The meteorites, which have come through the atmosphere to the Earth's surface, contain at least 25 known terrestrial elements. That they have not been found thus far to contain all of our elements is not surprising, for we should have difficulty in finding a piece of our Earth weighing a few kilograms which would contain 25 of our elements. We have not found any elements in meteorites which are unknown to our chemists. Our comets, which ordinarily show the presence of not more than three elements, carbon, nitrogen and oxygen, give certain evidence of sodium in their composition when they approach fairly near to the Sun; and the great comet of 1882, when very close to the Sun, developed in its spectrum many bright lines not previously seen in comet spectra, which Copeland said were due to iron. That the comets do not show a greater number of elements is not in the least surprising: they are not condensed bodies, and we think that their average temperature is low, too low generally to develop the luminous vapors of the more refractory elements. If their temperatures, approximated those which exist in the stars, their spectra would probably reveal the presence of many of the elements which exist in the meteorites. Of course the proof of this is lacking.
DESTINY OF THE STELLAR SYSTEM
We have said that the evolutionary processes depend primarily upon the loss of heat. This is to the best of our knowledge a genuine loss, except as some of the heat rays happen to strike other celestial bodies. The flow of heat energy from a star must be essentially continuous, always in one direction from hotter bodies to colder bodies, or into so-called unending and heatless space. Temperatures throughout the universe are apparently moving toward uniformity, at the level of absolute zero. Now, this uniformity would mean universal stagnation and death. It is possible to have life and to do work only when there are differences of temperature between the bodies concerned: work is done or accompanied by a flow of heat, always from the hotter to the colder body. We are not aware that any compensating principle exists. Several students of the subject, notably Arrhenius, have searched for such a principle, a fountain of youth so to speak, in accordance with which the vigor of stellar life should maintain itself from the beginning of time to the end of time; but I think that nothing approaching a satisfactory theory has yet been formulated. The stellar universe seems, from our present point of view, to be slowly "running down." The processes will not end, however, when all the heat generable WITHIN the stars shall have been radiated into an endless space. Every body within the universe, it is conceivable, could have cooled down to absolute zero, but the system might still be in its youth. So long as the stars, whether intensely hot or free from all heat, are rotating rapidly on their axes or are rushing through space with high speeds, the system will remain VERY MUCH ALIVE. Collisions or very close approaches of two stars are bound to occur sooner or later, whether the stars are hot or cold, and in all such cases a large share of the kinetic energy—the energy of motion—of the two bodies will be converted into heat. A collision, under average stellar conditions, should convert the two stars into a luminous gaseous nebula, or two or more nebulae, which would require hundreds or thousands of millions of years to evolve again into young stars, middle-aged stars, old stars, and stars absolutely cold. So long as any of these bodies retain motion with reference to other bodies, they retain the power of rebirth and another life. Not to go too far into speculative detail, the general effect of these processes would be the destruction of relative motions and the gradual decrease in the number of separate bodies, through coalescence. Assume further, however, that all existing bodies, widely scattered through the stellar system, are absolutely cold and absolutely at rest with reference to each other: the system might even then be only middle-aged. The mutual gravitations of the bodies would still be operative. They would pass each other closely, or collide, under high generated velocities: there would be new nebulae, and new and vigorous stellar life to continue through other long ages. The system would not run down until all the kinetic energy had been converted into heat, and all the heat generable had been dissipated. This would not occur until all material in the universe had been combined into one body, or into two bodies in mutual revolution. However, if there are those who say that the universe in action is eternal, through the operation of compensating principles as yet undiscovered, no man of science is at present equipped to prove the contrary.
THE NOVAE
The so-called new stars, otherwise known as temporary stars or novae, present interesting considerations. These are stars which suddenly flash out at points where previously no star was known to exist; or, in a few cases, where a faint existing star has in a few days become immensely brighter. Twenty-nine new stars have been observed from the year 1572 to date; 19 of them since 1886, when the photographic dry plate was applied systematically to the mapping of the heavens, and 15 of the 19 stand to the credit of the Harvard observers. This is an average of one new star in two years; and as some novae must come and go unseen it is evident that they are by no means rare objects. Novae pass through a series of evolutions which have many points in common; in fact, the ones which have been extensively studied by photometer and spectrograph have had histories with so many identities that we are coming to look upon them as standard products of evolutionary processes. These stars usually rise to maximum brilliancy in a few days: some of the most noted ones increased in brightness ten-thousand-fold in two or three days. All of them fluctuate in brightness irregularly, and usually in short periods of time. Several novae have become invisible to the naked eye at the end of a few weeks. With two or three exceptions, all have become invisible in moderate-sized telescopes, or have become very faint, within a few months. Two novae, found very early in their development, had at first dark line spectra, a night later bright lines appeared, and a night or two later the spectra contained the broad radiation and absorption bands characteristic of all recent novae. After the novae become fairly faint, the bright lines of the gaseous nebula spectrum are seen for the first time. These lines increase in relative brilliancy until the spectra are essentially the same as those of well-known nebulae, except that the novae lines are broad whereas the lines of the nebulae are narrow. In a few months or years the nebular lines diminish in brightness, and the continuous spectrum develops. Hartmann at Potsdam, and Adams and Pease with the 60-inch Mount Wilson reflector, have shown that the spectra of the faint remnants of four originally brilliant novae now contain some of the bright lines which are characteristic of Wolf-Rayet stars.[2]
[2] After this lecture was delivered Adams of Mount Wilson reported that in November, 1914, the chief nebular line (5007A) and another prominent nebular line (4363A) had entirely disappeared from the spectrum of Nova Geminorum No. 2, whereas the second nebular line in the green (4959A) remained strong; probably a step in progress from the nebular to the Wolf-Rayet spectrum.
Why the novae suddenly flare up, and what their relations to other celestial bodies may be, are questions which can not be regarded as settled. Their distribution on the celestial sphere is indicated in Figure 25 by the open circles. In this figure the densest parts of the Milky Way are drawn in outline. All of the novae have appeared in the Milky Way, with the exception of five: and these exceptions are worthy of note. One of the five appeared in the condensed nucleus of the great Andromeda nebula, not far from its center; another (zeta Centauri) was located close to the edge of a spiral nebula and quite possibly in a faint outlying part of the nebula; a third (tau Coronae) was observed to have a nebulous halo about it at the earliest stage of its observed existence; a fourth (tau Scorpii) appeared in a nebula; and the fifth (Nova Ophiuchi No. 2) in 1848 was not extensively observed. The other 24 novae appeared within the structure of the Milky Way. Keeping the story as short as possible, a nova is seemingly best explained on the theory that a dark or relatively dark star, traveling rapidly through space, has encountered resistance, such as a great nebula or cloud of particles would afford. While passing through the cloud the forward face of the star is bombarded at high velocities by the resisting materials. The surface strata become heated, the luminosity of the star increases rapidly. The effect of the bombardment by small particles can be only skin deep, and the brightness of the star should diminish rapidly and therefore the spectrum change speedily from one type to another. The new star of February, 1901, in Perseus, afforded evidence of great strength on this question. Wolf at Heidelberg photographed in August an irregular nebulous object near the nova. Ritchey's photograph of September showed extensive areas of nebulosity around the star. In October Perrine and Ritchey discovered that the nebular structure had apparently moved outward from the nova, from September to October. Going back to a March 29th photograph taken for a different purpose, Perrine found an irregular ring of nebulosity closely surrounding the star. Apparently, the region was full f nebulosity which is normally invisible to us. The rushing of the star through this resisting medium made the star the brightest one in the northern sky for two or three days. The great wave of light going out from the star when at its brightest traveled in five weeks as far as the ring of nebulosity, where, falling upon non-luminous nebulous materials, it made the ring visible. Continuing its progress, the wave of light illuminated the material which Wolf photographed in August, the materials which Ritchey photographed still farther away in September, and the still more distant materials which Perrine and Ritchey photographed in October, November, and later. We were able to see this material only as the very strong wave of light which left the star at maximum brightness made the material luminous in passing. That 24 novae should occur in the Milky Way, where the stars are most numerous, and where the resisting materials may preferably prevail, is not surprising; and it should be repeated that at least three of the five occurring outside of the Milky Way were located in nebulous surroundings.
The actual collision of two stars would necessarily be too violent in its effect to let the reduction of brilliancy occur so rapidly as to cause the disappearance of the nova in a few weeks or months. The close approach of two stars might conceivably produce the observed facts, but even this process seems too violent in its probable results. The chances for the collision of a rapidly traveling star with an enormously extended nebulous cloud are vastly greater, and the apparent mildness of the phenomenon observed is in better harmony with expectation.
RELATION OF NOVAE, PLANETARY NEBULAE AND WOLF-RAYET STARS
Although all recent novae have been observed to become planetary or stellar nebulae, they seem not to remain nebular for any length of time; they have gone further and become Wolf-Rayet stars. Whether any or all of the planetary nebulae that have been known since Herschel's day, and have remained apparently unchanged in form, have developed from new stars, is uncertain and doubtful. If they have, the disturbances which gave them their character must have been violent, such as would result from full or glancing collisions of two stars, in order to produce deep-seated effects which change slowly, rather than surface effects which change rapidly.