph, the individual stars can be distinctly seen and
counted. The enumeration has been made by Professor and Mrs. Bailey, and
a mean of their counts gives 6,389 for the number of stars in the
cluster, but they consider that the real number is considerably greater.
Another wonderful object is that known as 41 Toucani, which lies near
the smaller “Magellanic Cloud” in the Southern Hemisphere. Humboldt
found it very visible to the naked eye in Peru, and mistook it for a
comet.[133] Sir John Herschel describes it as “a most magnificent
globular cluster. It fills the field with its outskirts; but within its
more compressed part I can insulate a tolerably defined circular space
of 90″ diameter, wherein the compression is much more decided, and the
stars seem to run together, and this part has, I think, a pale pinkish
or rose colour, ... which contrasts evidently with the white light of
the rest.... The stars are equal, fourteen magnitude, immensely
numerous, and compressed.... It is _completely insulated_. After it has
passed, the ground of the sky is perfectly black throughout the whole
breadth of the sweep. There is a double star of eleventh magnitude
preceding the centre, ... condensation in three distinct stages.... A
stupendous object.” Dr. Gould calls it one of the most impressive, and
perhaps the grandest, of its kind in either hemisphere, and he estimated
its apparent magnitude at 4½, as seen with the naked eye.
Another remarkable globular cluster is that known as 22 Messier, which
lies about midway between Mu and Sigma Sagittarii. Sir John Herschel
says: “The stars are of two sizes, _viz._, 15 ... 16 and 12m; and, what
is very remarkable, the largest of these latter are visibly reddish, one
in particular, the largest of all (12–11m) south following the middle,
is decidedly a ruddy star, and so, I think, are all the other larger
ones ... very rich, very much compressed, gradually much brighter in the
middle, but not to a nucleus ... consists of stars of two sizes ... with
none intermediate, as if consisting of two layers, or one shell over
another. A noble object” I saw the larger stars well with a 3-inch
refractor in the Punjab.
Sir John Herschel remarks “the frequent association of nebulæ in pairs
forming double nebulæ,” and in his “Cape Observations” he figures
several examples of this class. One of these is evidently a globular
cluster, with two centres of condensation, one nucleus being much
brighter than the other. Two others, much smaller, show two distinct
nuclei. Another drawing shows apparently two globular clusters in
contact. There are other examples in the Northern Hemisphere. Dr. See
considers that some of these double nebulæ represent an early stage in
the evolution of binary or revolving double stars, and certainly some of
the drawings of these nebulæ are very remarkable and suggestive.
The actual dimensions of the globular clusters is an interesting
question. Are they composed of stars comparable in size and mass with
our sun? or are the component stars really small and comparatively close
together? This is a difficult question to answer satisfactorily, as the
distance of these objects from the earth has not yet been determined.
They may, on the one hand, be collections of suns similar to ours in
size and brightness, and situated at vast distances from the earth; or,
on the other hand, the stars composing them may be comparatively small
objects, lying at a distance from the earth not exceeding that of some
stars visible to the naked eye. Perhaps the latter hypothesis may be
considered the more probable of the two. But there is really no reason
to suppose that these collections of suns are comparatively near our
system. The probability seems to be in favour of their great distance
from the earth. The question of the absolute size of the component stars
is one which, I think, has not been hitherto sufficiently considered.
Let us examine both alternatives, and let us take the cluster Omega
Centauri as one in which the number of the component stars has been
_actually counted_. Assuming that the real number of stars in this
cluster is 10,000, and that they are individually equal, on an average,
to our sun in mass and volume, we may estimate the probable distance and
dimensions of the cluster. Taking the stellar magnitude of Omega
Centauri as four (as estimated at the Cordoba Observatory), I find that,
with the number 10,000, the average magnitude of the component stars
would be fourteen. This agrees with Sir John Herschel’s estimate of
thirteenth to fifteenth magnitude. Now, to reduce the sun to a star of
the fourteenth magnitude, I find that, assuming the sun to be 28
magnitudes brighter than an average star of the first magnitude, it
would be necessary to remove it to a distance of about 158,500,000 times
the sun’s distance from the earth—a distance so great that light would
take no less than 2,500 years to reach us from the cluster! Taking the
apparent diameter of the cluster at twenty minutes of arc, I find that
its real diameter would be 922,000 times the sun’s distance from the
earth—a distance so great that light would take over 14 years to pass
across the cluster. These results are certainly very startling, and
might lead us to suspect that these globular clusters are external
universes. Judging, however, from the average distance recently found
for stars of the first and second magnitude (see p. 423), the distance
of ordinary stars of the first magnitude—on the supposition that they
are of the same size and brightness as the sun, and that their light is
simply reduced by distance—would be about five times greater than that
found above for Omega Centauri. If, then, we increase the distance of
the cluster five times, it would be necessary to increase the diameters
of the component stars to five times that of the sun. This would give
them a volume 125 times that of our sun—a result which seems improbable.
If, on the other hand, we do not like to admit that each of the faint
points of light composing the cluster is equal in volume to our sun, let
us diminish the distance ten times. If we do so, we must also diminish
the diameter of the component stars ten times. This would make them
about the size of the planet Jupiter, and it seems improbable that such
comparatively small bodies could retain their solar heat for any great
length of time. They would probably have cooled down, as Jupiter has
done—at least to a great extent—ages ago, and would not now be visible
as a cluster of stars. Even this reduction of the distance to one-tenth
of the value first found would still leave the cluster at an immense
distance from the earth, a distance represented by 250 years of light
travel! A reduction of the distance to one-tenth of this again, or 25
years of light travel, would make the components about the size of the
earth, and that bodies of this small size could shine with stellar light
seems to be an untenable hypothesis. We seem, therefore, forced to
conclude that these globular star clusters lie at an immense distance
from the earth.
There is, however, another point to be considered with reference to the
size of the bodies composing a globular cluster. This is the character
of their light. I am not aware that the spectrum of a globular cluster
has yet been thoroughly examined, but if that of Omega Centauri is of
the first or Sirian type, it would modify the above conclusions to some
extent. It now seems probable that stars having a spectrum of the Sirian
type are intrinsically brighter than our sun, and I have shown already
that Sirius is considerably brighter than the sun would be if placed at
the same distance, although the mass of Sirius is but little more than
twice the sun’s mass. The components of a star cluster, therefore—if of
the Sirian type of stars—might be as bright as the sun, and at the same
time have a smaller mass and volume. This, however, would not make a
very great difference in the computed vast distance of the cluster, and
the calculations given above seem to point to the conclusion that these
globular clusters are probably composed of stars of average size and
mass, and that the faintness of the component stars is simply due to
their immense distance from the earth.
We will now consider the nebulæ, properly so-called, that is to say,
objects which the spectroscope shows to consist of glowing gas. These
are sometimes large and irregular in form, like the great nebula in the
“Sword” of Orion, sometimes with spiral convolutions, and sometimes of a
definite shape, like the planetary and annular nebulæ.
Of the large and irregular nebulæ, one of the most remarkable is that
known as “the great nebula in Orion.” It surrounds the multiple star,
Theta Orionis, which has been already referred to in a preceding
chapter. It is a curious fact that it escaped the searching eye of
Galileo, although he gave special attention to the constellation of
Orion, for even with a good opera-glass a nebulous gleam is distinctly
visible round the central star of the “Sword.” The nebula seems to have
been discovered by Cysat, a Swiss astronomer, in the year 1618, and it
was sketched by Huygens in 1656. Huygens says: “While I was observing
with a refractor of twenty-five feet focal length, the variable belts of
Jupiter, a dark central belt in Mars, and some phases of this planet, my
attention was attracted by an appearance among the fixed stars, which,
as far as I know, has not been observed by anyone else, and which,
indeed, could not be recognised, except by such powerful instruments as
I employ. Astronomers enumerate three stars in the Sword of Orion, lying
very near one another. On one occasion when, in 1656, I was accidentally
observing the middle one of these stars through my telescope, I saw
twelve stars instead of a single one, which, indeed, not unfrequently
happens in using the telescope. Three of this number were almost in
contact with one another, and _four_ of them shone as if through a mist,
so that the space around them, having the form drawn in the appended
figure, appeared much brighter than the rest of the sky, which was
perfectly clear, and looked almost black. This appearance looked,
therefore, almost as if there were a _hiatus_ or interruption. I have
frequently observed this phenomenon, and up to the present time, as
always unchanged in form; whence it would appear that this marvellous
object, be its nature what it may be, is very probably permanently
situated at this spot. I never observed anything similar to this
appearance in the other fixed stars.”[134] It has been called the
“fish-mouth” nebula, from the fancied resemblance of the centre portion
to the mouth of a fish. A number of small stars are visible over the
surface of the nebula, and at one time, Lord Rosse thought it showed
indications of resolution into stars when examined with his giant
telescope; but this is now known to have been a mistake, for Dr. Huggins
finds, with the spectroscope, that it consists of nothing but glowing
gas, of which hydrogen is certainly one constituent, and he has
succeeded in photographing the complete series of lines of this gas in
the spectrum of the nebula.
Referring to his earlier observations, Dr. Huggins says:—“The light from
the brightest parts of the nebula near the trapezium was resolved by the
prisms into three bright lines, in all respects similar to those of the
gaseous nebulæ. The whole of this great nebula, as far as lies within
the power of my instrument, emits light which is identical in character.
The light from one part differs from the light of another in intensity
alone.” The brightest line in the nebular spectrum—the “chief nebular
line,” as it is called—has not yet been identified with that of any
terrestrial substance. It was at first supposed to be identical with a
line of nitrogen, but this was afterwards disproved. It was then
incorrectly identified with a line of lead, and more recently by Lockyer
with the edge of a “fluting” in the magnesium spectrum. Dr. Huggins and
Professor Keeler, however, have shown conclusively that the nebular line
does not coincide with the magnesium fluting, although very close to it.
Observations by Dr. Copeland in 1886 showed the existence of the yellow
line, know as D_{3}, which is visible in the solar spectrum during total
eclipses of the sun, and indicates the existence of a gas in the sun’s
surroundings, to which the name “helium” has been given. Dr. Copeland
says:—“The recurrence of this line in the spectrum of a nebula is of
great interest, as affording another connecting link between gaseous
nebula and the sun and stars with bright line spectra, especially with
that remarkable class of stars of which the finest examples were
detected by M. M. Wolf and Rayet in the constellation of Cygnus.”[135]
As has been already mentioned in the chapter on variable and new stars,
the bright lines of hydrogen and helium have also been observed in the
spectra of these remarkable objects. The gas, giving the line D_{3} in
its spectrum, has quite recently been discovered by Professor Ramsay in
gases obtained by heating certain terrestrial minerals, so that the
objective existence of the gaseous element “helium”—previously only
suspected—is now definitely established. From recent spectroscopic
observations of the Orion nebula, Dr. Huggins thinks that “the stars of
the ‘trapezium’ are not merely optically connected with the nebula, but
are physically bound up with it, and are very probably condensed out of
the gaseous matter of the nebula.” With reference to this point,
Professor Keeler, who has carefully examined the spectra of the nebula
and the associated stars, says:—“The trapezium stars have spectra marked
by strong absorption bands; they have not the direct connexion with the
nebula that would be indicated by a bright line spectrum, but are, in
fact, on precisely the same footing (spectroscopically) as other stars
in the constellation of Orion. While their relation to the nebula is
more certain than ever, they can no longer be regarded as necessarily
situated _in_ the nebula, but within indefinite limits they may be
placed anywhere in the line of sight.” These results were confirmed by
Professor Campbell. He finds, “that of the twenty-five bright lines
known to exist in the spectrum of the Orion nebula, at least nineteen
are definitely matched by dark lines in the Orion stars, and at least
fifteen by dark lines in the six faint stars situated in the dense parts
of the nebula.”
Numerous drawings of this wonderful nebula have been made. Of these, the
best are those by Sir John Herschel, made at the Cape of Good Hope in
the years 1834–38, by Bond in America, and by Lassell at Malta. The
difficulty of accurately delineating so difficult and delicate an object
has given rise to discrepancies in the drawings, which have led to the
idea that changes of form have occurred, but this seems improbable. The
nebula has been very successfully photographed by Dr. Common and Dr.
Roberts, and these photographs confirm the general accuracy of the later
drawings.
From a consideration of the apparent size of the Orion nebula and its
probable mass and distance from the earth, the late Mr. Ranyard came to
the conclusion that its average density “cannot exceed one ten thousand
millionth of the density of atmospheric air at the sea-level.”[136]
Mr. W. H. Pickering and Dr. Max Wolf have photographed another nebula
surrounding the star Zeta Orionis—the southern star of the “Belt,” which
seems to be connected with the nebula in the “Sword”; and, Prof.
Barnard, using the “lens of a cheap oil lantern” of 1½ inch aperture,
and 3½ inches focal length, has photographed “an enormous curved
nebulosity” stretching over nearly the whole of the constellation of
Orion, and involving the “great nebula.”
[Illustration:
FIG. 14.—_The Orion Nebulæ._
(From “Worlds of Space.”)
]
Prof. Keeler has recently found, with the spectroscope, that the Orion
nebula is apparently receding from the earth at the rate of nearly
eleven miles a second, but this motion may be, in part at least, due to
the sun’s motion in space in the opposite direction. Prof. Pickering
considers that the parallax of the nebula is probably not more than
0·″003, which corresponds to a thousand years’ journey for light!
In the southern constellation, Argo is a magnificent nebula, somewhat
similar in appearance to the great nebula in Orion. It surrounds the
famous variable star Eta Argûs, whose remarkable fluctuations in light
have been already described in the chapter on variable stars. It is
sometimes spoken of as the “key-hole” nebula, owing to a curious opening
of that shape near its centre. It was carefully drawn by Sir John
Herschel at the Cape of Good Hope in the years 1834–38. It lies in a
very brilliant portion of the Milky Way, and Sir John Herschel thus
describes it: “It is not easy for language to convey a full impression
of the beauty and sublimity of the spectacle which the nebula offers as
it enters the field of view of a telescope, fixed in right ascension, by
the diurnal motion, ushered in as it is by so glorious and innumerable a
procession of stars, to which it forms a sort of climax, and in a part
of the heavens otherwise full of interest,” and he adds: “In no part of
its extent does this nebula show any appearance of resolvability into
stars, being, in this respect, analogous to the nebula of Orion. It has,
therefore, nothing in common with the Milky Way, on the ground of which
we see it projected, and may therefore be, and not improbably is, placed
at an immeasurable distance behind that stratum.” Sir John Herschel’s
conclusion as to its physical constitution has been fully confirmed by
the spectroscope, which shows it to consist of luminous gas. As in the
Orion nebula, there are numerous stars scattered over it. Some of these
may possibly have a physical connexion with the nebula, while others may
belong to the Milky Way. The nebula is of great extent, covering an
apparent space about five times the area of the full moon, and its real
dimensions must be enormous. It was photographed by Mr. Russell,
director of the Sydney Observatory, in July, 1890, and the photograph
shows that “one of the brightest and most conspicuous parts of the
nebula”—the swan-shaped form near the centre of Herschel’s drawing—has
“wholly disappeared,” and its place is now occupied by “a great, dark
oval.” Mr. Russell first missed the vanished portion of the nebula in
the year 1871, while examining it with a telescope of 11½ inches
aperture, and the photograph now confirms the disappearance, which is
very remarkable, and shows that changes are actually in progress in
these wonderful nebulæ, changes which may be detected after a
comparatively short interval of time.
[Illustration:
FIG. 15.—_Sir John Herschel’s drawing of the Nebula round Eta Argus._
(From Flammarion’s “Popular Astronomy.”)
]
Smaller than the nebula in Argo, but somewhat similar in general
appearance, is that known as 30 Doradus, which forms one of the numerous
and diverse objects which together constitute the greater Magellanic
Cloud. Sir John Herschel drew it carefully at the Cape of Good Hope, and
describes it as “one of the most singular and extraordinary objects
which the heavens present,” and he says “it is unique even in the system
to which it belongs, there being no other object in either nubecula to
which it bears the least resemblance.” It is sometimes called the
“looped nebula,” from the curious openings it contains. One of these is
somewhat similar to the “key-hole” opening in the Argo nebula. Near its
centre is a small cluster of stars, and scattered over the nebula are
many faint stars, of which Sir John Herschel gives a catalogue of 105
ranging from the ninth to the seventeenth magnitude. I do not know
whether this nebula has been examined with the spectroscope, but its
appearance would suggest that it is gaseous. It is remarkable as being
the only object of its class which is found outside the zone of the
Milky Way.
Among the nebula of irregular shape, although its spectrum is said to be
not gaseous, may be mentioned that known as the “trifid nebula,” or 20
Messier. It lies closely north of the star 4 Sagittarii in a magnificent
region of the heavens. As will be seen in the drawing made by Sir John
Herschel at the Cape of Good Hope, the principal portion consists of
three masses of nebulous matter separated by dark “lanes” or “rifts.”
Near the junction of the three “rifts” is a triple star. A beautiful
drawing of this nebula has also been made by Trouvelot. It agrees fairly
well with that of Sir John Herschel, but shows more detail.
[Illustration:
FIG. 16.—_The Trifid Nebula, Sagittarius._
(From “Scenery of the Heavens.”)
]
Among other gaseous nebula may be mentioned that called by Sir John
Herschel the “dumb-bell” nebula. It lies a little south of the sixth
magnitude star 14 Vulpeculæ, and was discovered by Messier in 1779,
while observing Bode’s comet of that year. In small telescopes it has
the appearance of a dumb-bell, or hour-glass, but in larger telescopes
the outline is filled in with fainter nebulous light, giving to the
whole an elliptical form. Several faint stars have been seen in it, but
these probably belong to the Milky Way, as Dr. Huggins finds the
spectrum gaseous. Dr. Roberts has photographed it, and he thinks that
“the nebula is probably a globular mass of nebular matter, which is
undergoing the process of condensation into stars, and the faint
protrusions of nebulosity in the _south following_ and _north preceding_
ends are the projections of a broad ring of nebulosity which surrounds
the globular mass. This ring, not being sufficiently dense to obscure
the light of the central region of the globular mass, is dense enough to
obscure those parts of it that are hidden by the increased thickness of
the nebulosity, thus producing the ‘dumb-bell’ appearance. If these
inferences are true, we may proceed yet a step, or a series of steps,
farther, and predict that the consummation of the life-history of this
nebula will be its reduction to a globular cluster of stars.”
Among the gaseous nebula may also be included those known as “annular
nebulæ.” These are very rare objects, only a few being known in the
whole heavens. The most remarkable is that known as 57 Messier, which
lies between the stars Beta and Gamma Lyræ, south of the bright star
Vega. It was discovered by Darquier, at Toulouse, in 1779, while
following Bode’s comet of that year. Lord Rosse thought it resolvable
into stars, and so did Chacornac and Secchi, but no stars are
perceptible with the great American telescopes, and Dr. Huggins finds it
to be gaseous. The central portion is not absolutely dark, but contains
some faint nebulous light. Examined with the great telescope of the Lick
Observatory, Professor Barnard finds that the opening of the ring is
filled in with fainter light “about midway in brightness between the
brightness of the ring and the darkness of the adjacent sky.”[137] “The
aperture was more nearly circular than the outer boundary of the nebula,
so that the ends of the ring were thicker than the sides.” The entire
nebula was of a milky colour. A central star, noticed by some observers,
was usually seen by Professor Barnard, but was never a conspicuous
object. He found the extreme dimensions of the nebula about 81″ in
length by about 59″ in width, or more than double the apparent area of
Jupiter’s disc. It has been beautifully photographed by Dr. Roberts, and
he says “the photograph shows the nebula and the interior of the ring
more elliptical than the drawings and descriptions indicate; and the
star of the _following_ side is nearer to the ring than the distance
given. The nebulosity on the _preceding_ and _following_ ends of the
ring protrudes a little, and is less dense than on the _north_ and
_south_ sides. This probably suggested the filamentous appearance which
Lord Rosse shows. Some photographs of the nebula have been taken between
1887 and 1891, and the central star is strongly shown on some of them,
but on others it is scarcely visible, which points to the star being
variable.” On a photograph taken by MM. Androyer and Montaugerand of the
Toulouse Observatory, with an exposure of nine hours (in multiple
exposures), about 4,800 stars are visible on and near the nebula in an
area of three square degrees.
Another object of the annular class will be found a little to the
south-west of the star Lambda Scorpii. It is thus described by Sir John
Herschel: “A delicate, extremely faint, but perfectly well defined,
annulus. The field crowded with stars, two of which are on the nebula. A
beautiful, delicate ring, of a faint, ghost-like appearance, about 40″
in diameter in a field of about 150 stars, eleven and twelve magnitude
and under.”
Near the stars 44 and 51 Ophiuchi is another object of the annular
class, which Sir John Herschel describes as “exactly round, pretty
faint, 12″ diameter, well terminated, but a little cottony at the edge,
and with a decided darkness in the middle, equal to a tenth magnitude
star at the most. Few stars in the field, a beautiful specimen of the
planetary annular class of nebula.”
The Planetary Nebulæ form an interesting class. They were so named by
Sir William Herschel from their resemblance to the discs of the planets,
but, of course, much fainter. They are generally of uniform brightness,
without any nucleus or brighter part in the centre. There are numerous
examples of this class, one of the most remarkable being that known as
97 Messier, which is situated about two degrees south-east of Beta Ursæ
Majoris—the southern of the two “pointers” in the Plough. It is of
considerable apparent size, and even supposing its distance to be not
greater than that of 61 Cygni, its real dimensions must be enormous.
Lord Rosse observed two openings in the centre with a star in each
opening, and from this appearance he called it the “owl nebula.” One of
the stars seems to have disappeared since 1850, and a photograph
recently taken by Dr. Roberts confirms the disappearance.
Another fine object of the planetary class is one which lies close to
the pole of the ecliptic. Webb saw it “like a considerable star out of
focus.” Smyth found it pale blue in colour. Dr. Huggins finds a gaseous
spectrum, the first discovery of the kind made. Professor Holden,
observing it with the great Lick telescope, finds its structure
extraordinary. He says it “is apparently composed of rings overlying
each other, and it is difficult to resist the conviction that these are
arranged in space in the form of a true helix,” and he ranks it in a new
class which he calls “helical nebulæ.”
A somewhat similar nebula lies a little to the west of the star Nu
Aquarii. Secchi believed it to be in reality a cluster of small stars,
but Dr. Huggins finds its spectrum gaseous. A small nebula on each side
gives it an appearance somewhat similar to the planet Saturn, with the
rings seen edgeways. The great Lick telescope shows it as a wonderful
object—“a central ring lies upon an oval of much fainter nebulosity.”
Professor Holden says “the colour is a pale blue,” and he compares the
appearance of the central ring “to that of a footprint left in the wet
sand on a sea beach.”
About two degrees south of the star Mu Hydræ is another planetary
nebula, which Smyth describes as resembling the planet Jupiter in “size,
equable light and colour.” Webb saw it of “a steady, pale blue light,”
and Sir John Herschel, at the Cape of Good Hope, speaks of its colour as
“a decided blue—at all events, a good sky-blue,” a colour which seems
characteristic of these curious objects. Although Sir William Herschel,
with his large telescopes, failed to resolve it into stars, Secchi
thought he saw it breaking up into stars with a “sparkling ring.” Dr.
Huggins, however, finds the spectrum to be gaseous, so that the luminous
points seen by Secchi could not have been stellar.
Sir John Herschel, in his “Cape Observations,” describes a planetary
nebula which lies between the stars Pi Centauri and Delta Crucis. He
says it is “perfectly round, very planetary, colour fine blue ... very
like Uranus, only about half as large again, and blue.... It is of the
most decided independent blue colour when in the field by itself, and
with no lamplight and no bright star. About 10′ north of it is an
orange-coloured star, eighth magnitude. When this is brought into view,
the blue colour of the nebula becomes intense ... colour, a beautiful
rich blue, between Prussian blue and verditer green.”
There are some rare objects called “nebulous stars.” The star Epsilon
Orionis—the centre star of Orion’s Belt—is involved in a great nebulous
atmosphere. The triple star Iota Orionis is surrounded by a nebulous
haze. The star Beta in Canes Venatici is a 4½ magnitude star surrounded
by a nebulous atmosphere.
The term elliptical nebulæ has been applied to those of an elliptical or
elongated shape. This form is probably due in many cases to the effect
of perspective, their real shape being circular, or nearly so. Perhaps
the most remarkable object of this class is the well-known “nebula in
Andromeda,” known to astronomers as 31 Messier. It can be just seen with
the naked eye, on a clear moonless night, as a hazy spot of light near
the star Nu Andromedæ, and it is curious that it is not mentioned by the
ancients, although it must have been very visible to their keen eyesight
in the clear Eastern skies. It was, however, certainly seen so far back
as 905 A.D., and it Is referred to as a familiar object by the Persian
astronomer, Al-Sûfi, who wrote a description of the heavens about the
middle of the tenth century. Tycho Brahé and Bayer failed to notice it,
but Simon Marius saw it in December, 1612, and described it “as a light
seen from a great distance through half-transparent horn plates.” It was
also observed by Bullialdus, in 1664, while following the comet of that
year. It has frequently been mistaken for a comet by amateur observers
in recent years. Closely north-west of the great nebula is a smaller one
discovered by Le Gentil in 1749, and another to the south, detected by
Miss Caroline Herschel in 1783. The great nebula is of an elliptical
shape and considerable apparent size. The American astronomer, Bond,
using a telescope of 15 inches aperture, traced it to a length of about
four degrees, and a width of two and a half degrees. A beautiful
photograph taken by Dr. Roberts in December, 1888 (see p. 398), shows an
extension of nearly two degrees in length, and about half a degree in
width, or considerably larger than the apparent size of the full moon.
Bond could not see any symptom of resolution into stars, but noticed two
dark rifts or channels running nearly parallel to the length of the
nebula. In Dr. Roberts’ photograph these rifts are seen to be really
dark intervals between consecutive nebulous rings into which the nebula
is divided. Dr. Roberts says: “A photograph which I took with the
20-inch reflector on October 10, 1887, revealed for the first time the
true character of the great nebula, and one of the features exhibited
was that the dark bands, referred to by Bond, formed parts of divisions
between symmetrical rings of nebulous matter surrounding the large
diffuse centre of the nebula. Other photographs were taken in 1887,
November 15; 1888, October 1; 1888, October 2; 1888, December 29;
besides several others taken since, upon all of which the rings of
nebulosity are identically shown, and thus the photographs confirm the
accuracy of each other, and the objective reality of the details shown
of the structure of the nebula.” Dr. Roberts adds: “These photographs
throw a strong light on the probable truth of the _Nebular Hypothesis_,
for they show what appears to be the progressive evolution of a gigantic
stellar system.”
The largest telescopes have hitherto completely failed to resolve this
wonderful object into stars. Dr. Huggins, however, finds that the
spectrum is _not_ gaseous, so that if the nebula really consists of
stellar points, they must be of very small dimensions. Assuming a
parallax of one-fiftieth of a second of arc—corresponding to 163 years
of “light travel”—I find that our sun, placed at this distance, would be
reduced in brightness to a star of about the eighth magnitude. If we
assume the components to have only one-hundredth of the sun’s diameter,
they would shine as stars of only the eighteenth magnitude, which no
telescope yet constructed would show as separate points of light. A more
probable explanation, however, seems to be that the nebula may consist
of masses of nebulous matter partially condensed into the solid form,
but not yet arrived at the stage in which our sun is at present. In
other words, the whole nebulous mass may be in a fluid or viscous state,
which might perhaps account for the continuous spectrum found by Dr.
Huggins.
The question may be asked, What is the probable size and distance of
this wonderful nebula? and could it be an external universe? Possibly
its distance from the earth may be even greater than that indicated by
the small parallax I have assumed above, but taking this parallax and
the apparent dimensions of the nebula as shown by Dr. Huggins’
photograph, I find that its real distance would be no less than 330,000
times the sun’s diameter from the earth, a diameter so great that light
would take over five years to pass from one side of the nebula to the
other! This result might lead us to imagine that the nebula may be
really an external universe. But let us consider the matter a little
further. The diameter found above is not very much greater than the
distance of the _nearest_ fixed star, Alpha Centauri, from the earth,
and the limits of _our_ universe are certainly far beyond Alpha
Centauri. If we diminish the parallax to, say ¹⁄₂₀₀th of a second, or a
“light journey” of 652 years, the diameter of the nebula would be
increased to 1,320,000 times the sun’s distance from the earth, or about
five times the distance of Alpha Centauri, and there are probably many
faint stars belonging to our system much farther from the earth than
this.
The temporary star which appeared near the nucleus of the nebula in
August, 1885—already referred to in the chapter on variable stars—was of
the seventh magnitude. I find that our sun, if placed at the distance
indicated by a parallax of ¹⁄₂₀₀th of a second, would be reduced to a
star of about the eleventh magnitude, or four magnitudes fainter than
the temporary star appeared to us. That is to say, the star would have
been—with the assumed distance—about forty times brighter than the sun.
With any greater distance, the star would have been proportionately
brighter, compared with the sun. This seems improbable, and tends to the
conclusion that the nebula is _not_ an external galaxy, but a member of
our own sidereal system, a system which probably includes all the stars
and nebulæ visible in our largest telescopes. Dr. Common, indeed,
suggests that it may be comparatively near our system. He says: “It is
difficult to imagine that such an enormous object, as the Andromeda
nebula must be, is not very near to us; perhaps it may be found to be
the nearest celestial object of all beyond the solar system. It is one
that offers the best chance of the detection of parallax, as it seems to
be projected on a crowd of stars, and there are well defined points that
might be taken as fiducial points for measurement,” and he adds: “Apart
from the great promise this nebula seems to give of determining
parallax, there is a fair presumption that in the course of time, the
rotation of the outer portion may perhaps be detected by observation of
the positions of the two outer detached portions in relation to the
neighbouring stars.”[138] Prof. Hall’s failure to detect any parallax in
the temporary star, as mentioned in the last chapter, is, of course,
against Dr. Common’s idea of its proximity to the earth. Referring to
the latter portion of Dr. Common’s remarks, Mr. C. Easton points
out[139] that a comparison of a drawing by Trouvelot, in 1874, with Dr.
Roberts’ photograph, suggests that the small elongated nebula—_h_
44—which lies to the north of the great nebula, “has turned about 15°
from left to right. The globular nebula (M 32), to the other side of M
31, seems to have slightly shifted its position.”
[Illustration:
FIG. 17.—_Spiral Nebula, 51 Messier._
(From “The Visible Universe.”)
]
The spiral nebulæ are wonderful objects, and were discovered by the late
Lord Rosse, with his great six-foot telescope. Their character has been
fully confirmed by photographs taken by Dr. Roberts. One of the most
remarkable of these extraordinary objects is that known as 51 Messier.
It lies about three degrees south-west of the bright star Eta Ursæ
Majoris—the star at the end of the Great Bear’s tail. It was discovered
by Messier while comet-hunting on October 13, 1773. Telescopes of
moderate power merely show two nebulæ nearly in contact, but Lord Rosse
saw it as a wonderful spiral, and his drawing agrees fairly well with a
photograph taken by Dr. Roberts in April, 1889. The nebula has also been
photographed by Dr. Common. Dr. Roberts says: “The photograph shows both
nuclei of the nebula to be stellar, surrounded by dense nebulosity, and
the convolutions of the spiral in this as in other spiral nebulæ are
broken up into star-like condensations with nebulosity around them.
Those stars that do not conform to the trends of the spiral have
nebulous trails attached to them, and seem as if they had broken away
from the spirals.” A tendency to a spiral structure in the smaller
nebula is also visible on the original negative. Dr. Huggins finds that
the spectrum is _not_ gaseous.
The nebulæ known as 99 Messier is of the spiral form. It lies on the
borders of Virgo and Coma Berenices, near the star 6 Comæ. In large
telescopes it somewhat resembles a “Catherine wheel.” D’Arrest and Key
thought it resolvable into stars. It has been photographed by M. Von
Gothard.
Among the clusters and nebulæ, we may class the Magellanic Clouds, or
Nubeculæ in the Southern Hemisphere, as they consist of stars, clusters,
and nebulæ. These very remarkable objects form two bright spots of milky
light, which, at first sight, look like luminous patches of the Milky
Way, but are in no way connected with the Galaxy. Sir John Herschel,
speaking of the larger cloud, says: “The immediate neighbourhood of the
Nubecula Major is somewhat less barren of stars than that of the Minor,
but it is by no means rich, nor does any branch of the Milky Way
whatever form any certain or conspicuous junction with, or include, it,”
and again he says, with reference to the smaller cloud: “Neither with
the naked eye, nor with a telescope, is any connexion to be traced
either with the greater Nubecula, or with the Milky Way.” The Nubeculæ
are roughly circular in form, and, viewed with the naked eye, they very
much resemble irresolvable nebulæ as seen in a telescope. The larger
cloud, or Nubecula Major, as it is called, is of considerable extent,
and covers about 42 square degrees, or over two hundred times the
apparent size of the full moon. It was called by the Arabs _el-baker_,
or “the White Ox,” and is referred to by Al-Sûfi in his “Description of
the Heavens,” written in the tenth century. When examined with a good
telescope, it is found to consist of about six hundred stars of the
sixth to the tenth magnitude, with many fainter ones, and about three
hundred clusters and nebulæ. Sir John Herschel, in his “Cape
Observations,” says: “The Nubeculæ Major, like the Minor, consists
partly of large tracts and ill-defined patches of irresolvable nebula,
and of nebulosity in every stage of resolution, up to perfectly resolved
stars like the Milky Way, as also of regular and irregular nebulæ
properly so-called, of globular clusters in every stage of
resolvability, and of clustering groups sufficiently insulated and
condensed to come under the designation of ‘clusters of stars.’... It is
evident, from the intermixture of stars and unresolved nebulosity, which
probably might be resolved with a higher optical power, that the
nubeculæ are to be regarded as systems _sui generis_, and which have no
analogues in our hemisphere.”
The smaller Magellanic Cloud, or Nubecula Minor, is fainter to the eye,
and not so rich in the telescope. It covers about 10 square degrees, or
about fifty times the area of the full moon. Sir John Herschel, in his
“Cape Observations,” describes it as “a fine large cluster of very small
stars, 12 ... 18 magnitude, which fills more than many fields, and is
broken into many knots, groups, and straggling branches, but _the whole_
(_i.e._, the whole of the clustering part) is clearly resolved.” It is
surrounded by a barren region remarkably devoid of stars. Sir John
Herschel says: “The access to the Nubecula Minor is on all sides through
a desert.”... “It is preceded at a few minutes in R. A. by the
magnificent globular cluster, 47 Toucani (Bode), but is completely cut
off from all connexion with it; and with this exception, its situation
is in one of the most barren regions in the heavens.” Herschel found the
middle of the cloud clearly resolved into stars, while its edges
remained irresolvable with his large reflector. He says: “The edge of
the smaller _cloud_ comes on as a mere nebula.... We are now _in the
cloud_. The field begins to be full of a faint light perfectly
irresolvable.... I should consider about this place to be the body of
the cloud which is here fairly resolved into excessively minute
stars.... It is not like the stippled ground of the sky. The borders
fade away, quite insensibly, and are less or not at all resolved.”
Herschel gives a catalogue of 244 objects in the Nubecula Minor. Of
these about 200 are stars, and the remainder nebula and clusters. From
this it appears that the smaller nubecula contains a much larger
proportion of stars than the larger cloud.
Judging from their roughly globular form, the dimensions of the
Magellanic Clouds are probably small compared with their distance from
the earth, so that in these remarkable objects—particularly in the
larger cloud—we see stars of the seventh, eighth, ninth, and tenth
magnitude, apparently mixed up with fainter stars, and “clusters of all
degrees of resolvability,” and Sir John Herschel says: “It must
therefore be taken as a demonstrated fact, that stars of the seventh or
eighth magnitude, and irresolvable nebulæ, may co-exist within limits of
distance not differing in proportion more than as 9 to 10.”[140] It
should be remembered, however, that possibly some of the fainter stars
may—as in the Pleiades—lie far out in space beyond the greater
Magellanic Cloud.
The Magellanic Clouds have recently been photographed by Mr. Russell at
the Sydney Observatory. He finds the larger cloud—the Nubecula Major—to
be of a most complex form, with evidence of a spiral structure, a
feature also traceable, but not so clearly, in a photograph of the
Nubecula Minor, or smaller cloud.
Dr. Dreyer’s new index catalogue of recent discoveries of nebulæ,
together with the general catalogue previously published, gives the
position of 9,369 nebulæ.[141] A very small proportion of the new
discoveries have been made by photography, and more than half of them
were found by M. Javelle with the great refractor of the Nice
Observatory. Most of the new objects are very small and faint, and form
probably “only a small portion of the number visible in large
telescopes.”
[Illustration:
FIG. 18.—_Magellanic Clouds._
(From “Worlds of Space.”)
]
Several nebulæ have been suspected of variation in light. One discovered
by Dr. Hind in 1852 near the variable star T Tauri was found to be an
easy object with the great Lick telescope in February, 1895, but in
September of the same year it had “entirely vanished.” In the same
instrument, “T Tauri was involved in a small hazy nebulosity, but the
definite nebula in which it shone in 1890 did not exist in September,
1895.”[142]
CHAPTER VII.
THE CONSTRUCTION OF THE HEAVENS.
The construction of the visible universe is one of great interest, but
of considerable difficulty. If we reflect that in viewing the starry
heavens we are placed at the centre of a hollow sphere of indefinite
extent, and that the distance of only a few of the stars from the earth
has hitherto been ascertained with any approach to accuracy, the great
difficulty of framing a satisfactory theory of the construction of the
heavens will be easily understood.
In considering the subject, let us first inquire as to the probable
number of stars visible in our largest telescopes. Are the visible stars
infinite or limited in number? The reply to this question is easy. As
the number of stars visible to the naked eye is limited, so the number
of stars visible in the largest telescopes is limited also. Those who do
not give the subject sufficient consideration seem to think that the
number of the stars is practically infinite, or at least that the number
is so great that it cannot be estimated. But this idea is totally
incorrect, and due to complete ignorance of telescopic revelations. It
is certainly true that, to a certain extent, the larger the telescope
used in the examination of the heavens, the more the number of the stars
seems to increase; but we now know that there is a limit to this
increase of telescopic vision. And the evidence clearly shows that we
are rapidly approaching this limit. Although the number of stars visible
in the Pleiades rapidly increases at first with increase in the size of
the telescope used, and although photography has still further increased
the number of stars in this remarkable cluster, it has recently been
found that an increased length of exposure—beyond three hours—adds very
few stars to the number visible on the photograph taken at the Paris
Observatory in 1885, on which over 2,000 stars can be counted. Even with
this great number on so small an area of the heavens, comparatively
large vacant spaces are visible between the stars, and a glance at the
original photograph is sufficient to show that there would be ample room
for many times the number actually visible. I find that, if the whole
heavens were as rich in stars as the Pleiades, there would be only 33
millions in both hemispheres.
On a photograph of the region surrounding Gamma Cassiopeiæ, taken by Dr.
Roberts in December, 1895, with a reflecting telescope of 20 inches
aperture, and an exposure of two hours and twelve minutes, he finds
17,100 stars on an area of four square degrees. This would give for the
whole area of the heavens—if equally rich in stars—a total of about 176
millions; but Gamma Cassiopeiæ lies in a rich region of the Milky Way,
and probably the great majority of the stars shown on Dr. Roberts’
photograph belong to the Galaxy, which we know to be especially rich in
stars. One thing is certain, that the heavens as a whole are not nearly
so rich as this particular spot. There may, perhaps, be richer spots
elsewhere in the Milky Way, but in other parts of the sky there are many
regions considerably poorer.
Let us consider a still more extreme case of stellar richness. On a
photograph of the great globular cluster, Omega Centauri, recently taken
in Peru, a count of the stars has been carefully made by Professor and
Mrs. Bailey, and, as stated in the last chapter, the number of stars
contained in the cluster may be taken as 10,000. Now, if the whole sky
were as thickly studded with stars as in this cluster, the total number
visible in the whole heavens would be 1,650 millions, a very large
number, of course, but not much in excess of the present population of
the earth, and I am not aware that the number of the earth’s inhabitants
has ever been described as “infinite.”
Clusters, such as the Pleiades and Omega Centauri, are, of course,
remarkable, and rare exceptions to the general rule of stellar
distribution, and the heavens in general are not—even in the richest
portions of the Milky Way—nearly so rich in stars as the globular
clusters. The fact of these clusters being remarkable objects, proves
that they are unusually rich in stars, and there is strong
evidence—evidence amounting to absolute proof in the case of the
globular clusters—that these collections of stars are really, and not
apparently, close, and that they are actually systems of suns, and
occupy a comparatively limited volume in space. We cannot, then,
estimate the probable number of the visible stars by counting those
visible in one of the globular clusters.
That the number of the visible stars will not probably be largely
increased by any increase in telescopic power, is indicated by the fact
that Celoria, using a small telescope, of power barely sufficient to
show stars to the eleventh magnitude, found that he could see almost
exactly the same number of stars near the north pole of the Milky Way as
were visible in Sir William Herschel’s great telescope! thus indicating
that, here at least, no increase of optical power will materially
increase the number of stars visible in that direction; for Herschel’s
large telescope certainly showed far fainter stars than those of the
eleventh magnitude in other portions of the heavens. It should therefore
have shown fainter stars at the pole of the Milky Way also, if such
stars existed in that region of space. Their absence, therefore, seems
certain proof that very faint stars do _not_ exist in that direction,
and that, here at least, our sidereal universe is limited in extent A
photograph, taken by Dr. Roberts not very far from the spot in question,
shows only 178 stars to the square degree. This rate of distribution
would give a total of only 7,343,000 stars for both hemispheres!
An examination by Miss Clerke of Professor Pickering’s catalogue of
stars surrounding the north pole of the heavens shows that “the small
stars are overwhelmingly too few for the space they must occupy, if of
average brightness; and they are too few in a constantly increasing
ratio.”[143] Here again, a “thinning out” of the stellar hosts seems
clearly indicated, and suggests that a limit will soon be reached,
beyond which our most powerful telescopes and photographic plates will
fail to reveal any further stars.
Let us now consider the number of stars actually visible. Maps of the
northern portion of the heavens have been published by Argelander and
Heis, and charts of the southern sky by Behrmann and Gould. Heis shows
stars to about magnitude 6⅓, and Behrmann to about the same brightness.
I find that the total number shown by both observers, as visible to the
naked eye, is 7,249. The total number, to the sixth magnitude inclusive,
shown by both observers, is 4,181. Argelander gives 5,000 stars to the
sixth magnitude inclusive, and for stars to the ninth magnitude, the
following numbers in each magnitude:—First magnitude, 20; second
magnitude, 65; third magnitude, 190; fourth magnitude, 425; fifth
magnitude, 1,100; sixth magnitude, 3,200; seventh magnitude, 13,000;
eighth magnitude, 40,000; and ninth magnitude, 142,000, or a total of
“200,000 for the entire number of stars from the first to the ninth
magnitude inclusive.”[144] This result agrees closely with an estimate
previously made by Struve. From a formula given by Dr. Gould, deduced
from observations in the Southern Hemisphere, I find the number of stars
to the ninth magnitude inclusive would be 215,674, so that Argelanders
estimate of 200,000 stars to the ninth magnitude inclusive cannot be far
from the truth. It will be seen from Argelanders figures that the number
of stars in each class of magnitude is roughly three times that in the
class one magnitude brighter. Supposing this progressive increase
continued to the seventeenth magnitude—the faintest visible in the great
Lick telescope—I find that the total number of stars would be nearly
1,400 millions, or less than the number found from a consideration of
the cluster Omega Centauri. But it is evident from Celoria’s
observation, referred to above, and from Professor Pickering’s
photographs of stars near the North Pole, that the fainter stars do
_not_ increase in the ratio assumed above. We must therefore conclude
that there is a “thinning out” of the fainter stars at some point below
the ninth magnitude. Taking into consideration the rich regions of the
Milky Way, and the comparatively poor portions of the sky, it is now
generally admitted by astronomers, who have studied this particular
question, that the probable number of stars visible in our largest
telescopes does not exceed 100 millions, a number which, large as it
absolutely is, may be considered as relatively very small, and even
utterly insignificant, when compared with an “infinite number.”
Let us see what richness of stellar distribution is implied by this
number of 100 millions of visible stars. It may be easily shown that the
area of the whole sky, in both hemispheres, is 41,253 square degrees, or
about 200,000 times the area of the full moon. This gives 2,424 stars to
the square degree. The moon’s apparent diameter being slightly over half
a degree (31′ 5″), the area of its disc is about one-fifth of a square
degree. Hence, for 100 millions of stars in the whole star sphere, we
have 485 stars to each space of sky, equal in area to the full moon.
This seems a large number, but stars scattered even as thickly as this
would appear at a considerable distance apart when viewed with a large
telescope and a high power. As the area of the moon’s disc contains
about 760 square minutes of arc, there would not be an average of even
one star to each square minute. A pair of stars half a minute, or 30
seconds, apart, would form a very wide double star, and with stars
placed at even this distance, the moon’s disc would cover about 3,000,
or over six times the actual number visible in the largest telescopes.
In Dr. Roberts’ photograph of the region surrounding Gamma Cassiopeiæ,
which shows over 17,000 stars, on four square degrees, or over 4,000
stars to the square degree, the stars do not seem very crowded, and
there is a good deal of black sky visible between them.
But, in addition to the conclusive evidence as to the limited number of
the visible stars derived from actual observation and the results of
photography, we have indisputable evidence from mathematical
considerations that the number of the visible stars _must necessarily_
be limited. For were the stars infinite in number, and scattered through
infinite space with any approach to uniformity, it may be proved that
the whole heavens would shine with the brightness of the sun. As the
surface of a sphere varies as the square of its radius, and light
inversely as the square of the distance (or radius of the star sphere at
any point), we have the diminished light of the stars exactly
counterbalanced by the increased number at any given distance. For a
distance of say ten times the distance of the nearest fixed star, the
light of each star would be diminished by the square of 10 or 100 times,
but the total number of stars would be 100 times greater, so that the
total star light would be the same. This would be true for _all_
distances. The total light would therefore—by addition—be proportional
to the distance, and hence, for an infinite distance we should have an
infinite amount of light For an infinite number of stars, therefore, we
should have a continuous blaze of light over the whole surface of the
visible heavens. Far from this being the case, the amount of light
afforded by the stars on the clearest nights is, on the contrary,
comparatively small, and the blackness of the background, “the darkness
behind the stars,” is very obvious. According to Miss Clerke (“System of
the Stars,” p. 7), the total light of all the stars, to magnitude 9½, is
about one-eightieth of full moonlight. M. G. l’Hermite found for the
total amount of starlight one-tenth of moonlight; but this estimate is
evidently too high. Assuming the sun’s brightness as 28 magnitudes
brighter than a star of the first magnitude,[145] and Zöllner’s estimate
that sunlight is 618,000 times that of moonlight, I find that the total
light of the stars to magnitude 9½, as stated by Miss Clerke, would be
equivalent to the combined light of about 320,000 stars of the sixth
magnitude, or 3,200 stars of the first magnitude. Even taking M.
l’Hermite’s high estimate of one-tenth of moonlight, the total starlight
would be represented by 25,600 stars of the first magnitude.
To explain the limited number of the visible stars, several hypothesis
have been advanced. If space be really infinite, as we seem compelled to
suppose, it would be reasonable to expect that the number of the stars
would be practically infinite also. But, as I have shown above, the
number of the _visible_ stars is certainly finite, and the number
visible and invisible must be finite also, for otherwise the amount of
starlight would be much greater than it is. To account for the limited
number of visible stars, it has been suggested that beyond a certain
distance in space, there may be an “extinction of light,” caused by
absorption in the luminiferous ether. In a recent paper on this subject,
Schiaparelli, the famous Italian astronomer, suggests that if any
extinction of light really takes place, it may probably be due, not to
absorption in the ether, but to fine particles of matter scattered
through interstellar space. In support of this hypothesis, he refers to
the supposed constitution of comets’ tails, of falling stars, and
meteorites, and he shows that the quantity of matter necessary to
produce the required extinction would be very small—so small, indeed,
that a quantity of this matter scattered through a volume equal to that
of the earth, if collected into one mass, would only form a ball of less
than one inch in diameter. We can readily admit the existence of such a
minute quantity of matter in a fine state of subdivision scattered
through space, but it seems to me much more probable that the limited
number of the visible stars is due, not to any extinction of their light
by absorption in the ether, or by fine particles scattered through
space, but to a real thinning out of the stars as we approach the limits
of our sidereal universe. Celoria’s observation, mentioned above, seems
to prove that near the pole of the Milky Way very few stars fainter than
the eleventh magnitude are visible, even in a large telescope, and Dr.
Roberts’ photographs, taken in the vicinity of the celestial pole,
confirm this conclusion. Now, this paucity of stars of the fainter
magnitudes cannot be due to any absorption of light in the ether, for
numerous stars of the sixteenth magnitude, or perhaps fainter, are
visible in other parts of the heavens, and if in one place, why not in
another? Sir John Herschel’s observations of the Milky Way in the
Southern Hemisphere appear to render the hypothesis of any extinction of
light very improbable. He says that the hypothesis, “if applicable to
any, is equally so to every part of the Galaxy. We are not at liberty to
argue that at one part of its circumference our view is limited by this
sort of cosmical veil, which extinguishes the smaller magnitudes, cuts
off the nebulous light of distant masses, and closes our view in
impenetrable darkness; while at another we are compelled, by the
clearest evidence telescopes can afford, to believe that star-strewn
vistas _lie open_, exhausting their powers, and stretching out beyond
their utmost reach, as is proved by that very phænomenon which the
existence of such a veil would render impossible, _viz._, infinite
increase of number and diminution of magnitude, terminating in complete
irresolvable nebulosity.”
How then are we to explain the limited number of the visible stars? If
space be infinite, as we seem compelled to suppose, the number of the
stars would probably be infinite also, or at least vastly greater than
the number actually visible. It has been suggested that, owing to the
progressive motion of light, the light of very distant stars may
probably not yet have reached the earth, although travelling through
space for thousands of years. But considering the vast periods of time
during which the stellar universe has probably been in existence, this
hypothesis seems very unsatisfactory. The most probable hypothesis seems
to be that all the stars, clusters and nebulæ, visible in our largest
telescopes, form together one vast system, which constitutes our visible
universe, and that this system is isolated by a starless void from other
similar systems which probably exist in infinite space. The distance
between these separate systems—or “island universes,” as they have been
called—may be very great, compared with the diameter of each system, in
the same way that the diameter of our visible universe is very great
compared with the diameter of the solar system. As the sun is a star,
and the stars are suns, and as our sun is separated from his neighbour
suns in space by a sunless void, so may our universe be separated from
other universes by a vast and starless abyss. On this hypothesis, the
supposed extinction of light—which may have little or no perceptible
effect within the limits of our visible universe—may possibly come into
play across the vast and immeasurable distances which probably separate
the different universes from each other, and may perhaps extinguish
their light altogether.
Another hypothesis which also seems possible is that the luminiferous
ether which extends throughout our visible universe may perhaps be
confined to this universe itself, and that beyond its confines, the
ether may thin out, as our atmosphere does at a certain distance from
the earth, and finally cease to exist altogether, ending in an
_absolute_ vacuum, which would, of course, arrest the passage of all
light from outer space, and thus produce “the darkness behind the
stars.”
Let us now consider the apparent distribution of the stars and nebulæ on
the celestial vault, and their probable relation to each other in space.
As already stated, Argelander considered the number of stars of the
first magnitude to be about twenty, but modern photometric measures have
reduced this number to thirteen or fourteen. According to the Harvard
measures, the fourteen brightest stars in the heavens, in order of
magnitude, are: Sirius, Canopus, Arcturus, Capella, Vega, Alpha
Centauri, Rigel, Procyon, Achernar, Beta Centauri, Betelgeuse, Altair,
Aldebaran and Alpha Crucis. Seven of these are in the Northern
Hemisphere, namely: Arcturus, Capella, Vega, Procyon, Betelgeuse,
Altair, and Aldebaran; and seven in the Southern Hemisphere: Sirius,
Canopus, Alpha Centauri, Rigel, Achernar, Beta Centauri, and Alpha
Crucis, so that the brightest stars are pretty evenly distributed
between the two hemispheres. Of these bright stars, no less than twelve
lie in or near the Milky Way, Arcturus and Achernar being the only two
at any considerable distance from the Galaxy. This is very remarkable
and suggestive, as the area covered by the Milky Way is probably not
more than one-fourth of the whole star sphere.
Of the stars fainter than the first magnitude, but brighter than
magnitude 2·0, there are about 10 in the Northern Hemisphere, of which 4
lie in or near the Milky Way, and about 19 in the Southern Hemisphere,
of which no less than 14 are situated in or near the Galaxy.
Of those brighter than magnitude 3·0, I find 33 stars in or near the
Milky Way out of a total of about 95 in both hemispheres. To extend this
investigation to all stars visible to the naked eye, I made, some years
since, an examination of all the stars in Heis’ atlas that lie in the
Milky Way, and found that number to be 1,186 out of a total of 5,356, or
a percentage of about 22. At my request, Col. Markwick, F.R.A.S., made a
similar count for the stars in Dr. Gould’s charts of the Southern
Hemisphere (_Uranometria Argentina_), and found that, down to the fourth
magnitude, there are 121 stars on the Milky Way out of 228, or a
percentage of 53, and for all stars to the seventh magnitude inclusive,
there are 3,072 on the Milky Way out of a total of 6,694, or a
percentage of nearly 46. Col. Markwick finds that the Milky Way in the
Southern Hemisphere, as shown on Gould’s charts, covers about one-third
of the whole hemisphere. As will be seen by the above figures, the
percentage of stars, even to the fourth magnitude, lying on the Milky
Way is considerably greater than this proportion.
The above results show that the brighter stars which are apparently
projected on the Milky Way probably belong to that zone, and are not
merely fortuitously scattered over the surface of the heavens.