THE YEAR'S PROGRESS IN ASTRONOMY.1

By P. PUISEUX,

Member of the Academy of Sciences, Professor at the Sorbonne, Astronomer at the Paris Observatory.

I.

It is an illogical peculiarity of the human mind that while it can not comprehend an infinite material universe it readily refuses to limit it. Those celestial objects which hide themselves from our curiosity by their almost inconceivable distances are to the astronomer the most attractive of all. He dislikes to realize the invariable appearance of the nebulæ, or that he can not measure their distances with any approach of accuracy. Because he can find no better mode of inquiry he improvises one based upon a reasonable evolutionary theory. These impalpable phantoms he puts to the tests of his laws of mechanics, of physics, to the reactions of chemistry, a legitimate procedure, since it suggests consequences the verification of which he may attempt.

The spiral nebula, with their sharply defined structure and a spectrum which is a weakened replica not too much altered of the sun, takes us into a region relatively well known. But when we consider the gaseous nebulæ, those pale, undefined flakes so constituted that they have defied the power of time, how can we establish any connection between them and our daily experiences? Their spectrum brings us little help. It shows in the most favorable circumstances four bright enigmatic lines with a few others which can be identified as due to hydrogen or helium.

We can not borrow any of the substance of the nebulæ for analysis in our laboratory. To look upon each unknown line as an indication of a new element would be easy but futile. It might be useful, on the other hand, to invent some molecular structure for some one single element (which we will call nebulium) capable of producing all the lines of unknown origin. That is just what J. W. Nicholson 2 has done, who applied the theory of Johnstone Stoney and Ritz regarding

1 Translated by permission from Revue générale des Sciences pures et appliquées, Paris, June 30, 1912, pp. 474-478.

* Session of the Royal Astronomical Society, Nov. 10, 1911.

the origin of spectrum limes. With four negative electrons revolving about a positive central body, 12 of the mysterious lines were explained, and with these several others which were attributed with no great. certainty to hydrogen and helium.

This demonstration, of course, leaves much to be desired. But it is evident in what direction we must proceed to complete it. It is very probable that hydrogen and helium, because of their complex spectra, are not simple elements. What glory it would be if by some well-chosen process we could decompose either of them and make the complete spectrum of nebulium appear in our laboratory! Astronomers could then hand over to the chemists a conquest even more precious than was the discovery of helium.

1

While waiting for the realization of this bold synthesis we continue to proceed by analysis and classification. All the conclusions formerly held about nebulæ demand revision because of the amount and value of the recently obtained photographic evidence. Recently A. R. Hinks has devoted himself to such an analysis. With his wonted precision he has defined the characteristics which distinguish the various classes of nebulæ and star clusters and has found for each class its law of distribution in the heavens. We must discard the convenient and very simple law which stated the frequency of the occurrence of nebulæ and clusters as a function of the galactic latitude only.

There is a clearly marked distinction between the stellar and planetary nebulæ as well as between the annular and spiral ones. This fact gave Stratton 2 a strong argument against the theory of T. J. J. See, who attributed a common origin to the last two kinds. The cosmogony of See, far outrunning the possibility of experimental evidence, creates other difficulties, and it seems to us that it has made no progress since its appearance toward being accepted. The same thing can be said of the mutual collisions of stars. Bickerton 3 has come forward as an advocate of them, and they should, according to him, give birth to double stars as well as to planetary systems. A hazardous theory may prove useful because of the verifications it necessitates. For instance, Monk' showed that we ought to keep tab on the displacements of the spectrum lines so as to know in advance of imminent collisions, and Forbes demonstrated that by a similar process we could obtain evidence concerning the rotation of certain stars about their centers. An attempt with encouraging results has already been made as to the latter problem at the Allegheny Observatory.

5

1 Monthly Notices, May, 1911.

2 The Observatory, September, 1911.

Session of the Royal Astronomical Society, Jan. 25, 1911.

The Observatory, vol. 24, p. 202.

· Monthly Notices, vol. 72, p. 378.

The probability of a collision sometime in the globular clusters seems especially great. They are considered, with good reason, among the most curious objects in the heavens. If we suppose that the closeness of the stars in these clusters depends on the distances of the stars from the centers of the clusters, then we may get their real distances from their apparent distances. H. C. Plummer 1 did so for M 13 and found groupings which would have been predicted by the theory of gases in convective and isothermal equilibrium. This is one more fact to make us believe that in clusters as well as in the nebula the force of gravitation is absent or held in check by some repulsive force.

We indeed go yet further and ask whether the law of Newton is always applicable among the stars relatively near for which we have been able to measure the parallaxes and proper motions. We possess decided evidence in favor of the affirmative from binary stars, in the fact that their proper motions follow directions more often parallel to the galaxy than perpendicular. But there are also motives for doubt.

W. W. Campbell, by means of his valuable catalogue of the radial velocities of stars, has shown that the Orion spectrum type is always associated with small velocities. This suggestion, resulting from no preconceived idea, quickly underwent broader developments. It has consequently become of philosophical interest. We seem to have gained now in the old system of classifying the stars, which was founded upon increasing complexity in their spectra, at the same time an ascending scale for their velocities and a descending one for their masses and distances from the sun. Of course there are often individual exceptions, and the above rules apply only when the stars are averaged in groups.

The consequences of these generalizations have been skilfully followed out by J. Halm. It was an advance to be able to use as criteria for a classification the masses and velocities, rather than the ages, temperatures, or spectrum types. The first two properties are more fundamental and more apt to enter into our formulæ. The existence of a correlation between the masses and the velocities is even more worthy of remark. It makes us wonder whether there is an equipartition of energy between the groups of stars just as there is between the molecules of a gas in equilibrium. Such a state would not have resulted under the influence of a Newtonian field of force including all the stars. Such movements are rather the final consequence of an initial velocity varying widely between neighboring stars. Further, the predominance of yellow stars near our sun and of white stars farther away, the existence of an ellipsoidal distribution of the trajectories in the central part of our universe, establishes Monthly Notices, vol. 71, p. 460. 2 Monthly Notices, vol. 72, p. 378. 3 Monthly Notices, vol. 71, p. 610.

between the milky way and the great spiral nebulæ a singularly closer analogy than we had felt warranted in supposing until quite recently.

The new star discovered December 30, 1910, in the constellation of the Lizard, has followed its predicted career, fading rather slowly. It becomes more and more certain that temporary stars, even when they show the ruddy aspect of certain periodic variables, show less difference between the visual and photographic magnitudes.

The polar star often used as a standard for photometric comparisons because it remains constantly at practically the same altitude, seems to have abdicated that role and passed into the ranks of the variable stars. In order to show its variability, Hertzsprung1 went through the discussion of 418 photographs, each having four exposures. The variation amounts to 0.2 of a magnitude and takes place in less than four days.

II.

It is espe

Measures upon

The step from variable stars to the sun is very easy. cially so because of the recent work of C. G. Abbot. the intensity of the solar radiation made simultaneously on Mount Wilson (1,800 meters altitude) and on Mount Whitney (4,420 meters) gave very concordant results and the parallel march of the numbers places beyond doubt a very decided variability of the sun which may amount to a tenth of the total radiation within a few days.

The work of Abbot tends also to show that the precision with which we may state the temperature of the sun has been exaggerated. There are in the sun sources of heat from 5,000° up to some 7,000°. However, the higher temperatures predominate. The infra-red radiation comes from the deeper layers.

Some years ago the researches of Halm appeared to indicate that the rate of rotation of the sun, varying as we knew with the solar latitude, varies also synchronously with the sun-spot cycle. The investigation of this matter remains upon the program of the Edinburgh Observatory. But between the results obtained by Storey and those by Adams at Mount Wilson there is a systematic difference. might be suspected that with one or the other the distance of the slit of the spectroscope from the edge of the sun was not correctly determined. Or the cause of the discrepancy may lie in the telluric oxygen lines used for comparisons.

It

The established but not absolutely regular correlation which exists between magnetic disturbances and the appearance of sun spots seems to have been made decidedly clearer by the researches of Bosler, of the

1 Astronomische Nachrichten, No. 4518.

Report of the Astrophysical Observatory of the Smithsonian Institution.

observatory at Meudon. Bosler has succeeded in proving that in each locality there is a definite direction not only for the earth current but also for the disturbed magnetic field and this takes place as if due to the direct action of a current upon a magnetized needle. Another result of the same research was to show yet more clearly that the years when the sun spots have been the most numerous have been those when Encke's comet has been the most brilliant.

The total solar eclipse of April 28, 1911, was observed by several expeditions sent for that purpose to Vavau of the Tonga Islands. The weather was unfavorable. We must mention in passing one new result, the photograph taken by Father Cortie of the extreme red end of the bright line spectrum.

The army of minor planets continues to grow. The most interesting without doubt was that observed on October 3 and 4 at Vienna and Copenhagen and designated by the letters MT. Its motion determined at that time indicated that its distance was as small as that of Eros. But it could not be found again on subsequent days. We are forced to believe that its brightness varies rapidly and that it was visible on the earlier dates because of an exceptional temporary brightness.

Birkland, while trying to reproduce the solar corona by means of the luminous phenomena about an electrified sphere, got a very close representation of the ring system of Saturn. He was thus led to propose a new theory of those singular objects. Particles of radiant matter, emitted from Saturn, reach a certain distance, make their revolutions according to the third law of Kepler, and serve as absorbers and resonators for the luminous energy coming from the sun.

The flattening of the planet Mars and the orientation in space of its axis of rotation rested until recently upon very discordant data from the micrometer. H. Struve, in a communication to the Berlin Academy of Sciences (Nov. 30, 1911), showed that by a very laborious but surer process depending on the variations in the orbits of its satellites, he had reached much better values. The best series was furnished by the powerful instruments of the Lick and the Yerkes Observatories. A very useful series of photographs of the satellites was obtained at the observatory at Pulkova by Kostinsky. The figure 190.4 for the reciprocal of the flattening and 202.7 for the ratio of the force of gravity to the centrifugal force at the equator will doubtless receive only insignificant changes.

The topography of the moon will now have a more solid basis as the result of the catalogue of 2,885 objects published by Saunder and based upon measures of plates taken at the Paris and the Yerkes Observatories. It is already in use at the Paris Observatory in a study of the libration of the moon. The uncertainty of a position derived from three plates appears to be less than 0.15". On our own