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Popular Science Monthly

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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

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