Laplace had one great advantage over the early speculators. Not only was he an able astronomer and mathematician, but by his time it was known that nebulae, or vast clouds of dispersed matter, actually existed in the heavens. Here was a solid basis for the speculation. Sir William Herschel, the most assiduous explorer of the heavens, was a contemporary of Laplace. Laplace therefore took the nebula as his starting-point.
A quarter of an ounce of solid matter (say, tobacco) will fill a vast space when it is turned into smoke, and if it were not for the pressure of the atmosphere it would expand still more. Laplace imagined the billions of tons of matter which constitute our solar system similarly dispersed, converted into a fine gas, immeasurably thinner than the atmosphere. This nebula would be gradually drawn in again by gravitation, just as the dust falls to the floor of a room. The collisions of its particles as they fell toward the centre would raise its temperature and give it a rotating movement. A time would come when the centrifugal force at the outer ring of the rotating disk would equal the centripetal (or inward) pull of gravity, and this ring would be detached, still spinning round the central body. The material of the ring would slowly gather, by gravitation, round some denser area in it; the ring would become a sphere; we should have the first, and outermost, planet circling round the sun. Other rings would successively be detached, and form the rest of the planets; and the sun is the shrunken and condensed body of the nebula.
So simple and beautiful a theory of the solar system could not fail to captivate astronomers, but it is generally rejected to-day, in the precise form which Laplace gave it. What the difficulties are which it has encountered, and the modifications it must suffer, we shall see later; as well as the new theories which have largely displaced it. It will be better first to survey the universe from the evolutionary point of view. But I may observe, in passing, that the sceptical remarks one hears at times about scientific theories contradicting and superseding each other are frivolous. One great idea pervades all the theories of the evolution of worlds, and that idea is firmly established. The stars and their planets are enormous aggregations of cosmic dust, swept together and compressed by the action of gravitation. The precise nature of this cosmic dust—whether it was gas, meteorites and gas, or other particles—is open to question.
As we saw in the first chapter, the universe has the word evolution written, literally, in letters of fire across it. The stars are of all ages, from sturdy youth to decrepit age, and even to the darkness of death. We saw that this can be detected on the superficial test of colour. The colours of the stars are, it is true, an unsafe ground to build upon. The astronomer still puzzles over the gorgeous colours he finds at times, especially in double stars: the topaz and azure companions in beta Cygni, the emerald and red of alpha Herculis, the yellow and rose of eta Cassiopeiae, and so on. It is at the present time under discussion in astronomy how far these colours are objective at all, or whether, if they are real, they may not be due to causes other than temperature. Yet the significance of the three predominating colours—blue-white, yellow, and red—has been sustained by the spectroscope. It is the series of colours through which a white-hot bar of iron passes as it cools. And the spectroscope gives us good ground to conclude that the stars are cooling.
When a glowing gas (not under great pressure) is examined by the spectroscope, it yields a few vertical lines or bars of light on a dark background; when a glowing liquid or solid is examined, it gives a continuous rainbow-like stretch of colour. Some of the nebulae give the former type of spectrum, and are thus known to be masses of luminous gas; many of the nebulae and the stars have the latter type of spectrum. But the stretch of light in the spectrum of a star is crossed, vertically, by a number of dark lines, and experiment in the laboratory has taught us how to interpret these. They mean that there is some light-absorbing vapour between the source of light and the instrument. In the case of the stars they indicate the presence of an atmosphere of relatively cool vapours, and an increase in the density of that atmosphere—which is shown by a multiplication and broadening of the dark lines on the spectrum—means an increase of age, a loss of vitality, and ultimately death. So we get the descending scale of spectra. The dark lines are thinnest and least numerous in the blue stars, more numerous in the yellow, heavy and thick in the red. As the body of the star sinks in temperature dense masses of cool vapour gather about it. Its light, as we perceive it, turns yellow, then red. The next step, which the spectroscope cannot follow, will be the formation of a scum on the cooling surface, ending, after ages of struggle, in the imprisonment of the molten interior under a solid, dark crust. Let us see how our sun illustrates this theory.
It is in the yellow, or what we may call the autumnal, stage. Miss Clerke and a few others have questioned this, but the evidence is too strong to-day. The vast globe, 867,000 miles in diameter, seems to be a mass of much the same material as the earth—about forty elements have been identified in it—but at a terrific temperature. The light-giving surface is found, on the most recent calculations, to have a temperature of about 6700 degrees C. This surface is an ocean of liquid or vaporised metals, several thousand miles in depth; some think that the brilliant light comes chiefly from clouds of incandescent carbon. Overlying it is a deep layer of the vapours of the molten metals, with a temperature of about 5500 degrees C.; and to this comparatively cool and light-absorbing layer we owe the black lines of the solar spectrum. Above it is an ocean of red-hot hydrogen, and outside this again is an atmosphere stretching for some hundreds of thousands of miles into space.
The significant feature, from our point of view, is the "sun-spot"; though the spot may be an area of millions of square miles. These areas are, of course, dark only by comparison with the intense light of the rest of the disk. The darkest part of them is 5000 times brighter than the full moon. It will be seen further, on examining a photograph of the sun, that a network or veining of this dark material overspreads the entire surface at all times. There is still some difference of opinion as to the nature of these areas, but the evidence of the spectroscope has convinced most astronomers that they are masses of cooler vapour lying upon, and sinking into, the ocean of liquid fire. Round their edges, as if responding to the pressure of the more condensed mass, gigantic spurts and mountains of the white-hot matter of the sun rush upwards at a rate of fifty or a hundred miles a second, Sometimes they reach a height of a hundred, and even two hundred, thousand miles, driving the red-hot hydrogen before them in prodigious and fantastic flames. Between the black veins over the disk, also, there rise domes and columns of the liquid fire, some hundreds of miles in diameter, spreading and sinking at from five to twenty miles a second. The surface of the sun—how much more the interior!—is an appalling cauldron of incandescent matter from pole to pole. Every yard of the surface is a hundred times as intense as the open furnace of a Titanic. From the depths and from the surface of this fiery ocean, as, on a small scale, from the surface of the tropical sea, the vapours rise high into the extensive atmosphere, discharge some of their heat into space, and sink back, cooler and heavier, upon the disk.
This is a star in its yellow age, as are Capella and Arcturus and other stars. The red stars carry the story further, as we should expect. The heavier lines in their spectrum indicate more absorption of light, and tell us that the vapours are thickening about the globe; while compounds like titanium oxide make their appearance, announcing a fall of temperature. Below these, again, is a group of dark red or "carbon" stars, in which the process is carried further. Thick, broad, dark lines in the red end of the spectrum announce the appearance of compounds of carbon, and a still lower