CHAPTER III

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.