1902 Encyclopedia > Sun
SUN. In the article ASTRONOMY (vol. ii. p. 768 sq.) the sun has been considered as a member of the solar system, and references are given to various discoveries which have been made from time to time relating to its physical and chemical constitution. In the present article we propose to consider the sun as a star, and to state as briefly as may be the views at present held regarding its structure, and subsequently to refer to the most recent observations dealing with the physics and chemistry of the various phenomena which are open to our study.
The sun as ordinarily visible to us, bounded by the photosphere, is only a small part of the real sun : from observations made during eclipses it is now known that outside the photosphere arefirst, an envelope, namely the chromosphere, which is mainly composed of hydrogen, and outside this another envelope, called the corona, while there is evidence that outside these, and especially along the plane of the sun's equator, there is a considerable extension of matter which may or may not be of the same nature as that of which the corona is composed.
These various parts of the solar economy have been examined by the spectroscope, and from this examination two widely divergent views have arisen.
Extent of the Sun's Atmosphere
According to the first view, the true atmosphere of the sun is limited by the chromosphere, and the constituents of that atmosphere consist essentially of the vapours of the chemical elements recognized on the earth. It will be seen that on this view the corona and the equatorial extension observed occasionally are merely solar appendages. In the other view the atmosphere of the sun is extended to the confines of the corona, the temperature naturally increasing as we descend; and it is held that towards the photosphere the temperature is so high that the chemical elements are dissociated into finer forms of matter, so that descending vapours get more simple, ascending vapours get more complex, and it is only in the cooler regions of the atmosphere that vapours resembling those of our ter-restrial elements can exist, while near the confines of the corona these vapours give place to solid particles and masses. Broadly stated, these divergent views have arisen from the application of two distinct methods of inquiry. In one method, light coming from every portion of the sun, and reflected, let us say, by a cloud into the spectroscope, gives us a spectrum full of absorption lines, and these lines are practically constant from year to year. In the other method, each minute portion of the solar economy has been examined bit by bit, and thus we have the spectrum of the spots, the spectrum of the promi-nences, the spectrum of the chromosphere, the spectrum of the corona. All these spectra vary enormously, not only among themselves, but from year to year; and, when we consider merely the spots and prominences, we may say that they vary from spot to spot and from prominence to prominence.
It will be obvious that the true mean density of the sun cannot be the same on the two hypotheses to which we have referred. If the atmosphere is practically limited by the photosphere, it has been found that the density of the sun is T444, water being taken as unity. If we include the corona in the sun's atmosphere, and assume that its height is half a million of miles above the photo-sphere, then the volume of the sun is ten times that bounded by the photosphere, and the density is reduced to a tenth of the value given above.
We next proceed to discuss the chemical results obtained by the first method of inquiry to which reference has been made. For these results we are of course dependent upon comparisons of the lines given by various incandescent vapours with the Fraunhofer lines seen in the ordinary spectrum of the sun. If by such means complete evidence is afforded of the existence of one of our chemical elements in the sun, it is obvious that no information is given as to its precise locality; further, if the high temperatures used in our laboratories to produce a spectrum should break up the molecules of the vapours as known to the chemist into finer ones, and if the temperature of the sun were to do the same, there would still be a considerable similarity between the solar and the terrestrial spectrum of any one substance.
Chemical Constitution of Atmosphere
The first (A) of the following tables gives the substances present in the sun's atmosphere according to (1) Kirchhoff, and (2) Angstrom and Thalen.
A subsequent method of inquiry, which was capable of tracing merely a small quantity, gave the additional substances shown in Table B.
When we come to bring the chemical evidence together which has been acquired by the examination of separate parts of the solar economy, we find, as has been already hinted, that the apparent similarity in chemical structure suggested by the foregoing tables entirely breaks down. Not only is the chemical nature of each separate solar phenomenon different from that of any other, but the facts of observation are in all cases entirely new and strange, so that very little light is obtained towards the understanding of them from ordinary laboratory work.
We will consider the chemistry of the chief solar features in order.
Chemistry of the Constituent Parts
Spectra of Spots
The spectrum of the spots differs from that of the ordinary surface of the sun chiefly by the widening of certain of the Fraunhofer lines in the spot spectrum,' some being excessively widened. The lines which are most widened change from spot to spot and from year to year. The most extensive sun-spot observations of this nature have been carried on in Kensington, and the conclusions derived from 700 observations on spots be-tween 1879 and 1885 are as follows :
(1) The spot spectra are very unlike the ordinary spectrum of the sun: some Fraunhofer lines are omitted; new lines appear; and the intensities of the old lines are changed.
(2) Only very few lines, comparatively speaking, of each chemical element, even of those which have many among the Fraunhofer lines, were seen to be most widened. It was as if on a piano only a few notes were played over and over again, always producing a different tune.
(3) An immense variation from spot to spot was observed be-tween the most widened lines seen in the first hundred observa-tions. Change of quality or density will not account for this variation. To investigate this point the individual observations of lines seen in the spectrum of iron were plotted out on strips of paper, and an attempt made to arrange them in order, but without success, for, even when the observations were divided into six groups, about half of them were left outstanding.
(4) If we consider the lines of any one substance, there is as much inversion between them as between the lines of any two metals. By the term "inversion" is meant that of any three lines, A, B, C, we may get A and B without C, A and C without B, B and C without A.
(5) Very few lines are strongly affected at the same time in the same spot, although a great many lines of the same substance may be affected, besides the twelve recorded as most widened on each day.
(6) Many of the lines seen in the spots are visible at low temper-atures (some in the oxy-hydrogen flame), and none are brightened or intensified when we pass from the temperature of the electric arc to that of the electric spark.
(7) Certain lines of a substance have indicated rest, while other adjacent lines seen in the spectrum of the same substance in the same field of view have shown change of wave-length.
(8) A large number of the lines seen in spots are common to two or more substances with the dispersion employed.
(9) The lines of iron, cobalt, chromium, manganese, titanium, calcium, and nickel seen in the spectra of spots are usually coinci-dent with lines in the spectra of other metals with the dispersion employed, whilst the lines of tungsten, copper, and zinc seen in spots are not coincident with lines in other spectra.
(10) The lines of iron, manganese, zinc, and titanium most fre-quently seen in spots are different from those most frequently seen in flames, whilst in cobalt, chromium, and calcium the lines seen in spots are the same as those seen in flames.
(11) Towards the end of the first series of investigations there appeared among the most widened lines a few wdiich are not re-presented, so far as is known, among the lines seen in the spectra of terrestrial elements. This change took place when there was a marked increase in the solar activity.
(12) The most widened lines in sun spots change with the sun-spot period.
(13) At and slightly after the minimum the lines are chiefly known lines of the various metals.
(14) At and slightly after the maximum the lines are chiefly of unknown origin.
(15) On the hypothesis under discussion the change indicates an increased temperature in the spots at the sun-spot maximum.
The general result is that in passing from minimum to maximum the lines most affected change from those of the ordinary chemical elements to lines whose significance are not known. The accompanying diagram represents graphically
the disappearance of the lines of iron, nickel, and titanium and the simultaneous appearance of unknown lines in the spot spectra in passing from minimum to maximum. In the region of the spectrum for which the curves are drawn six lines were recorded in each observa-tion, and therefore 600 in each series of 100 observations. In the curves the vertical ordinates represent not merely the number of individual lines recorded but the number of occurrences of lines of each substance. The dotted curve shows the variation in the frequency of the iron lines; at the minimum in 1879 practically all the 600 lines observed were iron lines; towards the end of 1881 they had dwindled down to 30; and during the three following years they fell to 10. The dot and dash curve shows a similar variation in the nickel lines, and the double line curve that of the titanium lines during the same periods. The continuous curve shows the gradual increase in the number of occurrences of unknown lines in passing from the minimum in 1879 to the maximum in 1884.
The chromosphere when quite quiescent merely gives us a spectrum of hydrogen together with a line in the yellow, which, from its proximity to D1 and D2, is called D3. The chromosphere is disturbed in two ways,first, by prominences, of which more hereafter, and second, by the formation gradually and peacefully of domes, which are of no great height but sometimes extend over large areas and last for weeks. These last-named phenomena have been termed " wellings up," the idea being that they were produced by the gradual uprise of vapours from below; but it is clear that the same phenomena might be produced by the very slow descent of matter from above. The spectrum of these higher portions of the chromo-sphere, whether produced from below or above, is more complicated than the ordinary one. The following table (C) gives the principal lines which have been recorded up to 1887 :
The first new line in this table is called in spectroscopic language 1474, because when this work was begun the only maps available were those made by Professor Kirch-hoff, and this particular line fell at 1474 on his scale. Since then these artificial scales have been discarded in favour of the natural one, which is given by the wavelengths of light of different colours. In this the reference number of the same line is 5315'9, which represents the wave-length in ten-millionths of a millimetre of that particular quality of light. After this we observe three lines of magnesium, only 3 out of 7 ; next a line of nickel, one ontyi however, out of 34; then two lines of sodium, although we might naturally expect to get all the 8 lines; then two lines of barium out of 26 ; and so on. Almost all the other lines have origins which are absolutely un-known : that is to say, we never get them in our terrestrial laboratories, and never, therefore, are able to match the bright lines in the chromosphere of the sun with any chemical substance. In 1871 the sun was more active, and this activity resulted in the addition of new lines, all, however, absolutely unknown to us, except one, which represents a line in the spectrum of titanium; but in that case we get one line out of 201 in exactly the same way as we get two only of iron out of 460. It is most important to note that practically none of the lines shown in table C are among those which are widened in spots.
The prominences are of two kinds-those which are nences. relatively quiet and give almost exclusively the lines of hydrogen and those in which the motions are as a rule very violent. The spectrum of the latter class generally includes a large number of metallic lines ; hence they are generally called metallic prominences. The first stage of metallic prominence is usually the appearance of three lines of the following wave-lengths4943, 503d, 5315'9. As the prominence increases in magnitude and violence other lines are added, until at times the spectrum seems full of lines. The rate of uprush of these prominences sometimes reaches 250 miles per second, or nearly a million miles an hour, figures which convey an idea of the enormous energies involved. The lines seen in these pro-minences, although many are present in the spectra of the metallic elements, appear with greatly changed intensities : the lines seen brightest in the prominences are frequently dim lines in the terrestrial spectrum. Again it may be remarked that these are not the lines which are most widened in spots. In the case of the spectrum of any one substance the number of lines seen usually in the promi-nences is very small.
The general conclusions which have been derived from a discussion of the prominence observations made by Profs. Tacchini and Ricco, in connexion with the sun-spot observa-tions already mentioned, are as follows.
(1) The chromospheric and prominence spectrum of any one, substance, except in the case of hydrogen, is unlike the ordinary spectrum of the substance. For instance, we get two lines of iron out of 460.
(2) There are inversions of lines in the same elements in the prominences, as there are inversions in the spots: in certain pro-minences we see certain lines of a substance without others ; in certain other prominences we see the other lines without the first.
(3) Yery few lines are strongly affected at once, as a rule, and a very small proportion altogether,smaller than in the case of spots.
(4) The prominences are less subject to sudden changes than spots, so far as lines of the same element are concerned.
(5) There is a change in the lines affected according to the sun's spot period.
(6) The lines of a substance seen in the prominences are those wdiich in our laboratories become considerably brightened when we change the arc spectrum for the spark spectrum.
(7) None of the iron lines ordinarily visible in prominences are seen at the temperature of the oxy-hydrogen flame. Some of the oxy-hydrogen flame lines are seen in the spots, but none have ever been seen in the prominences.
(8) A relatively large number of the lines ordinarily seen are of unknown origin.
(9) Many of the lines seen are not ordinarily seen amongst the Fraunhofer lines. Some are bright lines.
(10) As in the spots the H and K lines of calcium in the ultra-violet are always bright in the spot spectrum, the other lines of calcium and the other substances being darkened and widened, so it would appear that the lines H and K of calcium are always bright in the prominences in which the other lines are generally unaffected.
(11) Many of the lines are common to two or more elements with the dispersion which has been employed.
The spectrum of the inner corona indicates that it is chiefly composed of hydrogen. All the hydrogen lines are seen in it, and up to a certain height the H and K lines of calcium, proving the presence either of calcium or of something that exists in calcium which we cannot get at in our temperature.
In the outer corona most of the hydrogen lines dis-appear ; but one, the green line F, remains for a consider-able height side by side with the 1474 line, indicating, as far as we can see where everything is so doubtful, that the constituents of the outer corona consist most probably of hydrogen in a cool form and the unknown stuff which gives the 1474 line. We also know that the outer corona contains particles which reflect the ordinary sunlight to us, because in 1871 Dr Janssen, and in 1878 Professor Barker and others, saw the dark Fraunhofer lines in the spectrum of the corona. We must imagine, therefore, that some part of that spectrum depends for its existence on solid particles which not only give a spectrum like that of the lime-light but have the faculty of reflecting to us the light of the underlying photosphere. It was also put beyond all question in the eclipse of 1882 in Egypt that this corona has another spectrum of its own. There are bright bands in the spectrum, showing that with these additions it is not a truly continuous spectrum like that of the lime-light, and that its origin is therefore in all probability very complex.
Association and Distribution of Phenomena
Connexion of Phenomena
Observations of prominences, spots, and other phenomena which require continuous investigation have been carefully made from day to day for several years, and one conclusion arrived at is that when and where the (disturbed) spots are at the maximum the faculae and metallic prominences are also at the maximum. When the maximum changes from north to soutli latitude in the spots it also changes from north to south in the metallic
prominences and the facute. These observations, there-fore, establish not only an important connexion between spots, metallic prominences, and facuk* but also the fact of the wonderful localization of these phenomena upon the sun. The spots are never seen higher in latitude than 40° north or south, and they are invariably seen in smaller quantity at the equator. Similarly, the faculoe and metal-lic prominences do not go much beyond 40° north or south, and their minima are also at the equator. But this does not hold good for prominences of the quiet sort and the veiled spots,that is, spots without umbrae or very highly developed penumbrae. They extend from one pole of the sun to the other; hence there must exist a great difference between metallic and quiet prominences and between dis-turbed and veiled spots.
Association of Localized Phenomena
Although the more important of these solar phenomena are limited to certain zones of the sun's surface, and although they vary very violently, they have a cycle or regular succession of changes, during which the particular zone of the sun on which they appear alters. When there is the smallest number of spots on the sunthat is to say, when there is a sun-spot minimumthe spots that appear are seen in a high latitude, and the latitude decreases gradually until we arrive at the next minimum. Thus there are two perfectly distinct spotted areas, one corresponding to the end of the old period, the other to the beginning of the new period. At the maximum period of sun spots the latitude of the spot zone is about 15°. Activity in the solar atmosphere, therefore, appears to begin in a high latitudesay about 30° or 35°and very soon reaches the maximum in about latitude 15°; then it gradually dies away until spots, metallic prominences, and faculseall of reduced intensitycling pretty near to the solar equator, and at the same time we get a new wave of activity, beginning again in a high latitude. This asso-ciation of what may be called localized phenomena is quite in harmony with a similar association of phenomena which are more or less generally distributed over the whole sur-face of the sun.
Pores, which are in reality nothing but small sun spots, may occur in any part of the sun, and are always accom-panied by a slight waviness in the chromosphere. Veiled spotsspots which never attain full developmentare also universally distributed over the sun's surface and are accompanied by small prominences (see below).
Period of Solar Quietude
The main periodicity on the sun is that of about eleven years which elapses between two successive maxima or minima. When the sun is quietest, there are very few of the ordinary tree-like prominences visible, and there is an especial dearth of them near the poles and the equator. There are faculas, but they do not present their usual bright appearance, and are confined to the regions between latitudes 20° N. and 20° S. On examining the chemical nature of the materials in the chromosphere at such a period by means of a spectroscope, we see only the four lines of hydrogen and the line D3, whose chemical signifi-cance we do not know. Practically speaking, there are no spots visible and the disk appears to be perfectly pure, except the darkening towards the limb produced by absorption in the sun's atmosphere. As there are no spots, or only very small ones in high latitudes, it follows that there are no metallic prominences. The spectroscope searching right round the limb of the sun gathers no indications of violent actionno region giving many lines nothing but the simple spectrum of hydrogen. Obser-vations and photographs of the corona taken at solar eclipses occurring at minimum spot periods indicate that at two different sun-spot minima the appearances pre-sented by the corona are very much alike. A drawing made during the eclipse of 1867, before the application of photography to solar investigations, exhibits a similar appearance to an absolutely trustworthy photograph ob-tained at the eclipse of 1878. At the minimum period the chief feature is a very great extension of the corona in the direction of the solar equator, and a wonderfully exquisite outcurving right and left at both poles. It is probable that the equatorial extension pictured in the above-mentioned photograph is, after all, only a part of a much more extended phenomenon, one going to almost incredible distances from the sun itself. At the eclipse of 1878 precaution was taken to shield the eye of the observer from the intense light of the inner corona, which is sometimes so bright as to be mistaken for the sun's limb, by erecting a screen which covered the moon and a space 12' high around it. The observer, Professor New-comb, saw on both sides of the dark moon a tremendous extension of the sun's equator, far greater than that re-corded in the photographs taken at the same time. But the extended portions may have been so delicately illu-minated that they could not impress their image on the photographic plate during the time it was exposed, or that the light itself is poor in chemically active rays. The extension, as observed by the shielded eye, amounted to six or seven times the diameter of the dark moon. In a more favourable situation the same extension, but to a less extent, was observed without the aid of a screen. At a sun-spot minimum, therefore, there exists a great equa-torial extension of the corona east and west.
Transition to Maximum Period
The time between the minimum and the maximum sun-spot periods is three or four years, and that from maxi-mum to minimum seven or eight years, so that the sun increases in activity much more rapidly than it afterwards period decreases in passing to the next minimum. Starting, then, about half way between minimum and maximum, we find an increased activity in every direction. The quiet prominences, consisting of hydrogen, are more numerous, and the faculae are brighter. If at this time we examine the spectrum of the chromosphere, we find hydrogen and D3 are not the only constituents: we get other short lines, the chief being the three lines of magnesium b1, b2, b4. The spots are more numerous and are in a lower latitude, having moved from near 35° to about 25°. Metallic prominences now constantly accompany the spots; and the number of bright lines visible in their spectra gradually increases from month to month. These changes are accompanied by changes in the corona, whicli affect not only its form but also its spectrum. At the minimum spot period the corona gives an almost continu-ous spectrum, differing only in the presence of a few dark lines, and occasionally a few not very obvious bright lines, whence we conclude that at the minimum the corona is not entirely gaseous. In passing from the minimum to the maximum the spectrum is no longer continuous: bright lines begin to appear, emanating from the incandescent gaseous portions of the corona, and at the same time there is an increase in brilliancy. At this period there is no longer any remarkable equatorial extension, although here and there streamers of strange outlines occur. A drawing of the eclipse of 1858, a period between minimum and maximum, shows in middle latitudes, both north and south, four remarkable luminous cones standing with their bases on the chromosphere. The amount of light and structure in the corona has increased to such an extent that the beautiful double curves seen at the poles at the minimum are now hidden in a strong radiance.
During the maximum period all the solar forces are Maxi-doing their utmost, and we see in prominences and spots, »num and indeed in every outcome of action that we can refer Perl0cL to, indications of the most gigantic energies being at work. The ordinary prominences, instead of clinging to the equator, now occur most frequently at the poles. The faculae are brighter and are more widely distributed, and the chromosphere is richer in lines. The spots at this period occupy broad zones with mean latitudes of about 18" N. and 18° S. There are no spots near the poles and none near the equator; but large spots, indicating a state of violent agitation, surrounded by gigantic faculae, follow each other in these zones. Each of these indicators of solar activity is accompanied by a prominence. At this time also we note the greatest velocities of down-rush in the vapours which form the spots and of up-rush in those which form the prominences. These changes are accom-panied by corresponding changes in the corona; and, fortunately, we have photographic records for two periods of maximum,1871 and 1882. In these the streamers, instead of being limited to the equator or to mid-latitudes, exist in all latitudes, so that they practically extend to every part of the sun. Their directions, which may be called lines of force, are very varied, some being straight and some curved; but it is difficult to unravel the appear-ances, because what we see are only projections of the actual things, and this is especially the case when the sun's pole is tipped towards or away from the earth to the greatest extent. In the eclipse of 1882 the corona in-dicated a more equal distribution of action than that of 1871, but the general result was the same.
After the maximum period there is a gradual falling off of all the various energies, the mean latitudes of the spots decreasing until they reach 8° N. and 8° S.; then another series of spots breaks out about 35° N. and 35° S. lat., and the cycle begins anew.
It has been very generally accepted for some time that theory, sun-spots are depressions in the photosphere, produced by downfalls of cool material. The following sketch shows how, if we accept this view and also the hypothesis that the chemical elements are dissociated in the lower parts of the solar atmosphere, many of the more important solar phenomena may be explained and correlated.
We know that small meteorites in our own cold atmosphere are heated to incandescence by friction, that is, by the conversion of their kinetic energy into heat, and it is therefore not difficult to imagine that enormous masses, falling with great velocities through the sun's highly heated atmosphere, would be competent to give rise to such disturbances as those with which we are familiar on the sun's surface. This cool material is produced by the condensa-tion, in the upper cool regions of the sun's atmosphere, of the hot ascending vapours produced at the lower levels, and this is probably the main source of supply of spot-producing material. The faculse and other disturbances of the general surface do not precede but follow the formation of a spot, so that a spot may be considered as the initial disturbance of the photosphere in the region where it is observed. Large spots almost invariably appear first as little dots, frequently in groups, and then suddenly grow large. The little dots, according to the view of spot forma-tion now under discussion, are formed by small masses which precede the main fall. The heat produced by friction with the atmosphere and the arrested motion causes up-rushes of heated vapours, which eventually cool and con-dense, and afterwards fall to the photosphere and produce fresh disturbances. Down-rushes of cool material must take place all over the sun's surface, and, although the most violent results of such falls are restricted to certain regions, minor disturbances are distributed over the whole surface. These generally distributed phenomena are well known to be merely different degrees of the same kind of energies that operate in producing the more restricted ones.
We will now review the several phenomena in turn, beginning with the most widely distributed.
Effects of Down-Rush
Besides the general darkening near the edge of the sun's disk, the surface is seen to be strangely mottled near the poles, near the equator, and in fact universally. Moreover, small black specks, called granulations or pores, are everywhere visible, and spectro-scopic examination shows that every one of these is a true spot. The fine mottlings frequently indicate the existence of powerful currents in that they take definite directions, sometimes in straight lines, sometimes in lines suggesting cyclonic swirls. In addition to the pores spots of a smudgy kind, called veiled spots, are some-times seen, and it is probable that in such cases the force of the down-rush is insufficient to depress the photosphere to an extent competent to give rise to the ordinary dark spots. Some spots appear as large pores, that is, they consist of nothing but umbra ; others appear as well-developed veiled spots, consisting almost en-tirely of penumbra. The obvious large spots consisting of umbra and penumbra follow next in order of intensity, and, as has been previously pointed out, their appearance is confined to definite spot zones. Minute observation, therefore, shows that the whole of the sun's surface is traversed by down-rushes of varying intensities, from almost infinitesimal dimensions to the most powerful that we can conceive. Some of the ordinary spots do not appear to bo in any violent state of agitation : the penumbra and umbra are well de-fined, and the ridge of fáculas round such a spot does not indicate any disturbance by either lateral or convexión currents. Other spots, however, indicate very violent commotion, the penumbra and umbra being tremendously contorted and mixed up. In this kind of spot the disturbance often affects enormous areas of the sun's surface ; one spot in 1851 was 140,000 miles across, and the commotions were so great that they could be detected by eye observation with the telescope. It appears as if the material carried in the first instance below the level of the photosphere produces a disturbance in the interior regions, which exhibits itself at the surface by an increase in the quantity and brilliancy of the sur-rounding fáculas. As a spot dies away it is replaced by fáculas, and these remain long after the spot has closed up. It often happens that new spots break out in the places occupied by pre-vious spots. The spot-producing material in its descent is dis-sociated either before or when it reaches the photosphere, and the rapidity and energy of the dissociation depend upon the velocity with which it travels. Gravitation is of course the main factor operating in the production of a down-rush. The velocity produced by gravitation in matter falling from great heights above the photosphere must be very great, and in consequence the kinetic energy of the moving mass must also be great. The motion is impeded by friction with the gases in the sun's atmosphere, and some or perhaps all the kinetic energy becomes heat. The heat thus developed must produce sudden expansions, and the initial down-rush is surrounded by up-rushes along the lines of least resistance. The effects of such down-rushes vary in degree according to the quantity of matter falling and the height from which it falls.
Effects of Up-Rush
Equally too there are observed different degrees of the effects of up-rushes. All over the sun's surface are seen domes of faculae, either separate or in groups, and there is indication that they are hotter than the rest of the surface, for the bright lines of hydrogen are seen to surmount them. It is probably owing to this that the chromosphere exhibits a billowy outline when under con-ditions of little disturbance. The next condition of increased action exhibits itself in the growing complexity of the chemical nature and of the form of the chromosphere. Occasionally the whole level of the chromosphere over a large region seems to be quietly raised, and observation proves this to be due to the intrusion of other vapours. There is either a gradual evaporation from the photosphere or a gradual vaporization or expansion of slowly fall-ing material over large regions, raising the level of the sea of hydro-gen. The chromosphere then appears to contain different layers, and the lower we descend towards the photosphere the less we know about the substances that exist there. The next degree of disturb-ance is seen in what are called the quiet prominences, which very frequently occur in regions where the beginning of a disturbance has been previously indicated by the appearance of domes and metallic strata. As a rule the quiet prominences are not very highnot higher than 40,000 milesand many of them resemble trees. They are almost entirely composed of hydrogen, or at least of a substance which gives some of the lines observed in the spectrum of hydrogen. Such a prominence grows upwards from the photosphere, being first of a small height, then getting higher and often broader, and finally a kind of condensation cloud may form at the top. The upward velocity of the gases forming these prominences is seldom very great. When a prominence disappears it does not follow that the substances of which it was composed have also disappeared, and there is evidence to show that the apparent disappearance is due to a reduction of temperature. The most intense degree of action of an up rush is exhibited by the metallic prominences, which contain other substances in addition to hydrogen. They are seen mounting upwards to enormous heights with almost incredible velocities, and their ascent is accompanied by violent lateral motions. Such prominences have been seen with an upward velocity of 250 miles a second, and of a height as great as 400,000 miles. There is also evidence that some prominences consist of mixed up-rushes and down-rushes, and it may turn out eventually that this is the case in all the metallic prominences.
Interrelations of Phenomena
According to the gravitation-dissociation theory of the formation of spots, we ought to find that the effects, in 'various decrees, produced by down-rushes of associated matter are related to the effects, in like degrees, produced by the corresponding up-rushes of dissociated materials. Comparing, then, the facts already stated , we have:
It is a fact that the pores and domes are very closely associated over all parts of the sun, and that the domes are most prominent in places previously occupied by spots. All large spots are seen to be accompanied by metallic prominences, when observed at the edge of the sun. There is also a strict relationship between the intensity of action going on in a spot and the associated prominence, so much so that a very violent change in a spot on the disk some-times causes the bright prominence lines to become visible in its spectrum. The ordinary metallic prominences, as already stated, may consist of both ascending and descending material; this will be best understood by likening the whole phenomenon to a splash.
Physics of a Sun-Spot Cycle
We have previously seen that spots and metallic prominences are very intimately connected as regards their occurrence in zones, and this intimacy is easy to explain by supposing things to happen in the way here set forth. The height of the solar atmosphere is greater over the equator than at the poles ; particles condensed on the outside at the poles have therefore a relatively small velocity when they fall into the photosphere, and are able to produce only pores or veiled spots. Over the equator the particles attain a higher velocity in their fall, but they also have to pass through a much greater thickness of atmosphere and undergo so much dissociation that on reaching the photosphere they are incom-petent to produce spots. In mid-latitudes, therefore, the falls of condensed particles should be most effective in producing spots. In this way the absence of spots at the poles and equator is ex-plained,one of the best-known facts of solar physics. The falls of the condensed particles, or meteoric matter, into the sun increase the temperature of the atmosphere over the spots and prominences which they produce, so that other falls in the same region are not effective in producing spots on account of the increased dissociation which they must undergo before reaching the photosphere. If the material condensed in those regions is to produce a spot, it must be removed to some place where it can reach the photosphere with-out being dissociated. Hence from the first appearance of spots after a sun-spot minimum there is a continual change of latitude. From minimum to minimum there is a regular decrease in the latitude of spots ; hence it is clear that there must be currents from the poles towards the equator in the upper atmosphere of the sun, causing the removal of condensed materials to lower and relatively cooler latitudes. Assuming the existence of such currents, we ought to find that successive spots have a tendency to form along the same meridians, for the polar currents would carry the con-densed materials to lower latitudes in a nearly meridional direction. Examination of sun-spot records for 1878-79 shows that there is a marked tendency for spots to follow each other in meridians. The existence of such currents is further supported by the outcurving of the corona at the solar poles as observed in several eclipses. If these currents exist, there must also be compensating currents towards the poles in the lower parts of the sun's atmosphere, carrying incandescent vapours along with them. Small prominences often give indication of motion towards the poles which such currents would produce, and examination of sun-spot records also shows that the tendency of the proper motion of the spots is polewards. Hence, although the existence of these currents has not been definitely proved, there is strong evidence that there exists some circulation of this nature in the solar atmosphere.
When once the falls have commenced, if this hypothesis is true, they should rapidly increase in intensity, for, as it is the falls which increase the temperature of the lower atmosphere by the conversion of their kinetic energy into heat, the more falls there are the more material will be taken first to the poles and then towards the equa-tor, and therefore there will be more available spot-forming material. But we know that this increase iu intensity does not go on for ever, and th*re must therefore be some regulating influence. The in-crease of temperature and possibly of the height of the solar atmo-sphere, due to the increased falls, will eventually become such that the descending materials are dissociated before they reach the photosphere. The production of spots must therefore gradually diminish until they finally disappear and end the spot cycle. At the minimum period, therefore, pores and veiled spots, due to less powerful energies, are at a maximum.
Records of eclipses, occurring when the sun was quietest, show that the condensing and condensed materials brought to the equator by the polar currents probably extend far beyond the true atmosphere of the sun and are there collected, possibly in the form of a more or less regular ring the section of which widens towards the sun, the widest part being within the boundary of the sun's atmo-sphere. If we assume such a ring under absolutely stable conditions, there will be no fall of material, and therefore no prominences or spots. But suppose a disturbance caused, as before, by collisions, which most likely occur where the particles brought by the polar currents meet the surface of the ring. These particles then fall from where the ring first meets the atmosphere on to the photo-sphere, and form the first spots. Eclipse records show that this action takes place about 30° lat. According to this view, there are usually no spots above 30° lat., because there is no ring, and because the atmosphere is too low to give the height of fall necessary to produce spots. There are no spots at the equator for the reason that the condensed matter has to pass for perhaps millions of miles through strata of increasing temperature, and do not there-fore reach the photosphere before being dissociated. Accordingly, we ought to find that at and after the maximum the corona is brighter and more truly a gaseous body on account of the increased temperature. This is in strict accordance with eclipse observations extending over twenty years. According to this view of the solar economy, the sun ought to give out more heat at a maximum than at a minimum period, when the number of falls is greatest; on this point see the article METEOROLOGY (vol. xvi. p. 167 sq.).
The Sun's Place among the Stars
The relative nearness of the sun makes it convenient as a type of those stars which on account of their great dis-tance are less accessible to minute observation. If the sun were at a greater distance, its spectrum would become much fainter and would not show so much detail, but its general character would not be altered : its dark lines would not become bright ones. In the atmospheres of the various members of the solar system, including the earth, there is a very considerable absorption of blue light. We know also that this condition applies to the sun.
Star Absorption of Light
The light we receive under present conditions we call white; but, if its own atmosphere and ours were removed or became so changed as to no longer absorb blue light, the sun would appear blue. If, on the other hand, the blue absorption were enormously increased, so that it extended into the green, the sun would appear red, be-cause every other kind of light would be absorbed. If two kinds of absorptionone in the red, the other in the blue were going on together, as they sometimes do in our laboratories, the sun would then appear green. Although these changes are not of actual occurrence in the sun, we find each of these conditions represented among the stars. In the coloured stars, which may be red, green, or blue, we are simply dealing with this kind of absorption phenomena. This difference in the conditions of absorption in the stars, however, is by no means the most important one : the difference of temperature as indicated by the spectrum is of primary importance. As in our labora-tories the spectrum of a substance is changed by a varia-tion of temperature, and always in a regular way, so the nature of a star's spectrum furnishes a clue to its probable state as regards heat. For example, we may submit carbon vapour to a low temperature, and we shall then obtain what is called a spectrum of flutings; on increasing the temperature, the flutings are replaced wholly or partially by lines, according to the amount of increase. From hundreds of observations of this kind, both on carbon and other substances, it may be safely inferred that a fluted spectrum indicates a lower temperature than a line spectrum. There are doubtless substances in the sun's atmosphere which, although represented by lines in its spectrum, can be submitted to low conditions of tempera-ture so as to give fluted spectra. There can be little doubt, therefore, that a cooling of the sun would be followed by a change in its spectrum, which would cease to be one of lines and become one of flutings. While the sun was acquiring its present intensely heated state, it must at some period of its history have been in a condition of temperature in which its spectrum would consist of flutings, and similarly it must give a fluted spectrum at some future period when it has further cooled.
Solar Radiation and Spectra
The ordinary Fraunhofer spectrum gives the sum total of the line absorptions of all the various layers in the sun's atmosphere, but by examining individual layers just off the edge of the disk we can single out the absorption lines produced by the lower layers. Thus the absorption produced by the hottest layer, the chromosphere hottest because nearest the photosphereis indicated by its usually simple radiation spectrum when examined in this way. If the sun were made hotter, therefore, the gases which give the simple chromosphere spectrum would have a larger share in the absorp-tion, and the main features of the Fraunhofer spectrum would be the few dark lines corresponding to these bright ones. This being so, a star which gives practically the same absorption spectrum as the chromosphere of the sun must be hotter than the average temperature of the sun's atmosphere,as hot as the hottest part of it. The bright central part of the sun is not veiy much less than the whole volume, but it is so much hotter that it gives out thousands of times more light than the atmosphere. The cool vapours in the atmosphere give the dark Fraunhofer lines by their absorption, and even if they are hot enough to give bright lines when seen on the sun's edge they can only reduce the intensity of the dark lines. Here the difference of area between the disk representing the cen-tral mass and that representing the sun's atmosphere is very small, and, the light from the central mass being so much more intense, we do not ordinarily see the evidences of radiation, but, in place of it, the absorption of the atmosphere. If, however, we suppose the central mass to be very small compared with its atmosphere, the total radiation of the atmosphere may be sufficiently powerful to overcome the intensity of the light from the smaller central part, so that the spectrum of such a star would contain bright lines from the exterior mixed up with the dark lines from the interior. The spectrum of a star, therefore, does not always depend upon its total diameter, but upon the relative diameters of the central mass and the outer atmosphere. It is a question of sectional areas.
Observations of the spectra of a large number of stars show that, although there is a great difference between individual spectra, they still admit of arrangement in family groups. While some stars give line absorption spectra, others give fluted spectra, and others again give bright lines. They may be conveniently arranged as follows :
This classification probably represents the stars in order of tem-perature, class I. being the hottest.
Although different stars may contain lines of identical wave-lengths, the thickness of these li',es is very liable to variation in passing from one star to another. The thickest lines in the solar spectrum are H and E in the ultra-violet, both of equal thickness; on passing to some of the stars, however, we find H broad with K thin, and in others H without K. This is similar to what occurs in our laboratories when we study the spectrum of calcium, the substance which gives the lines H and K : at the temperature of the electric arc the blue line of calcium is very intense, while H and K are scarcely visible; but on passing to a higher temperature, that of the induction spark, H and K appear. In those stars which give H without K, namely, those in class I., it is probable that there is a very high temperature competent to separate H and K, just as H and K were conjointly separated from the blue line. A further indication of high temperature in the stars belonging to class I. is that the few lines which do occur in their spectra are almost the exact counterparts of those which occur in the hottest layer of the sun, hydrogen lines being especially prominent. The passage from class I. to class II. is by no means sudden : there are stars with every gradation of broad and fine lines. It will readily be understood that the stars of class II. are probably not so hot as those belonging to class I., and the change in the spectrum is supposed to be due to new combinations of the original substances, rendered possible by a reduction of temperature ; that is, new lines are formed at the expense of the old ones. The hydrogen lines are very prominent in class II., though not so intense as in class I. The stars of these two classes may be grouped together and called hydrogen stars. Stars belonging to class III. exhibit unmistakable evidence of carbon vapour. Sodium and iron are also often present. All the stars in this class, of which fifty-five are known, agree in having a reddish tint. They are usually faint, and seldom exceed the fourth magnitude. There is evidence of the existence of carbon vapour in the sun's atmosphere, depending upon one solitary fluting, and hence stars of this class probably represent what the sun would become if it were cooled. Class III. therefore represents a lower temperature than classes II. and I. Class IV., containing 475 known members, includes the stars giving fluted spectra with the darkest edges of the flutings towards the violet. The origin of the substances of which they are mainly composed is not at present known. All the principal bands are absolutely unchanging in position, although there is considerable variation in the inten-sities. The bands in the spectrum appear to result from the rhythmical vibrations of the same substance, probably a complex one. Besides this unknown substance, there are also metallic lines in many of the stars, the complete spectrum consisting of the handed spectrum superposed upon the line spectrum. The metallic lines are generally seen in the spectra of sodium, iron, magnesium, or calcium ; the hydrogen lines are very inconspicuous.
These considerations suggest the question of stellar evolution. Comets and nebulae are now supposed to consist of clouds of stones or small meteorites, and the difference between their spectra may be due to a difference of temperature, that of the nebulas being highest. Comets ordinarily give the spectrum of carbon, and, if we imagine such cometary matter to surround a central bright nucleus, we have the spectrum of a star of the third class. On the nebular hypothesis, starting with ordinary cometary materials, the small masses resulting from the first condensations gravitate towards each other, and their energy becomes heat by the retardation of their motion on coming in contact. As soon as the condensed mass is hot enough, it gives a fluted spectrum, like stars of the third class. As the energy of condensation increases, the temperature is raised and the spectrum passes from that of a third class star to that of a second class star, and then to that of a first class star. On the subsequent cooling of what is then a star the successive stages will be again passed through in inverse order. According to this view, we ought to find fewer hydrogen stars than carbon stars, because every star is a carbon star at two periods of its existence, but a hydrogen star only once. On this point, however, nothing definite can be stated, as the stars of classes I. and II. have, in con-sequence of their greater brightness, received more attention than carbon stars.
New and Variable Stars
In 1866 a star of the tenth magnitude in the constellation Corona suddenly flashed up into a star of nearly the first magnitude ; its spectrum as a tenth magnitude star differed from its spectrum as a first or second,the latter containing bright lines of hydrogen. In about a month it again became a tenth magnitude star and appeared as if nothing had happened to it. There can be little doubt that here there was a sudden increase of temperature, as evidenced by the spectrum becoming like that ofthe chromosphere of the sun. Ten years afterwards a new star appeared in Cygnus ; it had never been seen before, but appeared suddenly as a third or fourth magnitude star. In about a year it gradually dwindled down to the tenth magnitude, and its spectrum became that of a nebula. This mass was at a stellar distance, but it cannot be considered to have been a large mass of incandescent material, for in that case it would have taken millions of years, instead of only one, to cool down to the tenth magnitude. A possible explanation of most of the new and variable stars is to be found in the meteorite theory : the innumerable components of one group of meteorites colliding with those of another group would be competent to give out light sufficient to make the whole appear as a star. Each meteorite gives only a little light, but the total must be very considerable. The new star in Corona, and similarly all new stars, may have been the result of a collision of two groups of meteorites. They die out quickly because the components are small and far apart. The sudden increase in the brilliancy of the star in Cygnus would be produced by a collision of a meteor swarm with the star already existing. (J. N. L.)
The above article was written by: Sir Joseph Norman Lockyer, K.C.B., F.R.S.; Director of Solar Physics Observatory, South Kensington; Rede Lecturer at Cambridge, 1871; in charge of English Government Eclipse Expeditions, 1870, 1871, 1882, etc.; author of Chemistry of the Sun, Studies in Spectrum Analysis, and Recent and Coming Eclipses.