Myths and Marvels of Astronomy
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.
This evidence seems to me to suggest that the intense heat which
suddenly affected this star had its origin from without. At the same
time, I cannot agree with Meyer and Klein in considering that the cause
of the heat was either the downfall of a planetary mass on the star, or
the collision of the star with a star-cloudlet, or nebula, traversing
space in one direction while the star swept onwards in another. A planet
could not very well come into final conflict with its sun at one fell
swoop. It would gradually draw nearer and nearer, not by the narrowing
of its path, but by the change of the path's shape. The path would, in
fact, become more and more eccentric; until, at length, at its point of
nearest approach, the planet would graze its primary, exciting an
intense heat where it struck, but escaping actual destruction that time.
The planet would make another circuit, and again graze its sun, at or
near the same part of the planet's path. For several circuits this would
continue, the grazes not becoming more effective each time, but rather
less. The interval between them, however, would grow continually less
and less. At last the time would come when the planet's path would be
reduced to the circular form, its globe touching its sun's all the way
round, and then the planet would very quickly be reduced to vapour, and
partly burned up, its substance being absorbed by its sun. But all the
successive grazes would be indicated to us by accessions in the star's
lustre, the period between each seeming outburst being only a few months
at first, and becoming gradually less and less (during a long course of
years, perhaps even of centuries), until the planet was finally
destroyed. Nothing of this sort has happened in the case of any
so-called new star.
As for the rush of a star through a nebulous mass, that is a theory
which would scarcely be entertained by any one acquainted with the
enormous distances separating the gaseous star-clouds properly called
nebulae. There may be small clouds of the same sort scattered much more
densely through space; but we have not a particle of evidence that this
actually is the case. All we certainly _know_ about star-cloudlets
suggest that the distances separating them from each other are
comparable with those which separate star from star, in which case the
idea of a star coming into collision with a star-cloudlet, and still
more the idea of this occurring several times in a century, is wild in
the extreme.
On the whole, the theory seems more probable than any of these, that
enormous flights of large meteoric masses travel around those stars
which thus occasionally break forth in conflagration, such flights
travelling on exceedingly eccentric paths, and requiring enormously long
periods to complete each circuit of their vast orbits. In conceiving
this, we are not imagining anything new. Such a meteoric flight would
differ only in degree not kind from meteoric flights which are known to
circle around our own sun. I am not sure, indeed, that it can be
definitely asserted that our sun has no meteoric appendages of the same
nature as those which, if this theory be true, excite to intense
periodic activity the sun round which they circle. We know that comets