Outlines of the Earth's History: A Popular Study in Physiography

by Nathaniel Southgate Shaler

Below the corona and sharply separated from it the observer finds another body of very dense vapour, which is termed the chromosphere, and which has been regarded as the atmosphere of the sun. This layer is probably several thousand miles thick. From the manner in which it moves, in the way the air of our own planet does in great storms, it is not easy to believe that it is a fluid, yet its sharply defined upper surface leads us to suppose that it can not well be a mere mass of vapour. The spectroscope shows us that this chromosphere contains in the state of vapour a number of metallic substances, such as iron and magnesium. To an observer who could behold this envelope of the sun from the distance at which we see the moon, the spectacle would be more magnificent than the imagination, guided by the sight of all the relatively trifling fractures of our earth, can possibly conceive. From the surface of the fiery sea vast uprushes of heated matter rise to the height of two or three hundred thousand miles, and then fall back upon its surface. These jets of heated matter have the aspect of flames, but they would not be such in fact, for the materials are not burning, but merely kept at a high temperature by the heat of the great sphere beneath. They spring up with such energy that they at times move with a speed of one hundred and fifty miles a second, or at a rate which is attained by no other matter in the visible universe, except that strange, wandering star known to astronomers as "Grombridge, 1830," which is traversing the firmament with a speed of not less than two hundred miles a second.

Below the chromosphere is the photosphere, the lower envelope of the sun, if it be not indeed the body of the sphere itself; from this comes the light and heat of the mass. This, too, can not well be a firm-set mass, for the reason that the spots appear to form in and move over it. It may be regarded as an extremely dense mass of gas, so weighed down by the vast attraction of the great sphere below it that it is in effect a fluid. The near-at-hand observer would doubtless find this photosphere, as it appears in the telescope, to be sharply separated from the thinner and more vaporous envelopes—the chromosphere and the corona—which are, indeed, so thin that they are invisible even with the telescope, except when the full blaze of the sun is cut off in a total eclipse. The fact that the photosphere, except when broken by the so-called spots, lies like a great smooth sea, with no parts which lie above the general line, shows that it has a very different structure from the envelope which lies upon it. If they were both vaporous, there would be a gradation between them.

On the surface of the photosphere, almost altogether within thirty degrees of the equator of the sun, a field corresponding approximately to the tropical belt of the earth, there appear from time to time the curious disturbances which are termed spots. These appear to be uprushes of matter in the gaseous state, the upward movement being upon the margins of the field and a downward motion taking place in the middle of the irregular opening, which is darkened in its central part, thus giving it, when seen by an ordinary telescope, the aspect of a black patch on the glowing surface. These spots, which are from some hundred to some thousand miles in diameter, may endure for months before they fade away. It is clear that they are most abundant at intervals of about eleven years, the last period of abundance being in 1893. The next to come may thus be expected in 1904. In the times of least spotting more than half the days of a year may pass without the surface of the photosphere being broken, while in periods of plenty no day in the year is likely to fail to show them.

Fig. 6.—Ordinary Sun-spot, June 22, 1885.

It is doubtful if the closest seeing would reveal the cause of the solar spots. The studies of the physicists who have devoted the most skill to the matter show little more than that they are tumults in the photosphere, attended by an uprush of vapours, in which iron and other metals exist; but whether these movements are due to outbreaks from the deeper parts of the sun or to some action like the whirling storms of the earth's atmosphere is uncertain. It is also uncertain what effect these convulsions of the sun have on the amount of the heat and light which is poured forth from the orb. The common opinion that the sun-spot years are the hottest is not yet fully verified.

Below the photosphere lies the vast unknown mass of the unseen solar realm. It was at one time supposed that the dark colour of the spots was due to the fact that the photosphere was broken through in those spaces, and that we looked down through them upon the surface of the slightly illuminated central part of the sphere. This view is untenable, and in its place we have to assume that for the eight hundred and sixty thousand miles of its diameter the sun is composed of matter such as is found in our earth, but throughout in a state of heat which vastly exceeds that known on or in our planet. Owing to its heat, this matter is possibly not in either the solid or the fluid state, but in that of very compressed gases, which are kept from becoming solid or even fluid by the very high temperature which exists in them. This view is apparently supported by the fact that, while the pressure upon its matter is twenty-seven times greater in the sun than it is in the earth, the weight of the whole mass is less than we should expect under these conditions.

As for the temperature of the sun, we only know that it is hot enough to turn the metals into gases in the manner in which this is done in a strong electric arc, but no satisfactory method of reckoning the scale of this heat has been devised. The probabilities are to the effect that the heat is to be counted by the tens of thousands of degrees Fahrenheit, and it may amount to hundreds of thousands; it has, indeed, been reckoned as high as a million degrees. This vast discharge is not due to any kind of burning action—i.e., to the combustion of substances, as in a fire. It must be produced by the gradual falling in of the materials, due to the gravitation of the mass toward its centre, each particle converting its energy of position into heat, as does the meteorite when it comes into the air.

It is well to close this very imperfect account of the learning which relates to the sun with a brief tabular statement showing the relative masses of the several bodies of the solar system. It should be understood that by mass is meant not the bulk of the object, but the actual amount of matter in it as determined by the gravitative attraction which it exercises on other celestial bodies. In this test the sun is taken as the measure, and its mass is for convenience reckoned at 1,000,000,000.