the most eminent investigators in the domain of the mechanical theory
of heat, that, instead of the meteorites, parts of the sun itself
might fall towards its centre, or, in other words, that the sun was
shrinking. Owing to the high gravitation of the sun (27.4 times greater
than on the surface of the earth), the shrinkage would liberate a great
amount of heat. Helmholtz calculated that, in order to cover the heat
expenditure of the sun, a shrinkage of its diameter by 60 m. annually
would be required. If the sun’s diameter should only be diminished by
one-hundredth of one per cent.—a change which we should not be able
to establish—the heat loss would be covered for more than 2000 years.
That seems at first satisfactory. But if we proceed with our estimate,
we find that if the sun went on losing as much heat as at present for
seventeen million years it would have to contract within this period to
a quarter of its present volume, and would therefore acquire a density
like that of the earth. Long before that, however, the radiation from
the sun would have been decreased so powerfully that the temperature
on the earth’s surface would no longer rise above freezing-point.
Helmholtz, on this argument, limited the further existence of the
earth to about six million years. That is less satisfactory. But we
know nothing of the future and must be content with possibilities. Not
so, however, if we calculate back with the aid of Helmholtz’s theory.
According to this theory, and according to Helmholtz’s own data, a
state like the present cannot have existed for more than ten million
years. Since, now, geologists have come to the conclusion that the
petrefactions which we find in the fossil-bearing strata of the earth
have needed at least a hundred million years for their formation, and
more probably a thousand million years, and since, moreover, the still
more ancient formations—the so-called precambrian strata—have been
deposited in equally long or still longer periods, we see that the
theory of Helmholtz is unsatisfactory.
A somewhat peculiar way out of the dilemma has been suggested by a few
scientists. We know that one gramme of the wonderful element radium
emits about 120 calories per hour, or in the course of a year, in
round numbers, a million calories. This radiation seems to continue
unimpaired for years. If we now assume that each kilogramme of the mass
of the sun contains only two milligrammes of radium, that amount would
be sufficient to balance the heat expenditure of the sun for all future
ages. Without some further auxiliary hypothesis, we can, however, not
listen to this suggestion. It presupposes that heat is created out
of nothing. Some scientists, indeed, believe that radium may absorb
a radiation, coming from space, in some unknown manner and convert
it into heat. Before we enter seriously into a discussion of this
explanation we shall have to answer the questions where that radiation
comes from and where it takes its store of energy.
We must, therefore, again search for another source of heat energy for
the sun. Before we can hope to find it, we had better study the sun
itself a little.
All scientists are agreed that the sun is of the same constitution as
the thousands of luminous stars which we see in the sky. According to
the color of the light which they emit, stars are classified as white,
yellow, and red stars. The differences in their light become much
more distinct when we examine them spectroscopically. In the white
stars the helium and hydrogen lines predominate decidedly; the helium
stars contain, in addition, oxygen. Metals are comparatively little
represented; but they play a main part in the spectra of the yellow
stars, in which, further, some bands become visible. In the spectra
of the red stars we notice many bands which indicate that chemical
compounds are present in the outer portions. Everybody knows that the
platinum wire or the filament of an incandescent lamp which has been
heated to incandescence by the electric current first shines reddish,
then yellow when the current is increased, and finally more and more
white. At the same time the temperature rises. We can estimate the
temperature from the brightness of the glow. If we know the wave-length
of the radiations of that color which emits the greatest amount of
heat in the spectrum (it should be a normal spectrum), it is easy to
calculate the temperature of the star from Wien’s law of displacements.
We need only divide 2.89 by the respective wave-length expressed in
mm. to find the absolute temperature of the star; by deducting 273
from the result, we obtain the temperature in degrees Cent. on the
ordinary scale. For the sun the maximum of heat radiation lies near
wave-length 0.00055 (in the greenish-yellow light), and therefore the
absolute temperature of the radiating disk of the sun, the so-called
photosphere, should be 5255° absolute, or nearly 5000° Cent. But our
atmosphere weakens the sunlight, and it also causes a displacement of
the maximum radiation in the spectrum. The same applies to the sun’s
own atmosphere, so that we have to adopt a higher estimate than 5000°
Cent. By means of Stefan’s law of radiation, the solar temperature has
been estimated at about 6200°, which would correspond to a wave-length
of about 0.00045 mm. This correction is therefore significant. About
half of it has to be ascribed to the influence of the solar atmosphere,
the other half to the terrestrial atmosphere. A Hungarian astronomer,
Harkányi, has determined in the same way the temperature of several
white stars (Vega and Sirius), and found it to be about 1000° higher
than that of the sun, while the red star Betelgeuse, the most prominent
star in Orion, would have a temperature by 2500° lower than that of the
sun.
It must expressly be stated that in making these estimates we
understand by the temperature of the star in this case the temperature
of a radiating body which emits the same light as that which reaches
us from the star. But the stellar light undergoes important changes
on its way to us. We learn from observing new stars that a star may
be surrounded by a cloud of cosmical dust which sifts the blue rays
out and permits the red ones to pass. The star then shines with a
less brilliantly white light than in the absence of the cloud. The
consequence is that we estimate the temperature lower than it really
is. In the red stars bands have been noticed, indicating, as we have
already said, the presence of chemical compounds. The most interesting
of these are the compounds of cyanogen and of carbon, probably with
hydrogen, which appear to resemble those observed by Swan in the
spectrum of gas flames and which were named after him. It was formerly
thought that the presence of these compounds implied lower temperature.
But we shall see that this conclusion is not firmly established. Hale
has found during eclipses of the sun that exactly the same compounds
occur immediately above the luminous clouds of the sun. They are
probably more numerous below the clouds, where the temperature is no
doubt higher, than above them.
However that may be, we have reason to assume that the now yellow sun
was once a white star like the brilliant Sirius, that it has slowly
cooled down to its present appearance, and that it will some day shine
with the reddish light of Betelgeuse. The sun will then only radiate a
seventh of the heat which it emits now, and it is very likely that the
earth will have been transformed into a glacial desert long before that
time.
It has already been pointed out that the atmospheres of both the sun
and of the earth produce a strong absorption of the solar rays, and
especially of the blue and white rays. It is for this reason that the
light of the sun appears more red in the evening than at noon, because
in the former case it has to pass through a thicker layer of air, which
absorbs the blue rays. For the same reason the limb of the sun appears
more red in spectroscopic examinations than the centre of the sun.
This weakening of the sun’s light is due to the fine dust pervading
the atmospheres of the earth and the sun. When the products of strong
volcanic eruptions, like the eruptions of Krakatoa in 1883 and of Mont
Pelée in 1902, filled the atmosphere with a fine volcanic dust, the sun
appeared distinctly red when standing low in the horizon. It was this
dust that caused the red glow.
When we examine an image of the sun which has been thrown on a screen
by the aid of a lens or a system of lenses, we notice on the sun’s
disk a mottling of characteristic darker spots. These spots struck the
attention of Galileo, and they were discovered almost simultaneously by
him, by Fabricius, and by Scheiner (1610-1611). These spots have since
been the most diligently studied features of the sun. We carefully
determine their number and sizes, and combine these two data to make
the so-called sun-spot numbers. These numbers change from year to
year in a rather irregular way, the period amounting on an average to
11.1 years. The spots appear in two belts on the sun, and they glide
over the disk in the course of thirteen or fourteen days. Sometimes
they reappear after another thirteen or fourteen days. It is therefore
believed that they lie comparatively quiet on the surface of the sun,
and that the sun rotates about its own axis in about twenty-seven
days, so that after that period the same points are again opposite
the earth. This is the so-called synodical period. The great interest
which attaches to the study of these features lies in the fact that
simultaneously with these spots several other phenomena seem to vary
which attain their maxima at the same time. Such are, in the first
instance, the polar lights and the magnetic variations, and, to a
lesser degree, the cirrus clouds and temperature changes, as well as
several other meteorological phenomena (compare Chapter V.).
About the sun-spots we notice the so-called faculæ—portions which are
much brighter than their surroundings. When we carefully examine a
strongly magnified image of the sun, we find that it has a granulated
appearance (Fig. 18). Langley compares the disk to a grayish-white
cloth almost hidden by flakes of snow. The less bright portions are
designated “pores,” the brighter portions “granules.” It is generally
assumed that the granules correspond to clouds which rise like the
clouds of our atmosphere on the top of ascending convection currents.
But while the terrestrial clouds are formed of drops of rain or of
crystals of ice, the granules consist probably of soot—that is to
say, condensed carbon—and of drops of metals, iron, and others. The
smallest granule which we are able to discern has a diameter of about
200 km. (130 miles).
[Illustration: Fig. 18.—Sun-spot group and granulation of the
sun. (Photographed at the Meudon Observatory, near Paris, April
1, 1884)]
The faculæ are formed by very large accumulations of clouds which are
carried up by strong ascending currents and spread over large areas,
as in our cyclones. The spots correspond to descending masses of gas
with rising temperatures, which are therefore “dry” and do not carry
any clouds, as in terrestrial anticyclones. Through these holes in the
walls of solar clouds we peep a little farther into the gigantic masses
of gas, and we obtain an idea of the state of affairs in the deeper
strata of the sun. The depth of the wall of cloud is, of course, not
large compared to the radius of the sun.
[Illustration: Fig. 19.—Part of the solar spectrum of January
3, 1872. After Langley. The bright horizontal bands are due
to prominences. In the middle (at 208) the hydrogen line F,
strongly distorted by violent agitation]
The study of the spectra affords us the best insight into the nature
of the different parts of the sun. The spectra teach us not only the
constituents of these parts, but also the velocities with which they
move. We have learned in this way that, lying above the luminous
clouds of the sun which are radiating to us, there are great masses
of gas containing most of our terrestrial elements. We distinguish
particularly in them iron, magnesium, calcium, sodium, helium, and
hydrogen. The two last-mentioned constituents, being the least dense,
are found particularly in the outermost strata of the atmosphere. The
solar atmosphere becomes visible when, during an eclipse of the sun,
the disk of the moon has proceeded so far as to cover the intensely
luminous clouds in the so-called photosphere. Owing to its strong
percentage of hydrogen, the gaseous atmosphere generally shines in the
purple hue which is characteristic of this element. This stratum of gas
is also called the chromosphere (from the Greek word χρῶμα, meaning
color). Its thickness is estimated at from 7000 to 9000 km. (5000 to
6000 miles). From it rise rays of fire over the surrounding surface
like blades of grass on meadows, to which their appearance has been
likened.
[Illustration: Fig. 20.—Metallic prominences in vortex motion.
The white spot marks the size of the earth]
[Illustration: Fig. 21.—Fountain-like metallic prominences]
When these flames rise still higher, to about 15,000 km. (9300 miles)
or more, they are called protuberances or prominences. Their number
as well as their altitude grow with the number of sun-spots. They are
distinguished as metallic and as quiet prominences. The former are
characterized by particularly violent motion, as will become apparent
from Figs. 20 and 21, and they contain large amounts of metallic
vapor. They appear only within the belt of sun-spots which are most
pronounced at a distance of about 20° from the solar equator. Their
movements are so violent that they often traverse several hundreds of
kilometres in a second. The Hungarian Fényi observed, indeed, on July
15, 1895, a prominence whose greatest velocity in the line of sight,
measured spectroscopically, amounted to 862 km. (536 miles), and whose
maximum velocity at right angles to this direction was 840 km. per
second. These colossal velocities distinguish the highest parts, while
the lower portions, which are the most dense and which contain most
metallic vapor, are less mobile, as might be expected. Their altitude
above the sun’s surface may reach exceedingly high figures, and this
applies also to the quiet prominences. The above-mentioned prominence
of July 15, 1895, reached a height of 500,000 km., and Langley
observed, on October 7, 1880, one at an altitude of 560,000 km., whose
tip, therefore, nearly attained an elevation equal to that of a radius
of the sun, 690,000 km. above the limb of the sun’s photosphere. The
mean altitude of these prominences is 40,000 km. After their discovery
by Lector Vassenius, of Götheborg, in 1733, they could only be studied
during total solar eclipses, until Lockyer and Janssen taught us, in
the year 1868, how to observe them in full sunlight by means of the
spectroscope.
[Illustration: Fig. 22.—Quiet prominences of smoke-column type]
[Illustration: Fig. 23.—Quiet prominences, shape of a tree. The
white spot indicates the size of the earth]
[Illustration: Fig. 24.—Diagram illustrating the differences
in the spectra of sun-spots and of the photosphere. Some lines
in the spot spectrum are stronger, others fainter, than in the
photosphere spectrum. In the central portion, two reversals; to
the right, two bands. After Mitchell]
[Illustration: Fig. 25.—Spectrum of a sun-spot, the central
band between the two portions of the photosphere spectrum. The
spot spectrum is bordered with the half-shadows of the edge of
the spot. After Mitchell]
The quiet prominences consist almost exclusively of hydrogen and
helium; sometimes they contain also traces of metallic gases. They
resemble clouds floating quietly in the solar atmosphere, or masses
of smoke coming from a chimney. They may appear anywhere on the sun,
and their stability is so great that they have sometimes been watched
during a complete solar rotation (for about forty days); this is
possible only when they occur in the neighborhood of the poles, where
they always remain visible outside the sun’s limb. Figs. 22 and 23 show
several such prominences according to Young.
[Illustration: Fig. 26.—The great sun-spot of October 9,
1903. Taken with the photo-heliograph of Greenwich in the
usual manner. The spot is shown at mean level of the calcium
faculæ. The two following photographs show a lower-level and a
higher-level section through the calcium faculæ]
Sometimes the matter of the prominences seems to fall back upon the
surface of the sun between the smaller flames of fire which we have
likened to blades of grass (Fig. 21). In most cases, however, the
prominences appear slowly to dissolve. When their brilliant glow fades
owing to their intense radiation, they can no longer be observed. The
quiet prominences, which seem to float at heights of about 50,000 km.
and at still greater heights, must there be almost in a vacuum. Their
particles cannot be supported by any surrounding gases, after the
manner of the drops of water in terrestrial clouds. In order that they
may remain floating they must be pushed away from the sun by a peculiar
force—the radiation pressure (see Chapter IV.).
[Illustration: Fig. 27.—The great sun-spot of October 9, 1903.
Photograph of the low-level calcium faculæ with the aid of the
light of the calcium line H. The spot is not obscured by the
faculæ—at least, not so much as in the following illustrations]
The faculæ can be studied in the same way as the prominences, and
of late Deslandres and Hale have used for this purpose a special
instrument, the heliograph (compare Figs. 26 to 29). When the faculæ
approach the limb of the sun they appear particularly brilliant by
comparison with their surroundings. That seems to indicate that they
are lying at a great altitude, and that their light is hence not
weakened by the superposed hazy stratum. When they reach the sun’s
limb they appear to us like raised portions of the photosphere. The
clouds which form these faculæ are carried upward by powerful ascending
streams of gas whose expansion is due to the diminution of the gaseous
pressure.
[Illustration: Fig. 28.—The great sun-spot of October 9, 1903.
Photograph of the higher-level calcium faculæ, taken with the
light of the central portion of the line H (calcium). The
higher-level faculæ hide the spot, indicating that the faculæ
spread considerably during their ascent]
[Illustration: Fig. 29.—The great sun-spot of October 9, 1903.
Photograph of the hydrogen faculæ, taken with the light of the
spectral line F (hydrogen). Only the darkest portions of the
spot are visible. The other portions are obscured by masses of
the hydrogen, which were evidently in a restless state]
Sun-spots display many peculiarities in their spectra (Figs. 24 and
25). Very prominent is always the helium line; prominent likewise the
dark sodium lines, which are markedly widened and which show in their
middle portions a bright line—the so-called reversal of lines (Fig.
24). This occurrence indicates that the metal is lying in a deeper
stratum. In the red portion of the spectrum we find bands, just as in
the spectra of the red stars. These bands, which appear to be resolved
into crowds of lines by the aid of powerful instruments, indicate the
presence of chemical compounds. Since the spot is comparatively of
feeble intensity, its spectrum appears superposed like a less bright
ribbon upon the background of the spectrum of the more luminous
photosphere. The violet end of the sun-spot spectrum is particularly
weakened. Although the spot has the appearance of a pit in the
photosphere, and when on the sun’s limb makes it look as if a piece had
been cut out of the edge, it yet does not appear darker than the sun’s
edge. That points to the conclusion that the light emitted by the spot
emanates chiefly from its upper, cold portions.
[Illustration: Fig. 30.—Photograph of the solar corona of 1900.
(After Langley and Abbot.) Illustrating the appearance of the
corona in years of minimum sun-spot frequency]
The light coming from the deeper portions is distinctly absorbed to
a large degree by the higher-lying strata. The sun-spots also appear
to become narrower in their lower parts, owing to the compression of
the gases at greater depths, and one may regard their funnel-shaped
cloud-walls as “half-shadows,” which appear darker than the
surroundings, but brighter than the so-called core of the spot. The
weakening of the violet end of the spectrum is probably due to the
presence of fine particles of dust in the solar gases, just as they
cause the corresponding weakening of the violet end of the spectrum of
the sun’s limb. The bands in the red parts of the sun-spot spectrum may
originate from the deeper portions of the spot, because all the higher
parts of the solar atmosphere yield simple, sharp lines. The bands
suggest that chemical compounds can exist at the higher pressure of the
inner portions of the sun, and that these compounds are decomposed
in the outer parts of the sun, to give the line spectra of chemical
elements.
[Illustration: Fig. 31.—Photograph of the solar corona of 1870.
(After Davis.) The year 1870 was one of maximum sun-spot frequency]
The enigmatical corona lies farther out in the atmosphere of the sun.
It consists of streamers which may extend beyond the disk of the sun to
the length of several solar diameters. The corona can only be observed
at total eclipses of the sun. Figs. 30 to 32 illustrate the appearance
of this very peculiar phenomenon.
[Illustration: Fig. 32.—Photograph of the solar corona of 1898.
(After Maunder.) 1898 was a year of average solar activity]
When the number of sun-spots is small, the corona streamers extend like
huge brooms from the equatorial parts, and the feebler rays of the
corona near the solar poles are then bent downward to the equator, just
like the lines of force about the poles of a magnet (Fig. 30).
We suppose, for this reason, that the sun acts like a strong magnet,
whose poles are situated near the geographical poles of the sun. In
years which are richer in sun-spots the distribution of the streamers
of the corona is more uniform. At moderate sun-spot frequency, large
numbers of rays seem to emanate from the neighborhood of the maximum
belt of sun-spots, so that the corona often assumes a quadrangular
shape (compare Fig. 32).
These remarks hold for the “outer corona,” while the inner portion,
the so-called “inner corona,” shines in a more uniform light. The
spectroscopic examination demonstrates that the light consists
mainly of hydrogen gas and of an unknown gas designated coronium,
which particularly seems to occur in the higher parts of the inner
corona. The outer streamers of the corona, on the contrary, yield a
continuous spectrum which shows that the light is radiated by solid or
liquid particles. In the spectrum of the coronal rays at an extreme
distance from the disk, astronomers have sometimes fancied that they
discerned dark lines on a bright ground, just as in the spectrum of the
photosphere. It has been assumed that this light is reflected sunlight,
originating from small solid or liquid particles of the outer corona.
It must be reflected, because it is partly polarized. The radiating
disposition of the outer corona indicates the action of a force, the
radiation pressure, which drives the smaller particles away from the
centre of the sun.