THE SUN
The Sun is, of all the heavenly bodies, the most impressive, and has
necessarily, at all times, attracted the chief attention of men. There are
only two of the heavenly bodies that appear to be more than points of
light, only two that show a surface to the naked eye, and the Sun, being
so much the brighter of the two, and the obvious source of all our light
and heat, and the fosterer of vegetation, readily takes the premier place
in interest. In the present day we know too much about the Sun for anyone
to suppose that it can be the home of organic life; but it is not many
years since its habitability was seriously suggested even by so high an
authority as Sir William Herschel. He conceived that it was possible that
its stores of light and heat might be confined to a relatively thin shell
in its upper atmosphere, and that below this shell a screen of clouds
might so check radiation downward that it would be possible for an inner
nucleus to exist which should be cool and solid. This fancied inner globe
would then necessarily enjoy perpetual daylight, and a climate which knew
no variation from pole to pole. To its inhabitants the entire heavens
would be generally luminous, the light not being concentrated into any one
part of the vault; and it was supposed that, ignorant of time, a happy
race might flourish, cultivating the far-spread solar fields, in perpetual
daylight, and in the serenity of a perpetual spring that was distracted by
no storm.
The picture thus conjured up is a pleasing one, though probably, to the
restless sons of Earth, it would seem to suffer somewhat from monotony.
But we now know that it corresponds in not a single detail to the actual
facts. The study of solar conditions carried on through the last hundred
years has revealed to us, not serenity and peace, but storm, stress, and
commotion on the most gigantic scale. But though we now can dismiss from
our minds the possibility that the Sun can be inhabited, yet it is of such
importance to the maintenance of life on this planet, and by parity of
reasoning to life on any other planet, that a review of its conditions
forms a necessary introduction to our subject. Further, those conditions
themselves will bring out certain principles that are of necessary
application when we come to consider the case of particular planets.
The distance of the Sun from the Earth is often spoken of as the
"astronomical unit"; it is the fundamental measure of astronomy, and all
our information as to the sizes and distances of the various planets rests
upon it. And, as we shall shortly see, the particular problem with which
we are engaged--the habitability of worlds--is directly connected with
these two factors: the size of the world in question, and its distance
from the Sun.
The distance of the Sun has been determined by several different methods
the principles of which do not concern us here, but they agree in giving
the mean distance of the Sun as a little less than 93,000,000 miles; that
is to say, it would require 11,720 worlds as large as our own to be put
side by side in order to bridge the chasm between the two. Or a traveller
going round the Earth at its equator would have to repeat the journey 3730
times before he had traversed a space equal to the Sun's distance.
But knowing the Sun's distance, we are able to deduce its actual diameter,
its superficial extent, and its volume, for its apparent diameter can
readily be measured. Its actual diameter then comes out as 866,400 miles,
or 109.4 times that of the Earth. Its surface exceeds that of the Earth
11,970 times; its volume, 1,310,000 times.
But the weight of the Sun is known as well as its size; this follows as a
consequence of gravitation. For the planets move in orbits under the
influence of the Sun's attraction; the dimensions of their orbits are
known, and the times taken in describing them; the amount of the
attractive force therefore is also known, that is to say, the mass of the
Sun. This is 332,000 times the mass of the Earth; and as the latter has
been determined as equal to about
6,000,000,000,000,000,000,000 tons
that of the Sun would be equal to
2,000,000,000,000,000,000,000,000,000 tons.
It will be seen that the proportion of the volume of the Sun to that of
the Earth is greater than the proportion of its mass to the Earth's
mass--almost exactly four times greater; so that the mean density of the
Sun can be only one-fourth that of the Earth. Yet, if we calculate the
force of gravity at the surfaces of both Sun and Earth, we find that the
Sun has a great preponderance. Its mass is 332,000 times that of the
Earth, but to compare it with the attraction of the Earth's surface we
must divide by (109.4){2}, since the distance of the Sun's centre from its
surface is 109.4 times as great as the corresponding distance in the case
of the Earth, and the force of gravity diminishes as the square of the
increased distance. This gives the force of gravity at the solar surface
as 27.65 times its power at the surface of the Earth, so that a body
weighing one ton here would weigh 27 tons 13 cwt. if it were taken to the
Sun.[8]
This relation is one of great importance when we realize that the pressure
of the Earth's atmosphere is 14.7 lb. on the square inch at the sea
level; that is to say, if we could take a column of air one square inch in
section, extending from the surface of the Earth upwards to the very limit
of the atmosphere, we should find that it would have this weight. If we
construct a water barometer, the column of water required to balance the
atmosphere must be 34 feet high, while the height of the column of mercury
in a mercurial barometer is 30 inches high, for the weight of 30 cubic
inches of mercury or of 408 cubic inches of water (34 x 12 = 408) is 14.7
lb.
If, now, we ascend a mountain, carrying a mercurial barometer with us we
should find that it would fall about one inch for the first 900 feet of
our ascent; that is to say, we should have left one-thirtieth of the
atmosphere below us by ascending 900 feet. As we went up higher we should
find that we should have to climb more than 900 feet further in order that
the barometer might fall another inch; and each successive inch, as we
went upward, would mean a longer climb. At the height of 2760 feet the
barometer would have fallen three inches; we should have passed through
one-tenth of the atmosphere. At the height of 5800 feet, we should have
passed through one-fifth of the atmosphere, the barometer would have
dropped six inches; and so on, until at about three and a third miles
above sea level the barometer would read fifteen inches, showing that we
had passed through half the atmosphere. Mont Blanc is not quite three
miles high, so that in Europe we cannot climb to the height where half the
atmosphere is left below us, and there is no terrestrial mountain anywhere
which would enable us to double the climb; that is to say, to ascend six
and two-third miles. Could we do so, however, we should find that the
barometer had fallen to seven and a half inches; that the second ascent of
three and a third miles had brought us through half the remaining
atmosphere, so that only one-fourth still remained above us. In the
celebrated balloon ascent made by Mr. Coxwell and Mr. Glaisher on
September 5, 1861, an even greater height was attained, and it was
estimated that the barometer fell at its lowest reading to seven inches,
which would correspond to a height of 39,000 feet.
But on the Sun, where the force of gravity is 27.65 times as great as at
the surface of the Earth, it would, if all the other conditions were
similar, only be necessary to ascend one furlong, instead of three and a
third miles, in order to reach the level of half the surface pressure, and
an ascent of two furlongs would bring us to the level of quarter pressure,
and so on. If then the solar atmosphere extends inwards, below the
apparent surface, it should approximately double in density with each
furlong of descent. These considerations, if taken alone, would point to a
mean density of the Sun not as we know it to be, less than that of the
Earth, but immeasurably greater; but the discordance is sufficiently
explained when we come to another class of facts.
These relate to the temperature of the Sun, and to the enormous amount of
light and heat which it radiates forth continually. This entirely
transcends our power to understand or appreciate. Nevertheless, the
astonishing figures which the best authorities give us may, by their
vastness, convey some rough general impression that may be of service.
Thus Prof. C. A. Young puts the total quantity of sunlight as equivalent
to
1,575,000,000,000,000,000,000,000,000 standard candles.
The intensity of sunlight at each point of the Sun's surface is variously
expressed as
190,000 times that of a standard candle,
5300 times that of the metal in a Bessemer converter,
146 times that of a calcium light,
or, 3.4 times that of an electric arc.
The same authority estimates at 30 _calories_ the value of the _Solar
Constant_; that is to say, the heat which, if our atmosphere were removed,
would be received from the Sun in a minute of time upon a square metre of
the Earth's surface that had the Sun in its zenith, would be sufficient to
raise the temperature of a kilogram of water 30 degrees Centigrade. This
would involve that the heat radiation from each square metre of the Sun's
surface would equal 1,340,000 calories; or sufficient to melt through in
each minute of time a shell of ice surrounding the Sun to the thickness of
58.2 feet. Prof. Abbot's most recent determination of the solar constant
diminishes these estimates by one third; but he still gives the probable
temperature of the solar surface as not far short of 7000 degrees
Centigrade, or about 12,000 degrees Fahrenheit.
The Sun, then, presents us with temperatures and pressures which entirely
surpass our experience on the Earth. The temperatures, on the one hand,
are sufficient to convert into a permanent gas every substance with which
we are acquainted; the pressures, on the other hand, apart from the high
temperatures, would probably solidify every element, and the Sun, as a
whole, would present itself to us as a comparatively small solid globe,
with a density like that of platinum. With both factors in operation, we
have the result already given: a huge globe, more than one hundred times
the diameter of the Earth, yet only one-fourth its density, and gaseous
probably throughout the whole of its enormous bulk.
What effect have these two factors, so stupendous in scale, upon its
visible surface? What is the appearance of the Sun?
It appears to be a large glowing disc, sensibly circular in outline, with
its edge fairly well-defined both as seen in the telescope and as
registered on photographs. In the spectroscope, or when in an eclipse of
the Sun the Moon covers the whole disc, a narrow serrated ring is seen
surrounding the rim, like a velvet pile of a bright rose colour. This
crimson rim, the sierra or _chromosphere_ as it is usually called, is
always to be found edging the entire Sun, and therefore must carpet the
surface everywhere. But under ordinary conditions, we do not see the
chromosphere itself, but look down through it on the _photosphere_, or
general radiating surface. This, to the eye, certainly looks like a
definite shell, but some theorists have been so impressed with the
difficulty of conceiving that a gaseous body like the Sun could, under the
conditions of such stupendous temperatures as there exist, have any
defined limit at all, that they deny that what we see on the Sun is a real
boundary, and argue that it only appears so to us through the effects of
the anomalous refraction or dispersion of light. Such theories introduce
difficulties greater and more numerous than those that they clear away,
and they are not generally accepted by practical observers of the Sun.
They seem incompatible with the apparent structure of the photosphere,
which is everywhere made up of a complicated mottling: minute grains
somewhat resembling those of rice in shape, of intense brightness, and
irregularly scattered. This mottling is sometimes coarsely, sometimes
finely textured; in some regions it is sharp and well defined, in others
misty or blurred, and in both cases they are often arranged in large
elaborate patterns, the figures of the pattern sometimes extending for a
hundred thousand miles or more in any direction. The rice-like grains or
granules of which these figures are built up, and the darker pores between
them, are, on the other hand, comparatively small, and do not, on the
average, exceed two to four hundred miles in diameter.
But the Sun shows us other objects of quite a different order in their
dimensions. Here and there the bright granules of the photosphere become
disturbed and torn apart, and broad areas are exposed which are relatively
dark. These are _sunspots_, and in the early stages of their development
they are usually arranged in groups which tend to be stretched out
parallel to the Sun's equator. A group of spots in its later stages of
development is more commonly reduced to a single round, well-defined, dark
spot. These groups, when near the edge of the Sun, are usually seen to be
accompanied by very bright markings, arranged in long irregular lines,
like the foam on an incoming tide. These markings are known as the
_faculae_, from their brightness. In the spectroscope, when the serrated
edges of the chromosphere are under observation, every now and then great
_prominences_, or tongues and clouds of flame, are seen to rise up from
them, sometimes changing their form and appearance so rapidly that the
motion can almost be followed by the eye. An interval of fifteen or
twenty minutes has frequently been sufficient to transform, quite beyond
recognition, a mass of flame fifty thousand miles in height. Sometimes a
prominence of these, or even greater, dimensions has formed, developed,
risen to a great distance from the Sun, and completely disappeared within
less than half an hour. The velocity of the gas streams in such eruptions
often exceeds one hundred miles a second; sometimes, though only rarely,
it reaches a speed twice as great.
Sunspots do not offer us examples of motions of this order of rapidity,
but the areas which they affect are not less astonishing. Many spot groups
have been seen to extend over a length of one hundred thousand, or one
hundred and fifty thousand miles, and to cover a total area of a thousand
million square miles. Indeed, the great group of February, 1905, at its
greatest extent, covered an area four times as great as this. Again, in
the normal course of the development of a spot group, the different
members of the group frequently show a kind of repulsion for each other in
the early stages of the group's history, and the usual speed with which
they move away from each other is three hundred miles an hour.
The spots, the faculae, the prominences, are all, in different ways, of
the nature of storms in an atmosphere; that is to say, that, in the great
gaseous bulk of the Sun, certain local differences of constitution,
temperature, and pressure are marked by these different phenomena. From
this point of view it is most significant that many spots are known to
last for more than a month; some have been known to endure for even half a
year. The nearest analogy which the Earth supplies to these disturbances
may be found in tropical cyclones, but these are relatively of far smaller
area, and only last a few days at the utmost, while a hundred miles an
hour is the greatest velocity they ever exhibit, and this, fortunately,
only under exceptional circumstances. For a wind of such violence mows
down buildings and trees as a scythe the blades of grass; and were
tornadoes moving at a rate of 300 miles an hour as common upon the Earth
as spots are upon the Sun, it would be stripped bare of plants and
animals, as well as of men and of all their works.
It is not an accident that the Sun, when storm-swept, shows this violence
of commotion, but a necessary consequence of its enormous temperature and
pressures. As we have seen, the force of gravity at its surface is 27.65
times that at the surface of the Earth, where a body falls 16.1 feet in
the first second of time; on the Sun, therefore, a body would fall 445
feet in the first second; and the atmospheric motions generally would be
accelerated in the same proportion.
The high temperatures, the great pressures, the violent commotions which
prevail on the Sun are, therefore, the direct consequence of its enormous
mass. The Sun is, then, not merely the type and example of the chief
source of light and heat in a given planetary system; it indicates to us
that size and mass are the primary tokens by which we may judge the
temperature of a world, and the activity to be expected in its changes.