Astronomy for Everybody/Part 3/Chapter 2

In a description of the solar system its great central body is naturally the first to claim our attention. We see that the sun is a shining globe. The first questions to present themselves to us are about the size and distance of this globe. It is easy to state its size when we know its distance. We know by measurement, the angle subtended by the sun's diameter. If we draw two lines making this angle with each other, and continue them indefinitely through the celestial spaces, the diameter of the sun must be equal to the distance apart of the lines at the distance of the sun. The exact determination is a very simple problem of trigonometry. It will suffice at present to say that the measure of the apparent diameter of the sun, or the angle which it subtends to our eye, is thirty-two minutes, making this angle such that the distance of the sun is about 107.5 times its diameter in miles. If, then, we know the distance of the sun, we have only to divide it by 107.5 to get the sun's diameter.

The various methods of determining the distance of the sun will be described in our chapter stating how distances in the heavens are measured. The result of all the determinations is that the distance is very nearly ninety-three million miles, perhaps one or two hundred thousand miles more. Taking the round number, and ​dividing by 107.5, we find the diameter to be about 865,000 miles. This is about one hundred and ten times the diameter of the earth. It follows that the volume or bulk of the sun is more than one million three hundred thousand times that of the earth.

The sun's importance to us arises from its being our great source of heat and light. Were these withdrawn, not only would the world be enveloped in unending night, but, in the course of a short time, in eternal frost. We all know that during a clear night the surface of the earth grows colder through the radiation into space of the heat received from the sun during the day. Without our daily supply, the loss of heat would go on until the cold around us would far exceed that which we now experience in the polar regions. Vegetation would be impossible. The oceans would freeze over, and all life on the earth would soon be extinct.

The surface of the sun, which is all we can see of it, is called the photosphere. This term is used to distinguish the visible surface from the vast invisible interior of the sun. To the naked eye, the photosphere looks entirely uniform. But through a telescope we see that the whole surface has a mottled appearance, which has been aptly compared to that of a plate of rice soup. Examination under the best conditions shows that this appearance is due to minute and very irregular grains which are scattered all over the photosphere.

When we carefully compare the brightness of different regions of the photosphere, we find that the apparent centre of the disk is brighter than the edge. The differ​ence can be seen even without a telescope, if we look at the sun through a dark glass, or when it is setting in a dense haze. The falling off in the light is especially rapid as we approach the extreme edge of the disk, where it is little more than half as bright as at the centre. There is also a difference of colour, the light of the edge having a lurid appearance as compared with that of the centre.

All this shows that the light of the sun is absorbed by an atmosphere surrounding the sun. We readily see that, the sun being a globe, the light which we receive from the edge of its disk leaves it obliquely, while that from the centre leaves it perpendicularly. The more obliquely the light comes from the surface, the greater the thickness of the sun's atmosphere through which it must pass, and hence the greater the portion lost by the absorption of that atmosphere. The sun's atmosphere, like our own, absorbs the green and blue rays more than the red. For this reason the light has a redder tint when it comes from near the edge of the disk.

Careful observations show that the sun, like the planets, rotates on an axis passing through its centre. Using the same terms as in the case of the earth, we call the points in which the axis intersects the surface the poles of the sun, and the circle around it halfway between the poles the sun's equator. The period of rotation is about twenty-six days. As the distance around the sun is more than one hundred and ten times that ​round the earth, the speed of rotation must be more than four times that of the earth's rotation to make it complete the circuit in the time that it does. At the sun's equator the speed is more than a mile a second.

The most curious feature of this rotation is that it is completed in less time at the equator than at a distance on each side of the equator. Were the sun a solid body, like the earth, all its parts would have to rotate at the same time. Hence the sun is not a solid body, but must be either liquid or gaseous, at least at its surface.

The equator of the sun is inclined six degrees to the plane of the earth's orbit. Its direction is such that in our spring months the north pole is turned six degrees away from us and the central point of the apparent disk is about that amount south of the sun's equator. In our summer and autumn months this is reversed.

By the mean density of the sun we refer to the average specific gravity of the matter composing it, or the ratio of its weight to that of an equal volume of water. It is known that the density is only about one fourth that of the earth, and about four tenths greater than that of water. Stated with more exactness, the figures are:

Density of sun : Density of earth = 0.2554.

Density of sun: Density of water =: 1.4115.

The mass or weight of the sun is about 334,000 times that of the earth.

The force of gravity at the sun's surface is 27 times that of the earth. If it were possible for a human being ​to be placed there, an ordinary man would weigh two tons, and be crushed by his own weight.

When the sun is carefully examined with a telescope, one or more seemingly dark spots will generally, though not always, be seen on its surface. These are, of course, carried around by the rotation of the sun, and it is by means of them that the time of rotation is most easily determined. If a spot appears at the centre of the disk it will, in six days, be carried to the western edge, and there disappear. At the end of about two weeks it will reappear at the eastern edge unless it has, in the meantime, died away, which is frequently the case.

The spots have a wide range in size. Some are very minute points, barely visible in a good telescope, while on rare occasions one is large enough to be seen with the naked eye through a dark glass. They frequently appear in groups, and a group may sometimes be made out with the naked eye as a minute patch when the individual spots cannot be seen.

When the air is steady, and a good-sized spot is carefully examined with a telescope, it will be seen to be composed of a dark central region or nucleus, surrounded by a shaded border. If all the conditions are favourable, this border will appear striated, like the edge of a thatched roof. The appearance is represented in the cut, which also shows the mottling of the photosphere.

The spots are of the most varied and irregular forms, frequently broken up in many ways. The shaded border. ​or the thatched lines which form it, frequently encroaches on the nucleus or may, in places, extend quite across it.

A most remarkable law connected with the spots, which has been established by nearly three centuries of observation, is that their frequency varies in a regular period of eleven years and about forty days. During a certain year no spots will be visible for about half of the time.

This was the case in 1889 and again in 1900. The year following a slightly greater number will show themselves; and they will increase year after year for about five years. Then the frequency will begin to diminish, year after year, until the cycle is completed, when it will again begin to increase. These mutations have been traced back to the time of Galileo, although it was not till about 1825 that they were found by Schwabe to take place in a regular period.

​Years of greatest and least frequency, past and future are as follows:

Another noteworthy law connected with the sun's spots is that they are not found all over the sun; but only in certain regions of solar latitude.
Fig. 18.—Frequency of Sun-spots in Different Latitudes on the Sun. They are rather rare on the sun's equator, but become more frequent as we go north or south of the equator till we get to fifteen degrees of latitude, north or south. From this region to twenty degrees the frequency is greatest; then it falls off, so that beyond thirty degrees a spot is rarely seen. These regions are shown in the accompanying figure, where the shading is darker ​the more frequent the spots. If we made a white globe to represent the sun, and made a black dot on it for every spot during a number of years, the dotting would make the globe look as represented in the figure.

Collections of numerous small spots brighter than the photosphere in general are frequently seen on the sun. These are often seen in the neighbourhood of a spot, and occur most frequently in the regions of greater spot frequency, but are not entirely confined to those regions. They are, however, rare near the poles of the sun.

That the spots and faculæ proceed from some one general cause has been brought out by the spectro-heliograph, an instrument devised by Professor George E. Hale for taking photographs of the sun by the light of a single ray of the spectrum, that emitted by calcium, for example. The effect is the same as if we should look at the sun through a glass which would allow the rays of calcium vapour to pass, but would absorb all the others. We should then see the calcium light of the sun and no other.

When the sun is photographed by calcium light with this instrument, the result is wonderful. The sun-spot regions are now seen to be brighter than the others, and faculæ are found on every part of the sun. We thus learn that eruptions of gas, of which calcium is the best marked ingredient, are taking place all the time; but they are more numerous in the sun-spot zones than else​where. The sun-spots are therefore the effect of operations going on all the time, all over the sun, but giving rise to a spot only in the exceptional cases when they are very intense.

It was formerly supposed that the spots were openings or depressions in the photosphere, showing a darker region within. This view was based on the belief that, when a spot was near the edge of the sun's disk, the shaded border next the edge looked broader than the other. But this view is now abandoned. We cannot certainly say that a spot is either above or below the photosphere. We shall hereafter see that the latter is not a mere surface as it seems to us, but a shell or covering many miles, perhaps a hundred or more, in thickness. The spots doubtless belong to this shell, being cooler portions of it, but lying neither above nor below it.

The next remarkable feature of the sun to be described consists in the prominences. Our knowledge of these objects has an interesting history—which will be mentioned in describing eclipses of the sun. The spectroscope shows us that large masses of incandescent vapour burst forth from every part of the sun. They are of such extent that the earth, if immersed in them, would be as a grain of sand in the flame of a candle. They are thrown up with enormous velocity, sometimes hundreds of miles a second. Like the faculæ, they are more numerous in the sun-spot zones, but are not confined to those zones. The glare around the sun caused by the reflection of light ​by the air renders them entirely invisible to vision, even with the telescope, except when, during total eclipses of the sun, the glare is cut off by the intervention of the moon. They may then be seen, even with the naked eye, rising up as if from the black disk of the moon.

The prominences seem to be of two forms, the eruptive and the cloud-like. The first rise from the sun like immense sheets of flame; the latter seem to be at rest above it, like clouds floating in the air. But there is no air around the sun for these objects to float in, and we cannot certainly say what supports them. Very likely, however, it is a repulsive force of the sun's rays, which will be mentioned in a later chapter.

Spectrum analysis shows that these prominences are composed mostly of hydrogen gas, mixed with the vapours of calcium and magnesium. It is to the hydrogen that they owe their red colour. Continued study of the prominences shows them to be connected with a thin layer of gases which surrounds and rests upon the photosphere. This layer is called the chromosphere, from its deep red colour, similar to that of the prominences. As in the case of the latter, most of its light seems to be that of hydrogen; but it contains many other substances in seemingly varying proportions.

The last appendage of the sun to be considered is the corona. This is seen only during total eclipses as a soft effulgence surrounding the sun, and extending from it in long rays, sometimes exceeding the diameter of the sun in length. Its exact nature is still in doubt. It will be described in the chapter on eclipses.

Let us now recapitulate what makes up the sun as we see and know it.

We have first the vast interior of the globe which, of course, we can never see.

What we see when we look at the sun is the shining surface of this globe, the photosphere. It is not a real surface, but more likely a gaseous layer several hundred miles deep which we cannot distinguish from a surface. This layer is variegated by spots, and in or over it rise the faculæ.

On the top of the photosphere rests the layer of gases called the chromosphere, which can be observed at any time with a powerful spectroscope, but can be seen by direct vision only during total eclipses.

Through or from the red chromosphere are thrown up the equally red flames called the prominences.

Surrounding the whole is the corona.

Such is the sun as we see it. What can we say about what it really is? First, is it solid, liquid, or gaseous?

That it is not solid we have already shown by the law of rotation. It cannot be a liquid like molten metal, because it sends off from its surface such a flood of heat as would cool off and solidify molten metal in a very short time. For more than thirty years it has been understood that the interior of the sun must be a mass of gas, compressed to the density of a liquid by the enormous pressure of its superincumbent portions. But it was still supposed that the photosphere might be in the nature of a ​crust and the whole sun hke an immense bubble. This view, however, seems no longer tenable. It does not seem likely that there is any solid matter on the sun.

Attempts have sometimes been made to learn the temperature of the photosphere. It probably exceeds any that we can produce on earth, even that of the electric furnace, else how could calcium, the metallic base of lime, one of the most refractory of substances, exist there in a state of vapour? We all know that the air around us becomes cooler and rarer as we ascend above the surface of the earth, owing to the action of gravity and the consequent weight of the atmosphere, which gives rise to a constantly increasing pressure as we descend. Now, gravity at the sun is twenty-seven times as powerful as on the earth. Hence, going downward, temperature and pressure increase at a far more rapid rate on the sun than on the earth. Even in the photosphere the temperature is such that "the elements melt with fervent heat." And, as we go below the surface, the heat must increase by hundreds of degrees for every mile that we descend. The result is that in the interior the gases of the sun are subjected to two opposing forces which grow more and more intense. These are the expansive force of the heat and the compressing force of the gases above, produced by the enormous force of gravity of the sun.

The forces thus set in play merely in the outer portions of the sun's globe are simply inconceivable. Perhaps the explosion of the powder when a thirteen-inch cannon is fired is as striking an example of the force of ignited gases as we are familiar with. Now suppose every foot ​of space in a whole county covered with such cannon, all pointed upward and all being discharged at once. The result would compare with what is going on inside the photosphere about as a boy's popgun compares with the cannon.

Perhaps, from a practical point of view, the most comprehensive and important problem of science is: How is the sun's heat kept up? Before the laws of heat were fully apprehended this question was not supposed to offer any difficulties. Even to this day it is supposed by those not acquainted with the subject, that the heat which we receive from the sun may arise in some way from the passage of its rays through our atmosphere, and that, as a matter of fact, the sun may not radiate any actual heat at all—may not be an extremely hot body. But, modern science shows that heat cannot be produced except by the expenditure of some form of energy. The energy of the sun is necessarily limited in quantity and is continually being lost through radiation.

It is very easy to imagine the sun as being something like a white-hot cannon ball, which is cooling off by sending its heat in all directions, as such a ball does. We know by actual observation how much heat the sun sends to us. It may be expressed in the following way:

Imagine a shallow basin with a flat bottom, and a depth of one centimetre, that is, about four tenths of an inch. Let the basin be filled with water, the latter then being one centimetre deep. Expose such a basin to the ​rays of the vertical sun. The heat which the sun will radiate to them will be sufficient to warm the water about three and a half or four degrees Centigrade, or not very far from seven degrees Fahrenheit, in one minute. It follows that if we suppose a thin spherical shell of water, one centimetre thick, of the same radius as the earth's orbit, and having the sun in its centre, that shell of water will be heated with the rapidity just mentioned. The heat which it receives will be the total amount radiated by the sun. We can thus define how much heat the sun loses every minute, day and year.

A very simple calculation will show that if the sun were of the nature of a white-hot ball it would cool off so rapidly that its heat could not last more than a few centuries. But it has in all probability lasted millions of years. Whence, then, comes the supply? The answer of modern science to this question is that the heat radiated from the sun is supplied by the contraction of size as heat is lost. We all know that in many cases when motion is destroyed heat is produced. When a cannon shot is fired at the armour plate of a ship of war, the mere stroke of the shot makes both plate and shot hot. The blacksmith can make iron hot by hammering it.

These facts have been generalized into the statement that whenever a body falls and is stopped in its fall by friction, or by a stroke of any sort, heat is produced. From the law governing the case, we know that the water of Niagara, after it strikes the bottom of the falls, must be about one quarter of a degree warmer than it was during the fall. We also know that a hot body contracts ​in volume when cooled. The contraction of a gaseous body, such as we believe the sun to be, is greater than that of a solid or liquid. The heat of the sun is radiated from streams of matter constantly rising from the interior, which radiate their heat when they reach the surface. Being cooled they fall back again, and the heat caused by this fall is what keeps the sun hot.

It may seem almost impossible that heat sufficient to last for millions of years could be generated in this way; but the known force of gravity at the surface of the sun enables us to make exact computations on the subject. It is thus found that in order to keep up the supply of heat it is only necessary that the diameter of the sun should contract about a mile in twenty-five years—or four miles in a century. This amount would not be perceptible until after thousands of years. Yet the process of contraction must come to an end some time. Therefore, if this view is correct, the life of the sun must have a limit. What its limit may be we cannot say with exactness, we only know that it is several millions of years, but not many millions.

The same theory implies that the sun was larger in former times than it is now, and must have been larger and larger every year that we go back into its history. There was a time when it must have been as large as the whole solar system. In this case it could have been nothing but a nebula. We thus have the theory that the sun and solar system have resulted from the contraction of a nebula—through millions of years. This view is familiarly known as the nebular hypothesis.

​The question whether the nebular hypothesis is to be accepted as a proved result of science is one on which opinions differ. There are many facts which support it—such as the interior heat of the earth and the revolution and rotation of the planets all in the same direction. But cautious and conservative minds will want some further proof of the theory before they regard it as absolutely established. Even if we accept it, we still have open the question: How did the nebula itself originate, and how did it begin to contract? This brings us to the boundary where science can propound a question but cannot answer it.