Through the Telescope

by James Baikie

The spectrum of the reversing layer has since been photographed on several occasions—first by Shackleton, at Novaya Zemlya, on August 9, 1896—and its bright lines have been found to be true reversals of the dark lines of the normal solar spectrum. This layer may be described as a thin mantle, perhaps 500 miles deep, of glowing metallic vapours, surrounding the whole body of the sun, and normally, strange to say, in a state of profound quiescence. Its presence was of course an integral part of Kirchhoff's theory of the mode in which the dark lines of the solar spectrum were produced. Such a covering was necessary to stop the rays whose absence makes the dark lines; and it was assumed that the rays so stopped would be seen bright, if only the splendour of the solar light could be cut off. These assumptions have therefore been verified in the most satisfactory manner.

Thus, then, the structure of the sun as now known is very different from the conception of it which would be given by mere naked-eye, or even telescopic, observation. We have first the visible bright surface, or photosphere, with its spots, faculæ, and mottling, and surrounded by a kind of atmosphere which absorbs much of its light, as is evidenced by the fact that the solar limb is much darker than the centre of the disc (Plate [V.]); next the reversing layer, consisting of an envelope of incandescent vapours, which by their absorption of the solar rays corresponding to themselves give rise to the dark lines in the spectrum. Beyond these again lies the chromosphere, rising into gigantic eruptive or cloud-like forms in the prominences; and yet further out the strange enigmatic corona.

It must be confessed that the reversing layer, the chromosphere, and the corona lie somewhat beyond the bounds and purpose of this volume; but without mention of them any account of the sun is hopelessly incomplete, and it is not at all improbable that a few years may see the spectroscope so brought within the reach of ordinary observers as to enable them in great measure to realize for themselves the facts connected with the complex structure of the sun. In any case, the mere recital of these facts is fitted to convey to the mind a sense of the utter inadequacy of our ordinary conceptions of that great body which governs the motions of our earth, and supplies to it and to the other planets of our system life and heat, light and guidance. With the unaided eye we view the sun as a small tranquil white disc; the telescope reveals to us that it is a vast globe convulsed by storms which involve the upheaval or submersion, within a few hours, of areas far greater than our own world; the spectroscope or the total eclipse adds to this revelation the further conception of a sweltering ocean of flame surrounding the whole solar surface, and rising in great jets of fire which would dissolve our whole earth as a drop of wax is melted in the flame of a candle; while beyond that again the mysterious corona stretches through unknown millions of miles its streamers of silvery light—the great enigma of solar physics. Other bodies in the universe present us with pictures of beautiful symmetry and vast size: some even within our own system suggest by their appearance the presence within their frame of tremendous forces which are still actively moulding them; but the sun gives us the most stupendous demonstration of living force that the mind of man can apprehend. Of course there are many stars which are known to be suns on which processes similar to those we have been considering are being carried on on a yet vaster scale; but the nearness of our sun brings the tremendous energy of these processes home to us in a way that impresses the mind with a sense almost of fear.

'Is it possible,' says Professor Newcomb, 'to convey to the mind any adequate conception of the scale on which natural operations are here carried on? If we call the chromosphere an ocean of fire, we must remember that it is an ocean hotter than the fiercest furnace, and as deep as the Atlantic is broad. If we call its movements hurricanes, we must remember that our hurricanes blow only about 100 miles an hour, while those of the chromosphere blow as far in a single second. They are such hurricanes as, coming down upon us from the north, would, in thirty seconds after they had crossed the St. Lawrence, be in the Gulf of Mexico, carrying with them the whole surface of the continent in a mass not simply of ruin, but of glowing vapour.... When we speak of eruptions, we call to mind Vesuvius burying the surrounding cities in lava; but the solar eruptions, thrown 50,000 miles high, would engulf the whole earth, and dissolve every organized being on its surface in a moment. When the mediæval poets sang, "Dies iræ, dies illa, solvet sæclum in favilla," they gave rein to their wildest imagination without reaching any conception of the magnitude or fierceness of the flames around the sun.'

The subject of the maintenance of the sun's light and heat is one that scarcely falls within our scope, and only a few words can be devoted to it. It is practically impossible for us to attain to any adequate conception of the enormous amount of both which is continually being radiated into space. Our own earth intercepts less than the two thousand millionth part of the solar energy. It has been estimated that if a column of ice 2¼ miles in diameter could be erected to span the huge interval of 92,700,000 miles between the earth and the sun, and if the sun could concentrate the whole of his heat upon it, this gigantic pillar of ice would be dissolved in a single second; in seven more it would be vaporized. The amount of heat developed on each square foot of solar surface is 'equivalent to the continuous evolution of about 10,000 horse-power'; or, as otherwise stated, is equal to that which would be produced by the hourly burning of nine-tenths of a ton of anthracite coal on the same area of 1 square foot.

It is evident, therefore, that mere burning cannot be the source of supply. Lord Kelvin has shown that the sun, if composed of solid coal, would burn itself out in about 6,000 years.

Another source of heat may be sought in the downfall of meteoric bodies upon the solar surface; and it has been calculated that the inrush of all the planets of our system would suffice to maintain the present energy for 45,604 years. But to suppose the existence near the sun of anything like the amount of meteoric matter necessary to account, on this theory, for the annual emission of heat involves consequences which are quite at variance with observed facts, though it is possible, or even practically certain, that a small proportion of the solar energy is derived from this source.

We are therefore driven back upon the source afforded by the slow contraction of the sun. If this contraction happens, as it must, an enormous amount of heat must be developed by the process, so much so that Helmholtz has shown that an annual contraction of 250 feet would account for the total present emission. This contraction is so slow that about 9,500 years would need to elapse before it became measurable with anything like certainty. In the meantime, then, we may assume as a working hypothesis that the light and heat of the central body of our system are maintained, speaking generally, by his steady contraction. Of course this process cannot have gone on, and cannot go on, indefinitely; but as the best authorities have hitherto regarded the date when the sun shall have shrunk so far as to be no longer able to support life on the earth as distant from us by some ten million years, and as the latest investigations on the subject, those of Dr. See, point in the direction of a very large extension of this limit, we may have reasonable comfort in the conviction that the sun will last our time.