There are, however, some remarkable features in these clusters which afford possible indications of their origin and essential constitution. When closely examined most of them are seen to be less regular than they at first appear. Vacant spaces can be noted in them; even rifts of definite forms. In some there is a radiated structure; in others there are curved appendages; while some have fainter centres. These features are so exactly like what are found, in a more pronounced form, in the larger nebulæ, that we can hardly help thinking that in these clusters we have the result of the condensation of very large nebulæ, which have first aggregated towards numerous centres, while these agglomerations have been slowly drawn towards the common centre of gravity of the whole mass. It is suggestive of this origin that while the smaller telescopic nebulæ are far removed from the Milky Way, the larger ones are most abundant near its borders; while the star-clusters are excessively abundant on and near the Milky Way, but very scarce elsewhere, except in or near vast nebulæ like the Magellanic Clouds. We thus see that the two phenomena may be complementary to each other, the condensation of nebulæ having gone on most rapidly where material was most abundant, resulting in numerous star-clusters where there are now few nebulæ.
There is one striking feature of the globular clusters which calls for notice; the presence in some of them of enormous quantities of variable stars, while in others few or none can be found. The Harvard Observatory has for several years devoted much time to this class of observations, and the results are given in Professor Newcomb's recent volume on 'The Stars.' It appears that twenty-three clusters have been observed spectroscopically, the number of stars examined in each cluster varying from 145 up to 3000, the total number of stars thus minutely tested being 19,050. Out of this total number 509 were found to be variable; but the curious fact is, the extreme divergence in the proportion of variables to the whole number examined in the several clusters. In two clusters, though 1279 stars were examined, not a single variable was found. In three others the proportion was from one in 1050 to one in 500. Five more ranged up to one in 100, and the remainder showed from that proportion up to one in seven, 900 stars being examined in the last mentioned cluster of which 132 were variable!
When we consider that variable stars form only a portion, and necessarily a very small proportion, of binary systems of stars, it follows that in all the clusters which show a large proportion of variables, a very much larger proportion—in some cases perhaps all, must be double or multiple stars revolving round each other. With this remarkable evidence, in addition to that adduced for the prevalence of double stars and variables among the stars in general, we can understand Professor Newcomb adding his testimony to that of Professor Campbell already quoted, that 'it is probable that among the stars in general, single stars are the exception rather than the rule. If such be the case, the rule should hold yet more strongly among the stars of a condensed cluster.'
The Evolution of the Stars
So long as astronomers were limited to the use of the telescope only, or even the still greater powers of the photographic plate, nothing could be learnt of the actual constitution of the stars or of the process of their evolution. Their apparent magnitudes, their movements, and even the distances of a few could be determined; while the diversity of their colours offered the only clue (a very imperfect one) even to their temperature. But the discovery of spectrum analysis has furnished the means of obtaining some definite knowledge of the physics and chemistry of the stars, and has thus established a new branch of science—Astrophysics—which has already attained large proportions, and which furnishes the materials for a periodical and some important volumes. This branch of the subject is very complex, and as it is not directly connected with our present inquiry, it is only referred to again in order to introduce such of its results as bear upon the question of the classification and evolution of the stars.
By a long series of laboratory experiments it has been shown that numerous changes occur in the spectra of the elements when subjected to different temperatures, ranging upwards to the highest attainable by means of a battery producing an electric spark several feet long. These changes are not in the relative position of the bands or dark lines, but in their number, breadth, and intensity. Other changes are due to the density of the medium in which the elements are heated, and to their chemical condition as to purity; and from these various modifications and their comparison with the solar spectrum and those of its appendages, it has become possible to determine, from the spectrum of a star, not only its temperature as compared with that of the electric spark and of the sun, but also its place in a developmental series.
The first general result obtained by this research is, that the bluish white or pure white stars, having a spectrum extending far towards the violet end, and which exhibits the coloured bands of gases only, usually hydrogen and helium, are the hottest. Next come those with a shorter spectrum not extending so far towards the violet end, and whose light is therefore more yellow in tint. To this group our sun belongs; and they are all characterised like it by dark lines due to absorption, and by the presence of metals, especially iron, in a gaseous state. The third group have the shortest spectra and are of a red colour, while their spectra contain lines denoting the presence of carbon. These three groups are often spoken of as 'gaseous stars,' 'metallic stars,' and 'carbon stars.' Other astronomers call the first group 'Sirian stars,' because Sirius, though not the hottest, is a characteristic type; the second being termed 'solar stars'; others again speak of them as stars of Class I., Class II., etc., according to the system of classification they have adopted. It was soon perceived, however, that neither the colour nor the temperature of stars gave much information as to their nature and state of development, because, unless we supposed the stars to begin their lives already intensely hot (and all the evidence is against this), there must be a period during which heat increases, then one of maximum heat, followed by one of cooling and final loss of light altogether. The meteoritic theory of the origin of all luminous bodies in the heavens, now very widely adopted, has been used, as we have seen, to explain the development of stars from nebulæ, and its chief exponent in this country, Sir Norman Lockyer, has propounded a complete scheme of stellar evolution and decay which may be here briefly outlined:
Beginning with nebulæ, we pass on to stars having banded or fluted spectra, indicating comparatively low temperatures and showing bands or lines of iron, manganese, calcium, and other metals. They are more or less red in colour, Antares in the Scorpion being one of the most brilliant red stars known. These stars are supposed to be in the process of aggregation, to be continually increasing in size and heat, and thus to be subject to great disturbances. Alpha Cygni has a similar spectrum but with more hydrogen, and is much hotter. The increase of heat goes on through Rigel and Beta Crucis, in which we find mainly hydrogen, helium, oxygen, nitrogen, and also carbon, but only faint traces of metals. Reaching the hottest of all—Epsilon Orionis and two stars in Argo—hydrogen is predominant, with traces of a few metals and carbon. The cooling series is indicated by thicker lines of hydrogen and thinner lines of the metallic elements, through Sirius, to Arcturus and our sun, thence to 19 Piscium, which shows chiefly flutings of carbon, with a few faint metallic lines. The process of further cooling brings us to the dark stars.
We have here a complete scheme of evolution, carrying us from those ill-defined but enormously diffused masses of gas and cosmic dust we know as nebulæ, through planetary nebulæ, nebulous stars, variable and double-stars, to red and white stars and on to those exhibiting the most intense blue-white lustre. We must remember, however, that the most brilliant of these stars, showing a gaseous spectrum and forming the culminating point of the ascending series, are not necessarily hotter than, or even so hot as, some of those far down on the descending scale; since it is one of the apparent paradoxes of physics that a body may become hotter during the very process of contraction through loss of heat. The reason is that by cooling it contracts and thus becomes denser, that a portion of its mass falls towards its centre, and in doing so produces an amount of heat which, though absolutely less than the heat lost in cooling, will under certain conditions cause the reduced surface to become hotter. The essential point is, that the body in question must be wholly gaseous, allowing of free circulation from surface to centre. The law, as given by Professor S. Newcomb, is as follows:—
'When a spherical mass of incandescent gas contracts through the loss of its heat by radiation into space, its temperature continually becomes higher as long as the gaseous condition is retained.'