of the Whale, which also after a time disappeared. It was not noticed
again till the year 1637, when an observer rejoicing in the name of
Phocyllides Holwarda observed it, and, keeping a watch, after it had
vanished, upon the place where it had appeared, saw it again come into
view nine months after its disappearance. Since then it has been known
as a variable star with a period of about 331 days 8 hours. When
brightest this star is of the second magnitude. It indicates a somewhat
singular remissness on the part of the astronomers of former days, that
a star shining so conspicuously for a fortnight, once in each period of
331-1/3 days, should for so many years have remained undetected. It
may, perhaps, be thought that, noting this, I should withdraw the
objection raised above against Sir J. Herschel's idea that the star in
Cassiopeia may return to view once in 156 years, instead of once in 312
years. But there is a great difference between a star which at its
brightest shines only as a second-magnitude star, so that it has twenty
or thirty companions of equal or greater lustre above the horizon along
with it, and a star which surpasses three-fold the splendid Sirius. We
have seen that even in Tycho Brahe's day, when probably the stars were
not nearly so well known by the community at large, the new star in
Cassiopeia had not shone an hour before the country people were gazing
at it with wonder. Besides, Cassiopeia and the Whale are constellations
very different in position. The familiar stars of Cassiopeia are visible
on every clear night, for they never set. The stars of the Whale, at
least of the part to which the wonderful variable star belongs, are
below the horizon during rather more than half the twenty-four hours;
and a new star there would only be noticed, probably (unless of
exceeding splendour), if it chanced to appear during that part of the
year when the Whale is high above the horizon between eventide and
midnight, or in the autumn and early winter.
It is a noteworthy circumstance about the variable star in the Whale,
deservedly called Mira, or The Wonderful, that it does not always return
to the same degree of brightness. Sometimes it has been a very bright
second-magnitude star when at its brightest, at others it has barely
exceeded the third magnitude. Hevelius relates that during the four
years between October 1672 and December 1676, Mira did not show herself
at all! As this star fades out, it changes in colour from white to red.
Towards the end of September 1604, a new star made its appearance in
the constellation Ophiuchus, or the Serpent-Bearer. Its place was near
the heel of the right foot of 'Ophiuchus huge.' Kepler tells us that it
had no hair or tail, and was certainly not a comet. Moreover, like the
other fixed stars, it kept its place unchanged, showing unmistakably
that it belonged to the star-depths, not to nearer regions. 'It was
exactly like one of the stars, except that in the vividness of its
lustre, and the quickness of its sparkling, it exceeded anything that he
had ever seen before. It was every moment changing into some of the
colours of the rainbow, as yellow, orange, purple, and red; though it
was generally white when it was at some distance from the vapours of the
horizon.' In fact, these changes of colour must not be regarded as
indicating aught but the star's superior brightness. Every very bright
star, when close to the horizon, shows these colours, and so much the
more distinctly as the star is the brighter. Sirius, which surpasses the
brightest stars of the northern hemisphere full four times in lustre,
shows these changes of colour so conspicuously that they were regarded
as specially characteristic of this star, insomuch that Homer speaks of
Sirius (not by name, but as the 'star of autumn') shining most
beautifully 'when laved of ocean's wave'--that is, when close to the
horizon. And our own poet, Tennyson, following the older poet, sings how
the fiery Sirius alters hue,
And bickers into red and emerald.
The new star was brighter than Sirius, and was about five degrees lower
down, when at its highest above the horizon, than Sirius when _he_
culminates. Five degrees being equal to nearly ten times the apparent
diameter of the moon, it will be seen how much more favourable the
conditions were in the case of Kepler's star for those coloured
scintillations which characterised that orb. Sirius never rises very
high above the horizon. In fact, at his highest (near midnight in
winter, and, of course, near midday in summer) he is about as high above
the horizon as the sun at midday in the first week in February. Kepler's
star's greatest height above the horizon was little more than
three-fourths of this, or equal to about the sun's elevation at midday
on January 13 or 14 in any year.
Like Tycho Brahe's star, Kepler's was brighter even than Jupiter, and
only fell short of Venus in splendour. It preserved its lustre for about
three weeks, after which time it gradually grew fainter and fainter
until some time between October 1605 and February 1606, when it
disappeared. The exact day is unknown, as during that interval the
constellation of the Serpent-Bearer is above the horizon in the day-time
only. But in February 1606, when it again became possible to look for
the new star in the night-time, it had vanished. It probably continued
to glow with sufficient lustre to have remained visible, but for the
veil of light under which the sun concealed it, for about sixteen months
altogether. In fact, it seems very closely to have resembled Tycho's
star, not only in appearance and in the degree of its greatest
brightness, but in the duration of its visibility.
In the year 1670 a new star appeared in the constellation Cygnus,
attaining the third magnitude. It remained visible, but not with this
lustre, for nearly two years. After it had faded almost out of view, it
flickered up again for awhile, but soon after it died out, so as to be
entirely invisible. Whether a powerful telescope would still have shown
it is uncertain, but it seems extremely probable. It may be, indeed,
that this new star in the Swan is the same which has made its appearance
within the last few weeks; but on this point the evidence is uncertain.
On April 20, 1848, Mr. Hind (Superintendent of the Nautical Almanac,
and discoverer of ten new members of the solar system) noticed a new
star of the fifth magnitude in the Serpent-Bearer, but in quite another
part of that large constellation than had been occupied by Kepler's
star. A few weeks later, it rose to the fourth magnitude. But afterwards
its light diminished until it became invisible to ordinary eyesight. It
did not vanish utterly, however. It is still visible with telescopic
power, shining as a star of the eleventh magnitude, that is five
magnitudes below the faintest star discernible with the unaided eye.
This is the first new star which has been kept in view since its
apparent creation. But we are now approaching the time when it was found
that as so-called new stars continue in existence long after they have
disappeared from view, so also they are not in reality new, but were in
existence long before they became visible to the naked eye.
On May 12, 1866, shortly before midnight, Mr. Birmingham, of Tuam,
noticed a star of the second magnitude in the Northern Crown, where
hitherto no star visible to the naked eye had been known. Dr. Schmidt,
of Athens, who had been observing that region of the heavens the same
night, was certain that up to 11 P.M., Athens local time, there was no
star above the fourth magnitude in the place occupied by the new star.
So that, if this negative evidence can be implicitly relied on, the new
star must have sprung at least from the fourth, and probably from a much
lower magnitude, to the second, in less than three hours--eleven o'clock
at Athens corresponding to about nine o'clock by Irish railway time. A
Mr. Barker, of London, Canada, put forward a claim to having seen the
new star as early as May 4--a claim not in the least worth
investigating, so far as the credit of first seeing the new star is
concerned, but exceedingly important in its bearing on the nature of the
outburst affecting the star in Corona. It is unpleasant to have to throw
discredit on any definite assertion of facts; unfortunately, however,
Mr. Barker, when his claim was challenged, laid before Mr. Stone, of the
Greenwich Observatory, such very definite records of observations made
on May 4, 8, 9, and 10, that we have no choice but either to admit these
observations, or to infer that he experienced the delusive effects of a
very singular trick of memory. He mentions in his letter to Mr. Stone
that he had sent full particulars of his observations on those early
dates to Professor Watson, of Ann Arbor University, on May 17; but
(again unfortunately) instead of leaving that letter to tell its own
story in Professor Watson's hands, he asked Professor Watson to return
it to him: so that when Mr. Stone very naturally asked Professor Watson
to furnish a copy of this important letter, Professor Watson had to
reply, 'About a month ago, Mr. Barker applied to me for this letter, and
I returned it to him, as requested, without preserving a copy. I can,
however,' he proceeded, 'state positively that he did not mention any
actual observation earlier than May 14. He said he thought he had
noticed a strange star in the Crown about two weeks before the date of
his first observation--May 14--but not particularly, and that he did not
recognise it until the 14th. He did not give any date, and did not even
seem positive as to identity.... When I returned the letter of May 17, I
made an endorsement across the first page, in regard to its genuineness,
and attached my signature. I regret that I did not preserve a copy of
the letter in question; but if the original is produced, it will appear
that my recollection of its contents is correct.' I think no one can
blame Mr. Stone, if, on the receipt of this letter, he stated that he
had not the 'slightest hesitation' in regarding Mr. Barker's earlier
observations as 'not entitled to the slightest credit.'[33]
It may be fairly taken for granted that the new star leapt very quickly,
if not quite suddenly, to its full splendour. Birmingham, as we have
seen, was the first to notice it, on May 12. On the evening of May 13,
Schmidt of Athens discovered it independently, and a few hours later it
was noticed by a French engineer named Courbebaisse. Afterwards,
Baxendell of Manchester, and others independently saw the star. Schmidt,
examining Argelander's charts of 324,000 stars (charts which I have had
the pleasure of mapping in a single sheet), found that the star was not
a new one, but had been set down by Argelander as between the ninth and
tenth magnitudes. Referring to Argelander's list, we find that the star
had been twice observed--viz., on May 18, 1855, and on March 31, 1856.
Birmingham wrote at once to Mr. Huggins, who, in conjunction with the
late Dr. Miller, had been for some time engaged in observing stars and
other celestial objects with the spectroscope. These two observers at
once directed their telescope armed with spectroscopic adjuncts--the
telespectroscope is the pleasing name of the compound instrument--to the
new-comer. The result was rather startling. It may be well, however,
before describing it, to indicate in a few words the meaning of various
kinds of spectroscopic evidence.
The light of the sun, sifted out by the spectroscope, shows all the
colours but not all the tints of the rainbow. It is spread out into a
large rainbow-tinted streak, but at various places (a few thousand)
along the streak there are missing tints; so that in fact the streak is
crossed by a multitude of dark lines. We know that these lines are due
to the absorptive action of vapours existing in the atmosphere of the
sun, and from the position of the lines we can tell what the vapours
are. Thus, hydrogen by its absorptive action produces four of the bright
lines. The vapour of iron is there, the vapour of sodium, magnesium, and
so on. Again, we know that these same vapours, which, by their
absorptive action, cut off rays of certain tints, emit light of just
those tints. In fact, if the glowing mass of the sun could be suddenly
extinguished, leaving his atmosphere in its present intensely heated
condition, the light of the faint sun which would thus be left us would
give (under spectroscopic scrutiny) those very rays which now seem
wanting. There would be a spectrum of multitudinous bright lines,
instead of a rainbow-tinted spectrum crossed by multitudinous dark
lines. It is, indeed, only by contrast that the dark lines appear dark,
just as it is only by contrast that the solar spots seem dark. Not only
the penumbra but the umbra of a sun-spot, not only the umbra but the
nucleus, not only the nucleus but the deeper black which seems to lie at
the core of the nucleus, shine really with a lustre far exceeding that
of the electric light, though by contrast with the rest of the sun's
surface the penumbra looks dark, the umbra darker still, the nucleus
deep black, and the core of the nucleus jet black. So the dark lines
across the solar spectrum mark where certain rays are relatively faint,
though in reality intensely lustrous. Conceive another change than that
just imagined. Conceive the sun's globe to remain as at present, but the
atmosphere to be excited to many times its present degree of light and
splendour: then would all these dark lines become bright, and the
rainbow-tinted background would be dull or even quite dark by contrast.
This is not a mere fancy. At times, local disturbances take place in the
sun which produce just such a change in certain constituents of the
sun's atmosphere, causing the hydrogen, for example, to glow with so
intense a heat that, instead of its lines appearing dark, they stand out
as bright lines. Occasionally, too, the magnesium in the solar
atmosphere (over certain limited regions only, be it remembered) has
been known to behave in this manner. It was so during the intensely hot
summer of 1872, insomuch that the Italian observer Tacchini, who noticed
the phenomenon, attributed to such local overheating of the sun's
magnesium vapour the remarkable heat from which we then for a time
suffered.
Now, the stars are suns, and the spectrum of a star is simply a
miniature of the solar spectrum. Of course, there are characteristic
differences. One star has more hydrogen, at least more hydrogen at work
absorbing its rays, and thus has the hydrogen lines more strongly
marked than they are in the solar spectrum. Another star shows the lines
of various metals more conspicuously, indicating that the glowing
vapours of such elements, iron, copper, mercury, tin, and so forth,
either hang more densely in the star's atmosphere than in our sun's, or,
being cooler, absorb their special tints more effectively. But speaking
generally, a stellar spectrum is like the solar spectrum. There is the
rainbow-tinted streak, which implies that the source of light is glowing
solid, liquid, or highly compressed vaporous matter, and athwart the
streak there are the multitudinous dark lines which imply that around
the glowing heart of the star there are envelopes of relatively cool
vapours.
We can understand, then, the meaning of the evidence obtained from the
new star in the Northern Crown.
In the first place, the new star showed the rainbow-tinted streak
crossed by dark lines, which indicated its sun-like nature. _But,
standing out on that rainbow-tinted streak as on a dark background, were
four exceedingly bright lines--lines so bright, though fine, that
clearly most of the star's light came from the glowing vapours to which
these lines belonged._ Three of the lines belonged to hydrogen, the
fourth was not identified with any known line.
Let us distinguish between what can certainly be concluded from this
remarkable observation, and what can only be inferred with a greater or
less degree of probability.
It is absolutely certain that when Messrs. Huggins and Miller made their
observation (by which time the new star had faded from the second to the
third magnitude), enormous masses of hydrogen around the star were
glowing with a heat far more intense than that of the star itself within
the hydrogen envelope. It is certain that the increase in the star's
light, rendering the star visible which before had been far beyond the
range of ordinary eyesight, was due to the abnormal heat of the
hydrogen surrounding that remote sun.
But it is not so clear whether the intense glow of the hydrogen was
caused by combustion or by intense heat without combustion. The
difference between the two causes of increased light is important;
because on the opinion we form on this point must depend our opinion as
to the probability that our sun may one day experience a similar
catastrophe, and also our opinion as to the state of the sun in the
Northern Crown after the outburst. To illustrate the distinction in
question, let us take two familiar cases of the emission of light. A
burning coal glows with red light, and so does a piece of iron placed in
a coal fire. But the coal and the iron are undergoing very different
processes. The coal is burning, and will presently be consumed; the iron
is not burning (except in the sense that it is burning hot, which means
only that it will make any combustible substance burn which is brought
into contact with it), and it will not be consumed though the coal fire
be maintained around it for days and weeks and months. So with the
hydrogen flames which play at all times over the surface of our own sun.
They are not burning like the hydrogen flames which are used for the
oxy-hydrogen lantern. Were the solar hydrogen so burning, the sun would
quickly be extinguished. They are simply aglow with intensity of heat,
as a mass of red-hot iron is aglow; and, so long as the sun's energies
are maintained, the hydrogen around him will glow in this way without
being consumed. As the new fires of the star in the Crown died out
rapidly, it is possible that in their case there was actual combustion.
On the other hand, it is also possible, and perhaps on the whole more
probable, that the hydrogen surrounding the star was simply set glowing
with increased lustre owing to some cause not as yet ascertained.
Let us see how these two theories have been actually worded by the
students of science themselves who have maintained them.
'The sudden blazing forth of this star,' says Mr. Huggins, 'and then the
rapid fading away of its light, suggest the rather bold speculation that
in consequence of some great internal convulsion, a large volume of
hydrogen and other gases was evolved from it, the hydrogen, by its
combination with some other element,' in other words, by _burning_,
'giving out the light represented by the bright lines, and at the same
time heating to the point of vivid incandescence the solid matter of the
star's surface.' 'As the liberated hydrogen gas became exhausted' (I now
quote not Huggins's own words, but words describing his theory in a book
which he has edited) 'the flame gradually abated, and, with the
consequent cooling, the star's surface became less vivid, and the star
returned to its original condition.'
On the other hand, the German physicists, Meyer and Klein, consider the
sudden development of hydrogen, in quantities sufficient to explain such
an outburst, exceedingly unlikely. They have therefore adopted the
opinion, that the sudden blazing out of the star was occasioned by the
violent precipitation of some mighty mass, perhaps a planet, upon the
globe of that remote sun, 'by which the momentum of the falling mass
would be changed into molecular motion, or in other words into heat and
light.' It might even be supposed, they urge, that the star in the
Crown, by its swift motion, may have come in contact with one of the
star clouds which exist in large numbers in the realms of space. 'Such a
collision would necessarily set the star in a blaze and occasion the
most vehement ignition of its hydrogen.'
Fortunately, our sun is safe for many millions of years to come from
contact from any one of its planets. The reader must not, however, run
away with the idea that the danger consists only in the gradual
contraction of planetary orbits sometimes spoken of. That contraction,
if it is taking place at all, of which we have not a particle of
evidence, would not draw Mercury to the sun's surface for at least ten
million millions of years. The real danger would be in the effects which
the perturbing action of the larger planets might produce on the orbit
of Mercury. That orbit is even now very eccentric, and must at times
become still more so. It might, but for the actual adjustment of the
planetary system, become so eccentric that Mercury could not keep clear
of the sun; and a blow from even small Mercury (only weighing, in fact,
390 millions of millions of millions of tons), with a velocity of some
300 miles per second, would warm our sun considerably. But there is no
risk of this happening in Mercury's case--though the unseen and much
more shifty Vulcan (in which planet I beg to express here my utter
disbelief) might, perchance, work mischief if he really existed.
As for star clouds lying in the sun's course, we may feel equally
confident. The telescope assures us that there are none immediately on
the track, and we know, also, that, swiftly though the sun is carrying
us onwards through space,[34] many millions of years must pass before he
is among the star families towards which he is rushing.
Of the danger from combustion, or from other causes of ignition than
those considered by Meyer and Klein, it still remains to speak. But
first, let us consider what new evidence has been thrown upon the
subject by the observations made on the star which flamed out last
November.
The new star was first seen by Professor Schmidt, who has had the good
fortune of announcing to astronomers more than one remarkable
phenomenon. It was he who discovered in November 1866 that a lunar
crater had disappeared, an announcement quite in accordance with the
facts of the case. We have seen that he was one of the independent
discoverers of the outburst in the Northern Crown. On November 24, at
the early hour of 5.41 in the evening (showing that Schmidt takes time
by the forelock at his observatory), he noticed a star of the third
magnitude in the constellation of the Swan, not far from the tail of
that southward-flying celestial bird. He is quite sure that on November
20, the last preceding clear evening, the star was not there. At
midnight its light was very yellow, and it was somewhat brighter than
the neighbouring star Eta Pegasi, on the Flying Horse's southernmost
knee (if anatomists will excuse my following the ordinary usage which
calls the wrist of the horse's fore-arm the knee). He sent news of the
discovery forthwith to Leverrier, the chief of the Paris observatory;
and the observers there set to work to analyse the light of the
stranger. Unfortunately the star's suddenly acquired brilliancy rapidly
faded. M. Paul Henry estimated the star's brightness on December 2 as
equal only to that of a fifth-magnitude star. Moreover, the colour,
which had been very yellow on November 24, was by this time 'greenish,
almost blue.' On December 2, M. Cornu, observing during a short time
when the star was visible through a break between clouds, found that the
star's spectrum consisted almost entirely of bright lines. On December
5, he was able to determine the position of these lines, though still
much interrupted by clouds. He found three bright lines of hydrogen, the
strong (really double) line of sodium, the (really triple) line of
magnesium, and two other lines. One of these last seemed to agree
exactly in position with a bright line belonging to the corona seen
around the sun during total eclipse.[35]
The star has since faded gradually in lustre until, at present, it is
quite invisible to the naked eye.
We cannot doubt that the catastrophe which befell this star is of the
same general nature as is that which befell the star in the Northern
Crown. It is extremely significant that all the elements which
manifested signs of intense heat in the case of the star in the Swan,
are characteristic of our sun's outer appendages. We know that the
coloured flames seen around the sun during total solar eclipse consist
of glowing hydrogen, and of glowing matter giving a line so near the
sodium line that in the case of a stellar spectrum it would, probably,
not be possible to distinguish one from the other. Into the prominences
there are thrown from time to time masses of glowing sodium, magnesium,
and (in less degree) iron and other metallic vapours. Lastly, in that
glorious appendage, the solar corona, which extends for hundreds of
thousands of miles from the sun's surface, there are enormous quantities
of some element, whose nature is as yet unknown, showing under
spectroscopic analysis the bright line which seems to have appeared in
the spectrum of the flaming sun in the Swan.