The Catholic World, Vol. 07, April 1868 to September, 1868

by Various

But what is it that produces these spots in the sun? Here the difficulty is singularly complicated. To reply satisfactorily to this question, it would be necessary to become acquainted with what passes in the interior of the solar globe. But let us, without hesitation, and without attempting to delude ourselves, confess that our study of the sun is confined to its external stratum, and to the most striking phenomena of which it is the seat; whereas, with regard to the interior mass, it is only by the process of induction that we are enabled to arrive at any knowledge.

Observations which we have just made lead us to the conclusion that the spots are owing to emanations issuing from the solar body, almost similar to the way in which matter is ejected by our volcanoes. This is proved both by the form of the craters, which you have just seen, and by the columns of clouds, analogous to those arising out of volcanoes, or out of chimneys, observed during eclipses. Here, then, is how we explain the constitution of the photosphere and the formation of the spots. The exterior stratum cools itself constantly by radiation, passes into the gauze-like state, or state of vapor, and ends by precipitating itself in the liquid state, or even in the solid, remaining, however, suspended in the solar atmosphere, as clouds do in ours. It is this condensed matter that forms the photosphere, and it is from that principally we receive light and heat. From some cause or other, a movement from below takes place in the gauze-like mass which is situated underneath. By this movement the photospheric stratum, raised at first, spreads itself on all sides, forming a sort of cushion, and ends by separating itself, leaving a wide opening in the form of a crater. While the volcanic emission lasts, the spot remains open, and it disappears only at the moment when the equilibrium is reestablished, by the luminous matter filling up the void which was formed. If this theory is correct, the circumference of the spots ought to form the mountains above the exterior surface. Now, we have just seen that the outline of the spots is always surrounded by faculae, which constitute prominent elevations. Supposing it is true that the interior mass is the seat of violent action, this conclusion has nothing surprising in it, and we are led to it by a certain number of other phenomena equally remarkable. Thus, every time that a spot is produced, we remark that it is visibly projected with a quickness greater than that of the solar rotation. The projecting mass is then animated with a quickness greater than the surface of the photosphere; and, in order to explain this fact, we must admit that the matter of the interior stratum possesses a quickness greater than the superficial part.

This novel conclusion is supported by another fact. We know now that the rotation of the spots has not the same angular quickness under all the parallels. The quickness is sensibly greater in the equatorial zone than in the higher latitudes. This circumstance forces us to the conclusion that the sun is not a solid globe, but that its structure admits of the different strata of which it is formed having a movement of rotation independent of each other as regards velocity. In fact, the only explanation we can give of this difference of quickness is, that the interior mass is fluid, and that it is moved by a rotary process, more rapid than that of the external surface. We cannot, however, undertake the formal demonstration of this point on the present occasion.

This fluidity of the sun is calculated to surprise you; but you will cease to regard it as incredible when I remind you of certain ascertained facts about this luminary. The gravity of its surface is twenty-eight times greater than that of the surface of our globe, from which results an enormous pressure capable of condensing a large number of substances, or, at least, of singularly diminishing their volume. Looking simply at this fact, the mean density of the sun ought to be much greater than that of the earth. It is nothing of the kind, however, but just the contrary; for the specific gravity of the terrestrial globe is four times greater than that of the solar mass. We must admit the existence of a repulsive force capable of overcoming the molecular attraction, and of rarefying the substances which the weight tends to condense. This repulsive force is probably owing to the heat, and, in fact, the temperature of the sun is estimated at not less than five millions of degrees. At this temperature no matter could remain solid, even in spite of the enormous pressure of which we have already spoken. It is, then, impossible for us to admit the existence of a solid mass, and much more that of a cold centre in the interior of the sun.

And here an objection presents itself to which I ought to reply. If the interior mass of the sun is at a temperature so very elevated, how is it that, when the photosphere opens, a black spot is presented to our eyes? In examining this opening, we perceive a substance of which the temperature is extremely elevated, and which ought, consequently, to be very luminous. How is it, then, that, on the contrary, it presents to us the appearance of a very deep black? My reply is, that the black color of the spots is a purely relative matter; that it is owing to the contrast of the brilliant light which comes to us from the photosphere. If we could see those apparently dark parts away from the glittering mass of the sun, they would appear not only luminous, but dazzling with light.

But you will say to me, it still remains true that the interior mass of the sun is less luminous than the photosphere; but since the superficial part constantly cools by radiation, it follows that there ought to be less heat, and, consequently, less brilliancy in the photosphere than in the interior mass. With your permission, I will make a reply to this which might, at the first blush, appear paradoxical, but which is, nevertheless, the expression of truth. It is precisely because it is of so very high a temperature that the interior mass of the sun sends us a less degree of light and heat; it is precisely because it is cooled at the point of condensation, to precipitate itself in the liquid or solid state, that the photospheric matter becomes hotter and more luminous. To make this plain, we have only to recall certain well-known principles of physics. Two bodies equally hot may not emit the same quantity of heat. One of them may cool itself rapidly in heating the bodies which surround it; while the other may let its heat escape only very slowly, and heat but feebly the neighboring bodies. In this case, we say that the first has a more considerable radiating power. Now, philosophers know that gas has a very feeble radiating power, and that it may be consequently at a very high temperature without emitting around it a great quantity of light and heat. You have an illustration now before your eyes. This lamp, fed by lighted gas, gives a very brilliant flame, because the carbon remains there some time in suspension before burning. Let us throw into the flame a little oxygen; immediately the flame pales, becomes bluish, and ceases to be luminous. Its temperature, notwithstanding, has greatly increased, and it is now the celebrated gas by the aid of which M. Sainte-Claire Deville melts his platina so rapidly. The change results from the very rapid combustion of the carbon by the oxygen. As soon as this takes place, the flame, no longer containing any solid body, loses almost all power of emission, and ceases, in spite of its high temperature, to have the brilliancy which it possessed at a lower temperature. To convince you perfectly, let us put a solid body in this flame, now so pale, and you will see it become more brilliant than ever. We introduce, for example, a piece of lime, and the apartment is at once illuminated by the Drummond light, one of the most brilliant of our artificial lights.

But, leaving the earth, let us now return to the sun. The interior mass is undoubtedly at a very high temperature—so high, indeed, that all the substances composing it must be in the state of gas, possessing only a feeble radiating power; while the photosphere is composed of matter precipitated in a liquid or solid state, of which the radiating power must be considerable. Here is the explanation of what seemed paradoxical in my answer. The hottest part of the sun is not the part which warms and lights us most, because, being in the state of gas, it produces only a feeble radiation.

Two questions now present themselves. How is it that the sun preserves indefinitely so elevated a temperature in spite of the enormous amount of heat which it loses daily? Of what kind of matter is this luminary composed? And what the nature of the radiation which sends to us daily the light and heat which we need? It is undoubtedly impossible to give a complete and satisfactory answer to these questions. We may yet be able, however, to do so; and we are persuaded that science in its progress will only confirm and develop the explanations which we give to-day of first principles. In the first place, it is impossible to admit that the sun is simply a luminous globe, not possessing any means of renewing the heat which it loses at every moment; for, in that case, at the end of a few years its temperature would be lowered in a very appreciable manner; and it would not require an age to effect a complete change in the phenomena which are dependent on it. There must be, then, a source of heat in the sun.

We are in the habit of comparing things we do not know with those with which we are familiar. Thus we have been led to think of the solar globe as the seat of a combustion similar to that we witness on our hearths. This idea is deceptive.