For Our Boys & Girls

Te Aroha News, Volume VII, Issue 462, 12 April 1890, Page 3

For Our Boys & Girls

EDITED BY MRS FRANCES HODGSON BURNETT. [COPYRIGHT.] [All Rights Reserved.] The Spectroscope AS AN INSTRUMENT OF ASTRONOMICAL RESEARCH. By William Huggins, D.0.L., LL.D., F.R.S. IN TWO PARTS. PART 11. An analogy based upon our solar system had suggested with considerableprobability, before the spectroscope had a place among the instruments of an observatory, that the stars were suns more or less after the order of our sun. But there are other bodies in the heavens in respect of which analogy from our own system in its present state entirely failed us. Under favourable conditions there come forth from the darkness of the sky a multitude of faintly luminous patches and balls and wisps of feebly shining cloudy stuff, which are in strong contrast to the bright point-like stars, and are known as nebulas (little clouds). At present our catalogues contain some 10,000 of these objects, of which two or three, such as the nebula? in the girdle of Andromeda, and the nebula? in the sword-scabbard of Orion may be seen by the naked eye. Sir William Herschel, who discovered a large number of these bodies with his great telescope at Slough, suggested that they might be portions of the primordial material out of which the existing stars had been fashioned, and further that in the great variety of forms which these bodies present, we might have before us some of the stages through which suns and planets pass in their evolution from an original fiery mist. Later on, as with successive advances of telescopic power some of these bodies were resolved into clusters of closely aggregated stars, a change began to take place in astronomical opinion, and the nebulse came to be regarded as systems of stars too remote to be separately distinguishable, and in the supposed ‘ architecture of the heavens ’ galaxy of worlds was piled up behind galaxy until language, and imagination even, failed. The large telescopes which resolved some of these bodies brought into view other fainter nebula;, •often of vast extent and apparently of fantastic forms, which even the popular opinion of that time almost refused to believe to be ‘ sandheaps of stars.’ One of the most memorable epochs of the new science of Astronomical Physics was the night when for the first time the spectroscope was directed to one of these objects, and their true nature laid open to the astonished gaze of the observer. He saw not a complete spectrum, as in case of the stars, but three bright line 6 only, about the middle of the spectrum, and in these, nature's own handwriting, he read the new information that these bodies are masses of hot gas. Sir William Herschel’s guess was not far wrong. The marvellous celestial architecture of 1 galaxy behind galaxy ’ disappeared, but in it 3 place there arose before us the simple but, sublime spectacle of enormous gaseous masses, doubtless the birth-places of new suns and systems. ■ The spectroscope told us more. One element, so important to solar and terrestial chemistry that its absence would make the present state of things here, together -with human and all other life, impossible, ’namely, hydrogen, was certainly an impor tant constituent cf these hot gaseous bodies. Within a few nights of the memorable evening of August 29th, 1864, when the spectroscope revealed to man the true nature of the nebulae, the instrument was directed to many other nebulae, with the result that these bodies may be broadly divided into two classes. One class contains the nebulae which give a spectrum, which consists mainly of a few bright lines ; the nebulae of the other class give, a more complete spectrum in which the light appears sensibly continuous, though in reality crossed with bright or dark lines. It must not be supposed that the two classes of nebulae are independent orders of bodies, but that the gaseous nebulae, as it is convenient to call the first class, are probably at present in an earlier stage of evolution. These bright-lined nebulae are probably at or near the beginning of the evolutionary circle so far as we can know it. As to the conditions which have been anterior to the state of things we find in these nebulae, the spectroscope is silent. To the modern scientific mind the old philosophical idea of a primordial fiery mist no ilonger approves itself. Thermodynamics areiject the idea of the hot gas of the nebulae -'as an original condition of matter existing indefinite periods, and imperatively •demands the physical cause by which the heat and gaseiby have been brought about. -Gravitation is now regarded as the parent ■of heat. Whether the gaseous nebulae which we see have been produced by the impact.of ‘two or more cool solid bodies under the influence of gravitation alone, or whether the (gravitational force has been assisted by the original motions of the cool bodies, or indeed whether we are to look for their origin in any other direction, passes beyond the scope of this article into the domain ol pure hypothesis. There is little doubt that the nebulae which give a more complete spectrum are in a later stage of develop ment. Indeed, a photograph taken lasl year of the grand nebula in the girdle o Andromeda place? before us an advancec stage of the evolutionary process, similar tc one of the stages through which, to the theories of Kant and Laplace, oui system had to pass. The nebular hypo thesis, as well as some modern modification: of it, are not free from difficulties, but th< theory, nevertheless, without doubt re presents broadly the past history of ou system, and the condition of things nov existing in the nebula?. As we knot nothing of the parallax of the nebulae ii Andromeda, we are altogether ignorant c ithe scale of evolution going on there. T speak of this nebulae as a planetary syster tmay be to compare ‘things small wit great. ’ There are heavenly bodies of anothc order which we see from time to time as w look out from our little dwelling place int the space around us, namely, the blazin stars or comets which in the old. time wei regarded as the portents of all kinds of wo< The telescope had shown some of. thes bodies when within .the range of vision t pass rapidly under the sup’s injjuenci

from an almost invisible nebulosity, into a brilliant comet throwing its tail, many millions of miles in length, half across the heavens. Here, again, the spectroscope did not fail us. It answered a question long pending by showing that their light is mainly self-light, emitted as a. rule, mainly by carbon and hydrogen, in the same state as they exist in the blue base of an ordinary gas or candle flame. Photo* ‘graphs of the spectra of two comets make it very probable that part of their light in the visible region is due to carbon united with nitrogen. When a comet approaches very near the sun, in addition to the lines and bands due to those substances, metallic lines often make their appearance, especially those of sodium, ! magnesium, and probably iron. These gases and metals are found to exist in meteorites, with which, through the identity of the orbits of some comets of short period, with the orbits of swa ms of meteors which give us magnificent annual displays of shooting stars, comets had already been shown to be closely connected. It has been shown, too, that the substances which the spectroscope has revealed in comets are just those which would explain the principal types of their tails, if we admit a force of repulsion from the sun, probably electrical, which is a surface action only independent of mass. Then the velocity of projection would be inversely proportional to the molecular weights, each particle describing 'its own independent orbit, and reflecting solar light or giving out its own light as the case may be, but never to be regathered up again by the nucleus of the comet. We have now to consider a case in which the spectroscope has become an instrument of astronomical research, not by revealing to us the chemical nature of the body, the light of which it analyses, but by rendering visible to us objects which, without its aid, could not be seen. Our atmosphere in consequence of the scattering of the sun’s light from the finely divided matter always more or less suspended in it, becomes a luminous screen cutting off from view the fainter objects behind it, as for example the stars at midday. The sun in this way conceals an important part of itself. • When at rare intervals, for a few minutes at a time, this curtain of scattered light is lifted for us, and by the moon coming before the sun, and so screening the atmosphere from the sun’s light, a magnificent spectacle is revealed of appendages of the sun which are invisible at other times, namely the solar corona, and the bright flames, more or less red, close about the sun’s limb. It is these red flames which the spectroscope enables us to see without an eclipse. The power which the spectroscope possesses of revealing these objects to us whenever the sky is clear depends upon the circumstance that their light is resolved into a few bright lines only, while the light of the sky-glare is spread out by the prism into a complete continuous spectrum. It is obvious that the few bright lines between which the whole light of the red flame is divided may in this way become more brilliant relatively to the originally brighter sky-glare which has been very greatly weakened by being so much more sub-divided, as it must be when spread out into a continuous band extending from the red to the violet. When, therefore, the spectroscope is directed to the sun’s edge, the bright lines of any red-flames which may be present may be distinctly seen ; and further, as the sun’s limb has been brought to focus on the slit it is even possible by a small widening of the slit, which with a spectroscope of sufficient power can be done without reducing too much the brilliancy of the lines relatively to the spectrum-ground on which they are seen, to detect the shapes of the flames, and to study the rapid changes of form which take place in them. The daily observation and record of these objects has become part of the recognised routine of an observatory devoted to solar physics. Limitation of space makes it necessary to pass at once to another achievement which the spectroscope has enabled the astronomer to accomplish, and which, as the complete resolution of an apparently impossible problem, as well as by its refined delicacy, may, perhaps, be considered as one of the most remarkable recorded in the annals of science. The stars are ‘ fixed ’ to the common eye only. Telescopes provided with delicate measuring threads show that the stars are in motion relatively to each other. It is obvious that any apparent change observed in the position of star to star can show us alone the movement that is taking place transverse to the direction in which we are looking at them. It is utterly impossible that the stars are all really moving in a transverse direction only, and therefore what we can see and measure is only that component part of their true motions which at the time happens to be perpendicular to the line of sight. It is clear that we can never see directly that other component of their motions which is in the direction of vision. A bird or a train coming directly towards us, or receding directly from us, appears at rest. The great problem of the relative movements of the heavenly bodies outside the solar system seemed beyond human reach. In this case again the spectroscope made the impossible possible. The bright and dark lines of luminous bodies have absolutely fixed positions in thespeetrum, bubthefixity of these positions depends upon the etherwaves which give rise to them, being always of the same length, or in other words, recurring the same number of times in a second. Now this condition no longer holds if the luminous matter or the spectroscope is set in rapid motion approaching or receding from the other. To a musician standing at a railway station when an express with sounding whistle rushes past, the pitch of the whistle will immediately appear to fall the moment the train has passed. During the time the train was approaching a greater number of sound-waves would crowd into his ear than if the brain had been at rest, and when the train was receding from him the number of sound-waves in asecond would be fewer than that which belonged to the normal pitch of the whistle. In the former case the whistle would sound to him sharp, in the latter cases flab. A similar result takes place when a star and the earth from which the spectroscopist observes are in motion cowards or from each other. The ‘ fixed ’ lines are seen to shift towards the blue or towards the red, and from the amount of displacement, the motion of approach or of recession can be accurately measured. These measures give us at once the real motion of the stars in terrestrial units, whereas the angular measures of the motions transverse to the line of sight, can be reduced into real measures expressed in miles, in the case only of the few stars of which the distance has been previously determined by parallax. It will be seen how refined this method is, when we consider that the star motions to be measured are on an average somewhere about the ten thousandth part of the velocity of light, and can affect the positions of the lines in this small proportion only. The first measures of a star’s motions in the line of sight were made in 1868, when ' Sirius was found to be receding some 1 twenty miles a second. This method has > since been regularly carried on at Green wicb, and is now beine conducted at Pots--1 dam by means of photographed spectra instead of eye-observation. > Science belongs to no country, bub il * may perhaps be permitted to the writer tc i exjprees some satisfaction that after Kirch-

hoffs work in 1859, the more memorable achievements of the spectroscope as an instrument of astronomical research have been originally accomplished in this country, in spite of its fogs and its cloudy skies.