So far we have been concerned only with the smaller of the objects in space. Smallest of all were the pellets of matter which we describe as shooting-stars when they fall into the earth's atmosphere; these are so small that we could hold thousands of them in each hand. The largest object we have discussed so far has been the giant planet Jupiter, with about eleven times the diameter of the earth. A box big enough to hold Jupiter would hold II × II × II or 1331 earths—eleven each way. Yet even Jupiter is quite small in comparison with the sun, which we shall examine in the present chapter, and the sun is smaller still in comparison with the larger stars and other objects that we shall examine subsequently. Broadly speaking, the sun is as much bigger than Jupiter as Jupiter is bigger than the earth—Jupiter could contain more than a thousand earths, but the sun could contain more than a thousand Jupiters. To carry on the sequence, each of the blue stars we shall consider later could contain more than a thousand suns, while each of the “giant red” stars could contain more than a thousand blue stars. And each of certain nebulae which we shall discuss in our last chapter of all not only could contain, but actually does contain, thousands of millions of stars.

In giving a course of recent wireless talks, I assumed that my listeners had no previous scientific knowledge of any kind, and tried to introduce them to the fascination of modern astronomy and to the wonder of the universe we see through the giant telescopes of to-day.

The present book contains these talks expanded to double their original length, still in the informal conversational style and simple non-technical language of wireless talks. It is totally unambitious, aiming only at providing an easy, readable and not over-serious introduction to the most poetical of the sciences.

We have seen how the stars shew as great a range of candle-power as there is between a glow-worm and a searchlight; while their range of size is as that between a speck of dust and a motor-car. The range in their weights is much smaller, but still it is about equal to that between a feather and a football. And in every respect the sun is somewhere about average. It could hardly be expected to strike the exact happy mean in every way, but it never misses it badly. To put the same thing in another and less complimentary way, the sun is totally undistinguished in all respects—in weight, in size, in temperature and in candle-power.

Clearly, however, we get very little knowledge of the general nature of the stars from a mere mention of extremes and of one average star. We should not know much about the English population if we had only been told the heights and weights of the shortest dwarf and the tallest man, and that a particular man 5 feet 9 inches high was a good average Englishman in all respects. We want a more detailed knowledge as to the classification of the stars by size, candle-power and weight.

Suppose that all the entrants to a dog-show broke loose and ate their labels, and had to be reclassified.

We have already seen how important the force of gravitation is, both to astronomy and ourselves. It keeps the moon tied to the earth, and maps out the paths of all the planets and other members of the sun's family; it raises the tides in our oceans, and, we believe, raised those far greater tides in the sun which, some 2000 million years ago, brought our earth, and so ultimately ourselves, into being. Finally, it keeps us alive, by making the earth stay near the sun instead of running away into the icy depths of space.

Let us try to understand a little more as to what this force is.

The Force of Gravitation

No man can lift a ton weight; he is prevented by the force of gravitation—or gravity, as we usually call it when it acts on earth. This pulls the weight to the ground, and proves too strong for him.

Again, we find it impossible to throw a cricket ball for a mile; we are prevented by the same force, which continually pulls the ball towards the ground, and invariably succeeds in getting it down before it has travelled a mile. We can easily throw the ball out of our hands at twenty miles an hour, and if gravity did not draw it earthward, it would cover a mile every three minutes, and after a year it would be far out in space, 175,000 miles away from the earth.

A century ago astronomy was concerned with little beyond the sun, moon and planets—the small colony we have described as the sun's family. To-day it is mainly engaged in studying in detail the various other stars and colonies of stars, such as the three stars which form the system of Alpha Centauri, our nearest neighbours in space. The aggregate of all such stars and colonies constitutes the Galactic System, the vast conglomeration of stars whose rim is the Milky Way. At the same time, astronomy has discovered that even this huge system is only one of a vast number of somewhat similar systems. The present situation may be perhaps summed up in the three statements:

1. (1) The earth is only one member of the sun's family.

2. (2) The sun's family is only one member of the Galactic System.

3. (3) The Galactic System is only one member of the system of star-cities in space.

This is the furthest that astronomy has travelled so far, but we may well wonder what the situation will be, say, a thousand years hence. Will the above three statements still suffice, or will they have been supplemented by more statements of the same kind? In other words, shall we find that the whole system of star-cities only forms one unit in a still vaster assembly, and this assembly perchance a mere unit in something vaster still?

We cannot ourselves go and find out what the sun, moon or stars consist of, but our huge telescopes will, in a sense, bring them near to us, which comes to much the same thing. Thus the whole of space lies open for our exploration, at any rate until we are confronted by opaque substances which no telescope can penetrate. Even then the calculations of the mathematician are ready to carry on the story; for instance, quite a lot of work has been done in recent years on the constitution of the interiors of the stars. Telescopic observation and mathematical theory between them furnish us with a sort of magic rocket which will take us almost anywhere in space we desire to go.

Out in Space

Let us enter this magic rocket and persuade someone to shoot us towards the sun. We need only start with speed enough to carry us a short distance away from the earth—about 7 miles a second will do—and the sun's huge gravitational pull will do the rest. It will drag us down into the sun whether we like it or not. If we start at 7 miles a second, the whole journey will take about ten weeks.

Even in the first few seconds of our flight, we notice strange changes; the whole colour-scheme of the universe alters with startling suddenness.

Now that our rocket has brought us safely back to the earth of to-day, let us consider in more detail the small colony, almost completely isolated in space, which we believe to be the shattered fragments of what was once an ordinary star. It contains a great variety of objects, large, medium-sized, small and very small, which we must discuss in turn.

The Nine Planets

First let us look at its largest members, the nine principal planets. These move round the sun in almost circular paths, rather like circus horses trotting or galloping round the ring-master. They all go round in the same direction, and this must of course be the direction in which the wandering star, which brought them into being, moved round the sun. Because of the way it came into existence, the solar system has only one-way traffic—like Piccadilly Circus. The traffic nearest the centre moves fastest; that further out more slowly, while that at the extreme edge merely crawls—at least by comparison with the fast traffic near the centre. It is true that even the furthest and slowest of the planets covers nearly three miles every second, which is about 200 times the speed of an express train, but this is a mere crawl in astronomy. The planets Mercury and Venus, which constitute the fast traffic near the centre, move, the former ten and the latter seven, times as fast.

We inhabitants of the earth enjoy a piece of good fortune to which we give very little thought, which, indeed, we take almost as much for granted as the air we breathe—I mean the fact that we have a transparent atmosphere. Some of the other planets, for instance Venus and Jupiter, have atmospheres which are so thick with clouds as to be totally opaque. If we had been born on Venus or Jupiter, we should have lived our lives without ever seeing through the clouds, and so should have known nothing of the beauty and poetry of the night sky, and nothing of the intellectual excitement and joy of trying to decipher the meaning of the vast panorama of lights which are scattered round us in all directions in space.

It will not form a bad approach to our subject, if we imagine that until to-night our earth had also been covered in by an opaque blanket of clouds. Suddenly this is rolled back, and we see the glory, and the tantalising puzzle, of the night-sky for the first time.

Our first impression would probably be that the stars were some sort of illumination of lamps or lanterns suspended above our heads, perhaps at only a few miles, or even yards, distance—rather like the lights in the roof of a vast tent or hall.

We have seen how, when little was known about astronomy, it was natural to imagine that the stars went on for ever and ever, so that, however far one probed into space, one merely came to more stars. It was only like the town-bred child imagining that the lamp-posts go on for ever and ever. Yet we now know that if we go far enough out into space, we come to regions where the stars first begin to thin out, and then disappear altogether: we are now out in the depths of space beyond the Milky Way. The stars are like the lights of a vast city, but no city, however great, extends for ever, and if we go far enough, we get out of the city, and come at last to the dark open country beyond.

Yet this is not the whole story. We now know that the wheel-shaped system of stars bounded by the Milky Way is not the only system of stars in space. Far beyond the Milky Way are other cities, each with its own system of lights. The dark open country which surrounds our own city is not the end of everything; if we persevere through it for long enough, we shall come in time to another city whose lights are stars similar to those surrounding our sun. Let me explain the evidence for this statement.

When we discussed the face of the sky in our first chapter, the stars were nothing to us but a distant background of points of light. This background enabled us to fix our bearings in space, and we saw how we could pick out our near neighbours, the planets and other members of the sun's family, by their rapid motion against it.

Since then we have examined what the stars really are, and have discussed their various physical characteristics. Amongst other things, we have found that they shew great variety in their candle-powers. While some are thousands of times more luminous than our sun, others are thousands of times fainter. If we compare our sun to an ordinary candle, some stars must be compared to searchlights, and some, at the other end of the scale, to glow-worms or fireflies.

It has only recently been discovered how great a range there is in the candle-powers of the stars. For a long time it was supposed that the stars all had pretty much the same intrinsic brightness—like a row of street lamps—so that when a star looked very dim, it was only because it was very distant. In 1761 the astronomer Lambert argued that as all the stars had been made to serve the same purpose, there was no reason why some should have been made fainter than others; if some appeared fainter it could only be that they were more distant.