THE ROBE OF THE SUN

Evening Star, Issue 15907, 13 September 1915, Page 7

THE ROBE OF THE SUN

’ ' [By James Collier.]

(Special Rights Reserved by the ‘Star.’) , Some startling discoveries in the seventies on the constitution of the sun attracted the attention, especially of the English-speaking world, to the hitherto unknown name of Norman Lockyer. He was <then, if p rightly remember, a clerk in the War Office, like the organiser of our province of Canterbury, though doubtless of lower rank than J. R. Godley. Ho must soon have relinquished distasteful duties for more important tasks, and, still happily living, h© has won the homage of all the_ world for the discoveries he lias made or is making ,on the mighty luminary. Not only on “the eye of day,” but also on its eye-lashes, so to speak, nob only on the now chemical elements found in it and its sudden and amazing protuberances, hut also on the corona, or crown, on the atmosphere by which, in common with the earth and probably every other cooling celestial body, the sun is surrounded. It is to this atmosphere or robe of the sun, so recently discovered, and in whose constitution discoveries are still being made, that the attention of the reader is here invited. —Eclipses and the Atmosphere.— Mere analogy and a priori conjecture suggested, long before it was really found, that the sun would be ascertained to have an atmosphere of its own, and one of the earliest fruits of spectrum analysis, Fraunhofer’s dark lines, with, the decreasing density of the sun near its edges, proved the existence of a stratum of gas comparatively cold and chemically defined by these rays. But it is the eclipses of the sun that have revealed to us most of what we know about the Bun’s atmosphere. Usually, that atmosphere is as completely invisible as the stars by day, and for the same reasons—the splendor of the God of Day and the diffusion of Iris azure robe. A vast curtain, says Nordmann, hangs over the sky, and the brighter it is the less we sea of the source of illumination. Total eclipses of the sun have alone had the power to tear down this curtain. Twenty-four times in 58 years has it been rent, and an “immense luminous glory,” whose streamers extend for several millions of miles, bursts upon the vision. This glory has several layers, so to speak. First, all round the luminary lies a thin red-rose ring of rubylike brilliancy, which is called the chromosphere. From it leaps out the rosy flames of the protuberances. These are often some hundreds of thousands of miles in extent, and in the short duration of an eclipse they may be observed to change their positions and forms with a velocity that would be incredible were it not proved by calculation; sometimes it exceeds 50,000 miles jjer second. All round the chromosphere and its red protuberances stretches the immense coronal atmosphere, the immense green “crown,” that forms the outward atmosphere of the sun. —Spectrum of the Corona.— The study of these various strata by means of the spectroscope during total eclipses shows that both the chromosphere and its protuberances are composed of luminous hydrogen. It shows also, and the result is striking, that the green tint of the corona arises from a gas that has never yet been found on this earth; the gaa has been called coronium. Perhaps, like helium, it will yet be found in the earth’s atmosphere. Not till long after the discovery of helium in the sun’s atmosphere was it found on earth. The method that lias enabled ns to make these and other marvellous discoveries is still costly and imperfect. It- allows too little time fop observation, and the observations may be vitiated by a single cloud. The total eclipses of the sun between 1868 and 1906 number 24, and their average duration was only three minutes and six seconds. Assume that the same astronomer had been present at all of them, and ho would still have had the phenomenon under observation for only one horn’ and a-half. A first step to remedy the precarious means of observation was taken by Sir Norman Lockyer and M. Janssen. Simultaneously or conjointly they devised the celebrated method now generally used. It lias almost superseded the eclipse! 'Man, audacious man, has virtually invented eclipses for his own private" behoof 1 He is henceforth, independent of one of the grandest and most appalling phenomena of Nature. We must discriminate, however. It is only the chromosphere and its protuberances that can be independently observed. The corona cannot be thus observed. All the efforts made to e.xamine it by an independent method have completely failed. As a, consequence, we are unable to state in, what the “ glory ” that clothes as with a nimbus the surfaces of the moon and the sun consists. Still, we know that the corona, is composed of gases extremely tenuous, which hold in suspense very fine dust, and yet oO transparent, though they arc of enormous thickness, that the stars are- visible through them. —The Spectroheliograph.—

Though the corona of the sun cannot .yet be scientifically observed save during total solar eclipses the various strata of the atmosphere, which cannot be observed at all even during eclipses, because they are concealed behind the opaque disc of the moon, can now be at all times observed on the whole surface of the sun. The means of making such observations wo owe to the speetroheliograph. This ingenious instrument is the invention, at once independent and conjoint, of the American astrophysicist, Hale, of Mount Wilson Observatory, and the French astronomer Deslandres, successor to Janssen as director of the great solar observatory at Meudon. We shall follow Nordmann's account of the apparatus, which may bo founded on a very simple idea, but is not in itself very simple. At the focus of a mirror I have a fairly large image of the sim. There I set the slit of a powerful spectroscopy, adjusted to any diameter of that image—say, to the solar equator. At the other end of the spectroscope I shall have a complete spectrum, of the solar equator. Suppose, now. that I isolate a particular lins of that sp-ectrum—say, the red ray of hydrogen—and. that I cover all the rest of the spectrum by means of a screen pieieed with a fine slit which coincides with that ray. Now, if I place a photosrapbio plate, behind the screen I shall have on the plate a ray going from one end to the other of the solar equator. And the thickness and intensity of-the lay will not be uniform throughout, but these will depend on the distribution over the solar equa'.or of the masses of hydrogen whose absorption, in the atmosphere of the sun produces this ray. Next, leaving all the rest motionless, I displace the image of the sun on the slit, so as to have it swept entirely and successively by the latter. Let me,'finally, give the other end of the photographic plate a corresponding push, and I shall procure an image of the sun that arises solely from its atmospheric hydrogen. —Other Results.— What is more important still: by the agency of the speetroheliograph we can determine the vertical distribution of any chemical element, or its distribution in th-e various layers of the solar atmosphere projected on the sun's disc. Here the astrophysicists have far surpassed the chemists. We 'have no means of determining by a simple optical observation the composition of the different strata of our atmosphere taken vertically. Deslandres was, moreover, the first to obtain solar images produced bv th 6 black rays of the spectrum—a notable result. To discover the distribution of the rays of calcium in the various layers that surround the sim- is- the work of a photographer. The ends of rays H and K are tttilised far the lower strata, the central part for the highest strata, and their intermediate portion for the middle layers. The thing has not yet been done, however, we are-warned; ft is the task of the future, "though the firstseries of documents taken at Meudon and Mount Wilson have already led to remarkable result?. It has been demonstrated that the greater number of the 20,000 black rays found in the solar spectrum arise solely from the absence of an atmospheric layer that is comparatively thin, and is in immediate contact with the photosphere. The existence of this reversing .stratum, (reversing .the colors of the solar spectrum) has been confirmed by the

photograph of {is spectrum made during' eclipses. That spectrum ehxrivs as birilliaxiitr rays ail t!he rays that are' black in tlha solar spectrum. This is in perfect accordance with, Kirchhof's ideas. Farther,- our two astronomers have discovered in the solar atmosphere certain "flocculi," or cloudlets, filaments, and alignments—a whok series of strange pnenomena. presented by the various gaseous masses in motion in the solar atmosphere. These arc found to be of vast importance for the study of solar physics. Deslandres has devised a special apparatus thai instructs in the speed of the atmospheric currents of the sun—a thing not yet accomplished for our own atmosphere. These velocities are sometimes fantastic. Recently a mass of solar hydrogen has been observed to- be attracted, or to "aspire," towards a sunspot at a speed of 200,000 miles an hour —the distance being that of the moon from the earth. Such velocities, together with the mere existence of the luminescence of the soiar atmosphere and the characteristic curvature of the solax protuberances, have led astrophysicists to believe that the sum is the seat of powerful magnetic-and electric phenomena; A recent discovery, or series of discoveries, which, Nordmann calls magnificent, the outcome of hclingraphio researches, lends experimental support to the belief, It is chiefly the discovery that the' sunspots are magnetic, and the entire sun a magnetic field. —A Magnificent Discovery.— Some timo ago, examining the heliograms he had obtained -with the spectrokeliographic rays, Hale noticed t!hat the gas showed itself above the sunspots in curves that .regularly bent inwards towards the sunspots,. and also that those curves had the appearance that •would be given them by a violent maelstrom movement that had its centre in the sunspot. ' Further research proved the reality of the movement (of incandescent hydrogen), and gaseous masses were sometimes surprised, as it -were, in the violent rush towards the centre of any particular sunspot. The phenomena thus established assimilated the solar atmosphere above the sunspots to the vortices, tornadoes, and cydlones of •our own atmosphere. Out of this simple observation the American astronomer, by an ingenious combination of reasoning and experiment, has elicited one of the most beautiful and suggestive of modern astronomical discoveries. It is that of the magnetic character of the sunspots; it is the discovery of the whole sun as a magnetic field. If, reasons Mr Kale, matter rotates above a sunspot, and it is electrified, it must engender an electric current—that is, an electric field, of -which, the axis is sensibly perpendicular to the sun. This is mere reasoning, so far, but it can be proved. Hale applied to it the phenomenon discovered by the celebrated Dutch physicist, Zeemann, who followed on the track of Lorenz. The Zeemann phenomenon is an effect produced on light by magnetism, and generally by all magnetic fields; and we know, or believe, that light is produced by electrons, wliose combination makes up atoms. Availing himself of resources only to be found.in the United States, which furnished instruments of a prodigious delicapy, and making a long series of costly, ingenious experiments, MiHale at length succeeded in proving to demonstration that the sunspots constitute powerful magnetic fields, whose intensity is more than 6,000 times that which attracts the needle to the Bole. A second discovery made by tho same indefatigable savant is that the magnetic polarity of the sunspots depends on the direction in which they rotate. Lastly, and little more than a year ago, the 6ame keen observer and reasoner discovered by analytic methods that the entire surface of the sun is magnetic, like the earth itself, with its North and South Poles, and these situated, likewise, near the Poles of rotation. Deslandres has independently confirmed some of _ these results. Still further, for new and important discoveries crowd on us, the German physicist, Stark, lias proved that the rays of tho sunspots not only manifest the existence of magnetic fields, ,but show the existence of electrio fields that act on the sources of lisrht. j The vistas that these notable discoveries I open up to speculation and research appear dazzling and boundless.