in the year 1696, found the period from minimum to minimum to be
2 days, 20 hours, 48 minutes, 59½ seconds, and he came to the
conclusion that the diminution in the light of the star is probably
due to a partial eclipse by “a large body revolving round Algol.”
This hypothesis was fully confirmed in the years 1888-89 by Professor
Vogel with the spectroscope. As no close companion to Algol is
visible in the largest telescopes, we must conclude that either the
satellite is a dark body, or else so close to the primary that no
telescope could show it. Now, if the diminution in Algol’s light
is due to a dark body revolving round it, and periodically coming
between us and the bright star, it follows that both components
will be in motion, and both will revolve round the common centre
of gravity of the pair. A little before a minimum of light takes
place, the dark companion should therefore be approaching the eye,
and, consequently, the bright companion will be receding. During
the minimum there will be no apparent motion in the line of sight,
as the motion of both bodies will be at right angles to the visual
ray. After the minimum is over, the motion of the two bodies will
be reversed, the bright one approaching the eye, and the dark
one receding. Now, this is exactly what Vogel found. Before the
diminution in the light of Algol begins, the spectroscope showed that
the star is receding from the earth and after the minimum that it is
approaching the eye. That the companion is dark and not bright, like
the primary, is evident from the fact that the spectral lines are
merely shifted from their normal position and not doubled, as would
be the case were both components bright, as in the case of some of
the “spectroscopic binaries”—for example, Beta Aurigæ. Vogel found
that before the minimum of light, Algol is receding from the earth
with the velocity of 24½ miles a second, and after the minimum it is
approaching at the rate of 28½ miles a second. The difference between
the observed velocities indicates that the system is approaching
the earth with a velocity of about 2 miles a second. Knowing, then,
the orbital velocity, which is evidently about 26½ miles a second,
and assuming the orbit to be circular, it is easy, with the observed
period of revolution, or the period of light variation, to calculate
the diameter of the orbit in miles, although the star’s distance
from the earth remains unknown. Further, comparing its period of
revolution and the dimensions of the orbit with that of the earth
round the sun, it is easy to calculate, by Kepler’s third law of
motion, the mass of the system in terms of the sun’s mass, and the
probable size of the component bodies. Calculating in this way, Vogel
computes that the diameter of Algol is about 1,061,000 miles, and
that of the dark companion 830,300 miles, with a distance between
their centres of 3,230,000 miles, and a combined mass equal to
two-thirds of the sun’s mass, the mass of Algol being four-ninths,
and that of the companion two-ninths, of the mass of the sun. Taking
the diameter of the sun as 866,000 miles, and its density as 1.44
(water being unity), I find that the above dimensions give a mean
density for the components of Algol of about one-third that of water,
so that the components are probably gaseous bodies, as Hall has
already concluded.
[Illustration: Portion of the Sun’s Surface. Sunspot nearly 60,000
Miles Across]
It is a curious fact that Al-Sûfi, the Persian astronomer, in his
_Description of the Heavens_, written in the Tenth Century, speaks
distinctly of Algol as a red star (_étoile_, _brillant_; _d’un
éclat_, _rouge_), while at present it is white or at the most of a
yellow color. A similar change of color is supposed to have taken
place in the case of Sirius, but the change in Algol seems more
certain, as Al-Sûfi’s descriptions are generally most accurate and
reliable.
Stars of the Algol type of variable are very rare objects, only a
dozen or so having been hitherto discovered in the whole heavens.
Those visible to the naked eye, when at their normal brightness, are:
Algol, Lambda Tauri, Delta Libræ, R Canis Majoris, and U Ophiuchi.
A remarkable peculiarity about the variable stars in general is that
none of them has any considerable proper motion. As a large proper
motion is generally considered to indicate proximity to the earth,
we may conclude, with great probability, that the variable stars,
as a rule, lie at a great distance from our system. In other words,
it appears that the sun does not lie in a region of variable stars,
and, with the exception of Alpha Cassiopeiæ and Alpha Herculis, a
measurable parallax has not yet been found, so far as I know, for any
known variable star.
We now come to the interesting and mysterious class of objects known
as “new” or “temporary” stars. These phenomena are of very rare
occurrence, and but few undoubted examples of the class are recorded
in the annals of astronomy. Possibly in some cases they have been
merely variable stars, of irregular period and fitful variability;
but others may have been due to a real catastrophe, such as the
collision of two dark bodies in space, or, possibly, the passage of a
bright or dark body through a gaseous nebula.
The earliest temporary star of which we have any reliable
information seems to be one which is recorded in the Chinese annals
of Ma-tuan-lin, as having appeared in the year 134 B. C. in the
constellation Scorpio. Its position seems to have been somewhere
between the stars Beta and Rho of Scorpio. Pliny informs us that it
was the sudden appearance of a new star which induced the famous
astronomer Hipparchus to form his catalogue of stars, the first ever
constructed. As the date of Hipparchus’s catalogue is 125 B. C., it
seems highly probable that the new star referred to by Pliny was the
same as that recorded by the Chinese astronomer as having appeared
nine years previously.
A new star is said to have appeared in the year 76 B. C. between the
stars Alpha and Delta in the Plow, but the accounts are vague.
In 101 A. D., a small “yellowish-blue” star is said to have appeared
in the “sickle” in Leo, but its exact position is not known. In 107
A. D., a new star is mentioned near Delta, Epsilon and Eta in Canis
Major, three bright stars southeast of Sirius. In 123 A. D., another
new star is recorded by Ma-tuan-lin to have appeared between Alpha
Herculis and Alpha Ophiuchi.
The Chinese annals record that on December 10, 173 A. D., a brilliant
star appeared between Alpha and Beta Centauri in the Southern
Hemisphere. It remained visible for eight months, and is described
as resembling “a large bamboo mat!”—a curious description. There is
at present, close to the spot indicated, a known variable star—R
Centauri—of which the period seems to be long and the variation
of light irregular. Possibly an unusually bright maximum of this
variable star formed the star of the Chinese annals, or perhaps the
variable star is the remnant of the outburst which took place in the
First Century. The variable is a very reddish star, and at present
varies from about the sixth to the tenth magnitude.
A new star is recorded in the year 386 A. D. as having appeared
between Lambda and Phi Sagittarii. Near the position indicated,
Flamsteed observed a star, No. 65 of his catalogue, which is now
missing; and it has been conjectured that the star seen by Flamsteed
may possibly have been a return of the star mentioned in the Chinese
annals.
Cuspianus relates that a star as bright as Venus appeared near Altair
in 389 A. D., during the reign of the Emperor Honorius, and that he
had himself seen it. There is some doubt, however, about the exact
date, as other accounts give the year 388 or 398. The star seems to
have disappeared in about three weeks.
In the year 393 A. D., another strange star is recorded in the tail
of Scorpio. An extraordinary star is said to have been seen near
Alpha Crateris in 561 A. D. Here again a known variable and red
star—R Crateris—is close to the position indicated by the ancient
records.
The Chinese annals record a new star in 829 A. D., somewhere in the
vicinity of the bright star Procyon, and in this locality there are
several known variable stars.
The Bohemian astronomer, Cyprianus Leoviticus, mentions the
appearance of new stars in Cassiopeia in the years 945 A. D. and
1264, and it has been conjectured that perhaps these were apparitions
of Tycho Brahe’s famous star of 1572 (to be presently described),
forming a variable star with a period of over 300 years. Lynn and
Sadler, however, have shown that the supposed stars of 945 and 1264
were, in all probability, comets.
Extraordinary stars are recorded near Zeta Sagittarii in 1011 A. D.,
near Mu Scorpii in 1203, and near Pi Scorpii on July 1, 1584. It is
remarkable how many of these objects seem to have appeared in this
portion of the heavens.
A very brilliant star is mentioned by Hepidannus as having appeared
in Aries in May, 1012. He describes it as “dazzling the eye.” Other
temporary stars are mentioned in 1054 A. D., near Zeta Tauri, and in
1139 near Kappa Virginis; but the accounts of these are very vague,
and it seems by no means certain that they were really new stars.
No possible doubt, however, can be entertained with reference to the
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 Brahe, who has left us a
very elaborate account of its appearance, position, etc. Although
usually spoken of as Tycho Brahe’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 Brahe’s attention was first attracted to it on
November 11. His description of the new star is as follows—as quoted
by Humboldt: “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, wagoners 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, Alpha Lyræ, or Jupiter. For splendor, it was only
comparable to Venus when nearest to earth (that is, when only a
quarter of her disk is illuminated). Those gifted with keen sight
could, when the air was clear, discern the new star in the daytime,
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. Toward 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 magnitudes 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 afterward.) 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 Brahe compared it to Mars; afterward he thought it nearly
resembled Betelgeuse, the star in the right shoulder of Orion. The
color for the most part was like the red tint of Aldebaran. In the
spring of 1573, and especially in May, its white color 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.”
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 afterward disappeared. It was again seen by
Hevelius in November, 1655. 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 color is
yellow, and bright lines have been seen in its spectrum.
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.
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 two and one-half 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.
A remarkable and very interesting temporary star was discovered in
1892 in the constellation Auriga.
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 Plow, the star recorded
by Hepidannus in Aries, 1012 A. D., and the “Blaze Star” of 1866 in
Corona Borealis.
A WORLD ON FIRE—NOVA PERSEI.—ALEXANDER W. ROBERTS
In the small hours of the morning of 22d February, 1901, Dr.
Anderson of Bonnington, Edinburgh, saw a bright star shining in
the constellation of Perseus, where he knew no such star was ever
seen before. The circumstances connected with this discovery afford
another striking instance of how Nature keeps her secrets for her
true amateur, using the word in its highest sense.
The evening of 21st February was cloudy, and nine out of ten
astronomers would have gone to bed when there seemed little prospect
of the night clearing; but Dr. Anderson was the tenth man. At twenty
minutes to three in the morning the clouds rolled away from over
the old gray Scottish capital, and the trained eye of the patient
observer saw right in the heart of Perseus a new star. Never before
had its light, blue-white, like an unpolished diamond, shone down on
this strange earth of ours.
Next day the news of the wonderful discovery was flashed to all the
great observatories of the world, and telescopes and spectroscopes,
cameras and photometers, were directed toward the strange phenomenon,
and by testing, measuring, examining, sought to wrest its secrets
from it.
Much is still a mystery; but what has been ascertained during the
period that the rhythm of its light-waves beat upon our shores
is of great interest and importance as bearing directly on the
life-history of each individual star in the heavens, and of our own
sun and planet among them.
The first and simplest question that arises for settlement is the
date when the new star blazed forth in our terrestrial sky. The
curious reader will notice the reservation: in _our_ terrestrial
sky. When the star _actually_ burst forth into resplendent
light is another matter, as we shall discover later on. It was
certainly before Dr. Anderson was born, and probably before another
Scotsman—Ferguson by name—combined, like many another sage, counting
and watching sheep with counting and watching stars.
With regard to the date of the appearance in our sky of the new
star, Nova Persei, as it is called in astronomical literature, when
Dr. Anderson discovered it at twenty minutes to three o’clock on
the morning of 22d February, it was bright enough to be straightway
evident to a trained astronomer. In these later days of strenuous
scientific activities every portion of the sky is constantly being
examined and charted, and no sooner was the discovery of Nova Persei
announced than a searching of records began, in order to ascertain
if, at any time, the star had ever been seen before.
[Illustration: Fig. 19.—Chart Showing Position of Nova Persei]
It so chanced that on the evenings of 18th and 19th February two
photographs of the very spot where three days later the new star
appeared were taken at Harvard Observatory. On neither of these
photographs is there the slightest evidence of the star’s existence.
It was, therefore, on these dates non-existent so far as our earth
was concerned. On the evening of 20th February a well-known English
observer, Mr. Stanley Williams, had also taken a photograph of the
same portion of the sky; and again there was no trace of the star.
Mr. Williams’s photograph was taken twenty-eight hours before Dr.
Anderson saw it. Still more strange is the fact that on the evening
of 21st February three observers on the Continent testify that they
had the constellation Perseus under observation from seven o’clock
to eleven, and had the new star then been visible they could not have
failed to see it. The star, therefore, blazed out some time between
eleven o’clock and three on the night of its discovery.
Now, what does this mean? It means this: that by some cause a star,
quite dark before, or so faint that it could not be seen even by
means of a powerful telescope, in a few hours, or perhaps in a few
minutes, blazed forth as a star of conspicuous brightness. In this
brief space of time a dark and probably chill globe became a seething
mass of fire, a million times hotter than it was before. Fierce,
fervent heat lit up the orb with a glow that reached from rim to rim
of the stellar universe. We have here a catastrophe that goes beyond
our wildest conceptions: the conflagration of a world, the ruin of
a star. What guarantee have we for an assumption of this kind? What
of certitude is there in our vision of such a Day of Doom for any
part of our universe? Let us consider the salient facts regarding the
recent changes in the appearance and structure of this star. We shall
relate only those facts that are beyond controversy, as far as our
present knowledge goes.
Nova Persei did not reach its maximum brightness till the evening of
25th February, when it was probably the most conspicuous object in
the midnight sky. It was then at least six times brighter than at the
time of its discovery. After this date it began to wane slowly. At
intervals there were spurts of brightness lasting for two or three
days, as if the fires had not exhausted themselves. On the whole,
however, the light of the star waned, and by the end of the year its
enfeebled light was just bright enough to be evident to the naked
eye; twelve months after its appearance it could only be seen with
the aid of a telescope.
Now, one of the most powerful instruments of research in the
new astronomy is the spectroscope. It takes hold of the rays of
light that come to us from a star, and makes these rays reveal
the condition of things in the world they come from. One of the
spectroscopes turned on the new star in Perseus was Professor
Copeland’s magnificent instrument at Blackford Hill Observatory,
Edinburgh. Professor Copeland described the new star as “a feebly
developed” sun. As the star, however, increased in brightness the
spectroscope chronicled the fact that great physical changes were
taking place in its composition and structure. The star soon ceased
to be a feebly developed sun, for development had gone on apace with
the increase of light. Round the solid or semi-molten mass there was
rapidly aggregating an ocean of fiery gases, probably thrown up from
the nucleus.
Put simply, Nova Persei, for long ages a cold, dark, solid globe,
was in the brief space of a few days transformed from circumference
to core into a luminous, heated gaseous sphere. By what chance or
circumstance this vast change came about may be inquired into later
on. We only note here that this was the story spelled out by those
skilled in deciphering the observations recorded by the spectroscope.
In July, 1901, Professor Pickering of Harvard Observatory announced
that the star had become a nebula; that, indeed, its once solid
globe had practically dissolved into thinnest air. Not only had
its elements become molten with fervent heat, but they had become
transformed into shimmering wisps of matter more diaphanous than a
gossamer web.
Everything connected with the history of this star is of exceptional
interest; but all that had already been ascertained was completely
overshadowed by the astonishing discovery made in November, 1902,
that nebulous prominences were observed darting out from the star
with a velocity of at least 100,000 miles every second of time. These
astonishing changes have been confirmed at the two great American
observatories, the Yerkes and the Lick.
Whence and how had destruction come upon this particular star? At
one hour the star is dark, cold, solid. A few hours later this dark,
solid, cold body is a blazing world, its solid mass blown apparently
into countless fragments; from every fragment, big or little, there
pour streams of fiery vapor; for millions of miles round the star
there is a whirlpool of fire, a tempest of flame; and from end to end
of this great universe of ours the brightness of the burning star
pulsates. Three explanations have been given.
The one that naturally arises in our mind is that it was struck by
another star. Two worlds, each moving at the rate of twenty miles
a second, come into collision, and the result is the annihilation
of both. The force of their impact, changed into heat, drives their
elements into vapor. Such a catastrophe is quite possible in a
universe like ours, where stars and worlds, millions and millions
in number, sweep down the great avenues of space with a velocity far
beyond our comprehension.
We take it that when the crack of doom comes to this earth of ours
it will be in this fashion. Some great dark star will strike our sun
fair and square, and then in the twinkling of an eye, before the
inhabitants of earth know what has taken place, sun and moon and
planet will be wrapped up and dissolved in an atmosphere of fire.
We can in a certain rough way compute the increase in temperature
that would arise from the collision of two great orbs. Thus, let
us suppose that Nova Persei was moving onward through space with a
velocity of ten miles a second—a moderate velocity, be it noted, for
a star—when it collided with the body that wrought its destruction.
The impact would be terrific, and the result of it would be not
only the complete disintegration of both stars, but a sudden rise
in temperature of about five hundred thousand degrees, an increase
sufficient to vaporize the hardest adamant.