Chapter 5

But there are many things that point to absence of air on the moon. Even the photographs we reproduce tell the same story. The edges of the shadows are all hard and black. If there had been an appreciable atmosphere it would have scattered the sun's light on to the edges and produced a gradual shading off such as we see on the earth. This relative absence of air must give rise to some surprising effects. There will be no sounds on the moon, because sounds are merely air waves. Even a meteor shattering itself to a violent end against the surface of the moon would make no noise. Nor would it herald its coming by glowing into a "shooting star," as it would on entering the earth's atmosphere. There will be no floating dust, no scent, no twilight, no blue sky, no twinkling of the stars. The sky will be always black and the stars will be clearly visible by day as by night. The sun's wonderful corona, which no man on earth, even by seizing every opportunity during eclipses, can hope to see for more than two hours in all in a long lifetime, will be visible all day. So will the great red flames of the sun. Of course, there will be no life, and no landscape effects and scenery effects due to vegetation.

The moon takes approximately twenty-seven of our days to turn once on its axis. So for fourteen days there is continuous night, when the temperature must sink away down towards the absolute cold of space. This will be followed without an instant of twilight by full daylight. For another fourteen days the sun's rays will bear straight down, with no diffusion or absorption of their heat, or light, on the way. It does not follow, however, that the temperature of the moon's surface must rise enormously. It may not even rise to the temperature of melting ice. Seeing there is no air there can be no check on radiation. The heat that the moon gets will radiate away immediately. We know that amongst the coldest places on the earth are the tops of very high mountains, the points that have reared themselves nearest to the sun but farthest out of the sheltering blanket of the earth's atmosphere. The actual temperature of the moon's surface by day is a moot point. It may be below the freezing-point or above the boiling-point of water.

The Mountains of the Moon

The lack of air is considered by many astronomers to furnish the explanation of the enormous number of "craters" which pit the moon's surface. There are about a hundred thousand of these strange rings, and it is now believed by many that they are spots where very large meteorites, or even planetoids, splashed into the moon when its surface was still soft. Other astronomers think that they are the remains of gigantic bubbles which were raised in the moon's "skin," when the globe was still molten, by volcanic gases from below. A few astronomers think that they are, as is popularly supposed, the craters of extinct volcanoes. Our craters, on the earth, are generally deep cups, whereas these ring-formations on the moon are more like very shallow and broad saucers. Clavius, the largest of them, is 123 miles across the interior, yet its encircling rampart is not a mile high.

The mountains on the moon (Fig. 16) rise to a great height, and are extraordinarily gaunt and rugged. They are like fountains of lava, rising in places to 26,000 and 27,000 feet. The lunar Apennines have three thousand steep and weird peaks. Our terrestrial mountains are continually worn down by frost acting on moisture and by ice and water, but there are none of these agencies operating on the moon. Its mountains are comparatively "everlasting hills."

The moon is interesting to us precisely because it is a dead world. It seems to show how the earth, or any cooling metal globe, will evolve in the remote future. We do not know if there was ever life on the moon, but in any case it cannot have proceeded far in development. At the most we can imagine some strange lowly forms of vegetation lingering here and there in pools of heavy gas, expanding during the blaze of the sun's long day, and frozen rigid during the long night.

METEORS AND COMETS

We may conclude our survey of the solar system with a word about "shooting stars," or meteors, and comets. There are few now who do not know that the streak of fire which suddenly lights the sky overhead at night means that a piece of stone or iron has entered our atmosphere from outer space, and has been burned up by friction. It was travelling at, perhaps, twenty or thirty miles a second. At seventy or eighty miles above our heads it began to glow, as at that height the air is thick enough to offer serious friction and raise it to a white heat. By the time the meteor reached about twenty miles or so from the earth's surface it was entirely dissipated, as a rule in fiery vapour.

Millions of Meteorites

It is estimated that between ten and a hundred million meteorites enter our atmosphere and are cremated, every day. Most of them weigh only an ounce or two, and are invisible. Some of them weigh a ton or more, but even against these large masses the air acts as a kind of "torpedo-net." They generally burst into fragments and fall without doing damage.

It is clear that "empty space" is, at least within the limits of our solar system, full of these things. They swarm like fishes in the seas. Like the fishes, moreover, they may be either solitary or gregarious. The solitary bit of cosmic rubbish is the meteorite, which we have just examined. A "social" group of meteorites is the essential part of a comet. The nucleus, or bright central part, of the head of a comet (Fig. 19) consists of a swarm, sometimes thousands of miles wide, of these pieces of iron or stone. This swarm has come under the sun's gravitational influence, and is forced to travel round it. From some dark region of space it has moved slowly into our system. It is not then a comet, for it has no tail. But as the crowded meteors approach the sun, the speed increases. They give off fine vapour-like matter and the fierce flood of light from the sun sweeps this vapour out in an ever-lengthening tail. Whatever way the comet is travelling, the tail always points away from the sun.

A Great Comet

The vapoury tail often grows to an enormous length as the comet approaches the sun. The great comet of 1843 had a tail two hundred million miles long. It is, however, composed of the thinnest vapours imaginable. Twice during the nineteenth century the earth passed through the tail of a comet, and nothing was felt. The vapours of the tail are, in fact, so attenuated that we can hardly imagine them to be white-hot. They may be lit by some electrical force. However that may be, the comet dashes round the sun, often at three or four hundred miles a second, then may pass gradually out of our system once more. It may be a thousand years, or it may be fifty years, before the monarch of the system will summon it again to make its fiery journey round his throne.

THE STELLAR UNIVERSE

§ 1

The immensity of the Stellar Universe, as we have seen, is beyond our apprehension. The sun is nothing more than a very ordinary star, perhaps an insignificant one. There are stars enormously greater than the sun. One such, Betelgeux, has recently been measured, and its diameter is more than 300 times that of the sun.

The Evolution of Stars

The proof of the similarity between our sun and the stars has come to us through the spectroscope. The elements that we find by its means in the sun are also found in the same way in the stars. Matter, says the spectroscope, is essentially the same everywhere, in the earth and the sun, in the comet that visits us once in a thousand years, in the star whose distance is incalculable, and in the great clouds of "fire-mist" that we call nebulæ.

In considering the evolution of the stars let us keep two points clearly in mind. The starting-point, the nebula, is no figment of the scientific imagination. Hundreds of thousands of nebulæ, besides even vaster irregular stretches of nebulous matter, exist in the heavens. But the stages of the evolution of this stuff into stars are very largely a matter of speculation. Possibly there is more than one line of evolution, and the various theories may be reconciled. And this applies also to the theories of the various stages through which the stars themselves pass on their way to extinction.

The light of about a quarter of a million stars has been analysed in the spectroscope, and it is found that they fall into about a dozen classes which generally correspond to stages in their evolution (Fig. 21).

The Age of Stars

In its main lines the spectrum of a star corresponds to its colour, and we may roughly group the stars into red, yellow, and white. This is also the order of increasing temperature, the red stars being the coolest and the white stars the hottest. We might therefore imagine that the white stars are the youngest, and that as they grow older and cooler they become yellowish, then red, and finally become invisible--just as a cooling white-hot iron would do. But a very interesting recent research shows that there are two kinds of red stars; some of them are amongst the oldest stars and some are amongst the youngest. The facts appear to be that when a star is first formed it is not very hot. It is an immense mass of diffuse gas glowing with a dull-red heat. It contracts under the mutual gravitation of its particles, and as it does so it grows hotter. It acquires a yellowish tinge. As it continues to contract it grows hotter and hotter until its temperature reaches a maximum as a white star. At this point the contraction process does not stop, but the heating process does. Further contraction is now accompanied by cooling, and the star goes through its colour changes again, but this time in the inverse order. It contracts and cools to yellow and finally to red. But when it again becomes a red star it is enormously denser and smaller than when it began as a red star. Consequently the red stars are divided into two classes called, appropriately, Giants and Dwarfs. This theory, which we owe to an American astronomer, H. N. Russell, has been successful in explaining a variety of phenomena, and there is consequently good reason to suppose it to be true. But the question as to how the red giant stars were formed has received less satisfactory and precise answers.

The most commonly accepted theory is the nebular theory.

THE NEBULAR THEORY

§ 2

Nebulæ are dim luminous cloud-like patches in the heavens, more like wisps of smoke in some cases than anything else. Both photography and the telescope show that they are very numerous, hundreds of thousands being already known and the number being continually added to. They are not small. Most of them are immensely large. Actual dimensions cannot be given, because to estimate these we must first know definitely the distance of the nebulæ from the earth. The distances of some nebulæ are known approximately, and we can therefore form some idea of size in these cases. The results are staggering. The mere visible surface of some nebulæ is so large that the whole stretch of the solar system would be too small to form a convenient unit for measuring it. A ray of light would require to travel for years to cross from side to side of such a nebula. Its immensity is inconceivable to the human mind.

There appear to be two types of nebulæ, and there is evidence suggesting that the one type is only an earlier form of the other; but this again we do not know.

The more primitive nebulæ would seem to be composed of gas in an extremely rarified form. It is difficult to convey an adequate idea of the rarity of nebular gases. The residual gases in a vacuum tube are dense by comparison. A cubic inch of air at ordinary pressure would contain more matter than is contained in millions of cubic inches of the gases of nebulæ. The light of even the faintest stars does not seem to be dimmed by passing through a gaseous nebula, although we cannot be sure on this point. The most remarkable physical fact about these gases is that they are luminous. Whence they derive their luminosity we do not know. It hardly seems possible to believe that extremely thin gases exposed to the terrific cold of space can be so hot as to be luminous and can retain their heat and their luminosity indefinitely. A cold luminosity due to electrification, like that of the aurora borealis, would seem to fit the case better.

Now the nebular theory is that out of great "fire-mists," such as we have described, stars are born. We do not know whether gravitation is the only or even the main force at work in a nebula, but it is supposed that under the action of gravity the far-flung "fire-mists" would begin to condense round centres of greatest density, heat being evolved in the process. Of course the condensation would be enormously slow, although the sudden irruption of a swarm of meteors or some solid body might hasten matters greatly by providing large, ready-made centres of condensation.

Spiral Nebulæ

It is then supposed that the contracting mass of gas would begin to rotate and to throw off gigantic streamers, which would in their turn form centres of condensation. The whole structure would thus form a spiral, having a dense region at its centre and knots or lumps of condensed matter along its spiral arms. Besides the formless gaseous nebulæ there are hundreds of thousands of "spiral" nebulæ such as we have just mentioned in the heavens. They are at all stages of development, and they are visible to us at all angles--that is to say, some of them face directly towards us, others are edge on, and some are in intermediate positions. It appears, therefore, that we have here a striking confirmation of the nebular hypothesis. But we must not go so fast. There is much controversy as to the nature of these spiral nebulæ. Some eminent astronomers think they are other stellar universes, comparable in size with our own. In any case they are vast structures, and if they represent stars in process of condensation, they must be giving birth to huge agglomerations of stars--to star clusters at least. These vast and enigmatic objects do not throw much light on the origin of our own solar system. The nebular hypothesis, which was invented by Laplace to explain the origin of our solar system, has not yet met with universal acceptance. The explanation offers grave difficulties, and it is best while the subject is still being closely investigated, to hold all opinions with reserve. It may be taken as probable, however, that the universe has developed from masses of incandescent gas.

THE BIRTH AND DEATH OF STARS

§ 3

Variable, New, and Dark Stars: Dying Suns

Many astronomers believe that in "variable stars" we have another star, following that of the dullest red star, in the dying of suns. The light of these stars varies periodically in so many days, weeks, or years. It is interesting to speculate that they are slowly dying suns, in which the molten interior periodically bursts through the shell of thick vapours that is gathering round them. What we saw about our sun seems to point to some such stage in the future. That is, however, not the received opinion about variable stars. It may be that they are stars which periodically pass through a great swarm of meteors or a region of space that is rich in cosmic dust of some sort, when, of course, a great illumination would take place.

One class of these variable stars, which takes its name from the star Algol, is of special interest. Every third night Algol has its light reduced for several hours. Modern astronomy has discovered that in this case there are really two stars, circulating round a common centre, and that every third night the fainter of the two comes directly between us and its companion and causes an "eclipse." This was until recently regarded as a most interesting case in which a dead star revealed itself to us by passing before the light of another star. But astronomers have in recent years invented something, the "selenium-cell," which is even more sensitive than the photographic plate, and on this the supposed dead star registers itself as very much alive. Algol is, however, interesting in another way. The pair of stars which we have discovered in it are hundreds of trillions of miles away from the earth, yet we know their masses and their distances from each other.

The Death and Birth of Stars

We have no positive knowledge of dead stars; which is not surprising when we reflect that a dead star means an invisible star! But when we see so many individual stars tending toward death, when we behold a vast population of all conceivable ages, we presume that there are many already dead. On the other hand, there is no reason to suppose that the universe as a whole is "running down." Some writers have maintained this, but their argument implies that we know a great deal more about the universe than we actually do. The scientific man does not know whether the universe is finite or infinite, temporal or eternal; and he declines to speculate where there are no facts to guide him. He knows only that the great gaseous nebulæ promise myriads of worlds in the future, and he concedes the possibility that new nebulæ may be forming in the ether of space.

The last, and not the least interesting, subject we have to notice is the birth of a "new star." This is an event which astronomers now announce every few years; and it is a far more portentous event than the reader imagines when it is reported in his daily paper. The story is much the same in all cases. We say that the star appeared in 1901, but you begin to realise the magnitude of the event when you learn that the distant "blaze" had really occurred about the time of the death of Luther! The light of the conflagration had been speeding toward us across space at 186,000 miles a second, yet it has taken nearly three centuries to reach us. To be visible at all to us at that distance the fiery outbreak must have been stupendous. If a mass of petroleum ten times the size of the earth were suddenly fired it would not be seen at such a distance. The new star had increased its light many hundredfold in a few days.

There is a considerable fascination about the speculation that in such cases we see the resurrection of a dead world, a means of renewing the population of the universe. What happens is that in some region of the sky where no star, or only a very faint star, had been registered on our charts, we almost suddenly perceive a bright star. In a few days it may rise to the highest brilliancy. By the spectroscope we learn that this distant blaze means a prodigious outpour of white-hot hydrogen at hundreds of miles a second. But the star sinks again after a few months, and we then find a nebula round it on every side. It is natural to suppose that a dead or dying sun has somehow been reconverted in whole or in part into a nebula. A few astronomers think that it may have partially collided with another star, or approached too closely to another, with the result we described on an earlier page. The general opinion now is that a faint or dead star had rushed into one of those regions of space in which there are immense stretches of nebulous matter, and been (at least in part) vaporised by the friction.

But the difficulties are considerable, and some astronomers prefer to think that the blazing star may merely have lit up a dark nebula which already existed. It is one of those problems on which speculation is most tempting but positive knowledge is still very incomplete. We may be content, even proud, that already we can take a conflagration that has occurred more than a thousand trillion miles away and analyse it positively into an outflame of glowing hydrogen gas at so many miles a second.

THE SHAPE OF OUR UNIVERSE

§ 4

Our Universe a Spiral Nebula

What is the shape of our universe, and what are its dimensions? This is a tremendous question to ask. It is like asking an intelligent insect, living on a single leaf in the midst of a great Brazilian forest, to say what is the shape and size of the forest. Yet man's ingenuity has proved equal to giving an answer even to this question, and by a method exactly similar to that which would be adopted by the insect. Suppose, for instance, that the forest was shaped as an elongated oval, and the insect lived on a tree near the centre of the oval. If the trees were approximately equally spaced from one another they would appear much denser along the length of the oval than across its width. This is the simple consideration that has guided astronomers in determining the shape of our stellar universe. There is one direction in the heavens along which the stars appear denser than in the directions at right angles to it. That direction is the direction in which we look towards the Milky Way. If we count the number of stars visible all over the heavens, we find they become more and more numerous as we approach the Milky Way. As we go farther and farther from the Milky Way the stars thin out until they reach a maximum sparseness in directions at right angles to the plane of the Milky Way. We may consider the Milky Way to form, as it were, the equator of our system, and the line at right angles to point to the north and south poles.