The Children's Book of Stars

by G. E. Mitton

To begin at the beginning, we must go back to the time when the sun was only a great gaseous nebula filling all the space included in the orbit of Neptune. This nebula was not in itself hot, but as it rotated it contracted. Now, heat is really only a form of energy, and energy and heat can be interchanged easily. This is a very startling thing when heard for the first time, but it is known as surely as we know anything and has been proved again and again. When a savage wants to make a fire he turns a piece of hard wood very very quickly between his palms—twiddles it, we should say expressively—into a hole in another piece of wood, until a spark bursts out. What is the spark? It is the energy of the savage's work turned to heat. When a horse strikes his iron-shod hoofs hard on the pavement you see sparks fly; that is caused by the energy of the horse's leg. When you pump hard at your bicycle you feel your pump getting quite hot, for part of the energy you are putting into your work is transformed into heat; and so on in numberless instances. No energetic action of any kind in this world takes place without some of the energy being turned into heat, though in many instances the amount is so small as to be unnoticeable. Nothing falls to the ground without some heat being generated. Now, when this great nebula first began its remarkable career, by the action of gravity all the particles in it were drawn toward the centre; little by little they fell in, and the nebula became smaller. We are not now concerned with the origin of the planets—we leave that aside; we are only contemplating the part of the nebula which remained to become the sun. Now these particles being drawn inward each generated some heat, so as the nebula contracted its temperature rose. Throughout the ages, over the space of millions and millions of miles, it contracted and grew hotter. It still remained gaseous, but at last it got to an immense temperature, and is the sun as we know it. What then keeps it shining? It is still contracting, but slowly, so slowly that it is quite imperceptible to our finest instruments. It has been calculated that if it contracts two hundred and fifty feet in diameter in a year, the energy thus gained and turned into heat is quite sufficient to account for its whole yearly output. This is indeed marvellous. In comparison with the sun's size two hundred and fifty feet is nothing. It would take nine thousand years at this rate before any diminution could be noticed by our finest instruments! Here is a source of heat which can continue for countless ages without exhaustion. Thus to all intents and purposes we may say the sun's shining is inexhaustible. Yet we must follow out the train of reasoning, and see what will happen in the end, in eras and eras of time, if nothing intervenes. Well, some gaseous bodies are far finer and more tenuous than others, and when a gaseous body contracts it is all the time getting denser; as it grows denser and denser it at last becomes liquid, and then solid, and then it ceases to contract, as of course the particles of a solid body cannot fall freely toward the centre, as those of a gaseous body can. Our earth has long ago reached this stage. When solid the action ceases, and the heat is no more kept up by this source of energy, therefore the body begins to cool—surface first, and lastly the interior; it cools more quickly the smaller it is. Our moon has parted with all her heat long ago, while the earth still retains some internally. In the sun, therefore, we have an object-lesson of the stages through which all the planets must have passed. They have all once been glowing hot, and some may be still hot even on the surface, as we have seen there is reason to believe is the case with Jupiter.

By this marvellous arrangement for the continued heat of the sun we can see that the warmth of our planets is assured for untold ages. There is no need to fear that the sun will wear out by burning. His brightness will continue for ages beyond the thoughts of man.

Besides this, a few other things have been discovered about him. He is, of course, exceptionally difficult to observe; for though he is so large, which should make it easy, he is so brilliant that anyone regarding him through a telescope without the precaution of prepared glasses to keep off a great part of the light would be blinded at once. One most remarkable fact about the sun is that his surface is flecked with spots, which appear sometimes in greater numbers and sometimes in less, and the reason and shape of these spots have greatly exercised men's minds. Sometimes they are large enough to be seen without a telescope at all, merely by looking through a piece of smoked or coloured glass, which cuts off the most overpowering rays. When they are visible like this they are enormous, large enough to swallow many earths in their depths. At other times they may be observed by the telescope, then they may be about five thousand miles across. Sometimes one spot can be followed by an astronomer as it passes all across the sun, disappears at the edge, and after a lapse of time comes back again round the other edge. This first showed men that the sun, like all the planets, rotated on his axis, and gave them the means of finding out how long he took in doing so. But the spots showed a most surprising result, for they took slightly different times in making their journey round the sun, times which differed according to their position. For instance, a spot near the equator of the sun took twenty-five days to make the circuit, while one higher up or lower down took twenty-six days, and one further out twenty-seven; so that if these spots are, as certainly believed, actually on the surface, the conclusion is that the sun does not rotate all in one piece, but that some parts go faster than others. No one can really explain how this could be, but it is certainly more easily understood in the case of a body of gas than of a solid body, when it would be simply impossible to conceive. The spots seem to keep principally a little north and a little south of the equator; there are very few actually at it, and none found near the poles, but no reason for this distribution has been discovered. It has been noted that about every eleven years the greatest number of spots appears, and that they become fewer again, mounting up in number to the next eleven years, and so on. All these curious facts show there is much yet to be solved about the sun. The spots were supposed for long to be eruptions bursting up above the surface, but now they are generally held to be deep depressions like saucers, probably caused by violent tempests, and it is thought that the inrush of cooler matter from above makes them look darker than the other parts of the sun's surface. But when we use the words 'cooler' and 'darker,' we mean only by comparison, for in reality the dark parts of the spots are brighter than electric light.

Royal Observatory, Greenwich. SUN-SPOTS.

The fact that the spots are in reality depressions or holes is shown by their change of appearance as they pass over the face of the sun toward the edge; for the change of shape is exactly that which would be caused by foreshortening.

It sounds odd to say that the best time for observing the sun is during a total eclipse, for then the sun's body is hidden by the moon. But yet to a certain extent this is true, and the reason is that the sun's own brilliance is our greatest hindrance in observing him, his rays are so dazzling that they light up our own atmosphere, which prevents us seeing the edges. Now, during a total eclipse, when nearly all the rays are cut off, we can see marvellous things, which are invisible at other times. But total eclipses are few and far between, and so when one is approaching astronomers make great preparations beforehand.

THE EARTH AS IT WOULD APPEAR IN COMPARISON WITH THE FLAMES SHOOTING OUT FROM THE SUN.