1941: Astronomy

Solar Discoveries.

Studies of the sun are of especial importance for we can hope to learn much more about it than we can about distant stars. Whatever details we learn about the sun aid us in interpreting the 'cues' and 'hints' from the far more distant stars. Any detailed study of the solar atmosphere requires, among other data, a knowledge of the intensities of absorption lines in the solar spectrum. For many years spectroscopists have had to depend on estimated intensities of these lines, primarily on the estimates of Jewell and Rowland. The Rowland scale of intensities has been carefully calibrated, and the intensities of a few hundred of the stronger lines have been measured, but there has remained an urgent need for a precise photometric atlas of the Fraunhofer lines. This need has now been met by a prodigious volume prepared by Minnaert and his colleagues at Utrecht. This work, Photometric Atlas of the Solar Spectrum, contains 174 pages (over 300 feet) of microphotometer tracings of the solar spectrum on a direct-intensity scale, covering the spectral region λ3332 to λ8771. The scale of the tracings is approximately 2 centimeters per Ångstrom so that wave-lengths, based on the Revised Rowland Table, may be read from the curves with an accuracy of approximately 0.01 Å. Measures of central intensities taken from the Atlas are correct to about 5 per cent.

The great majority of the lines in the spectrum of the solar disk have now been identified. There has, however, remained one group of lines which defied identification. These are 22 lines in the spectrum of the solar corona. That these could not be produced by any unknown chemical element has been accepted, but it remained for Edlen, from his investigation of the spectra of highly ionized atoms, to determine the identity of the brightest of these lines. He found that two of them are produced by highly ionized calcium atoms, seven by highly ionized iron atoms, and six by highly ionized nickel. Since these fifteen lines which are now identified by Edlen produce 97 per cent of the corona's total radiation, his identifications are of particular importance.

Facilities for the detailed study of solar prominences at the McMath-Hulbert Observatory of the University of Michigan have been greatly improved during the year. A new spectroheliograph, which can be operated in conjunction with the tower telescope, now gives the rate of motion of prominence material in the line of sight. Since this instrument and the spectroheliokine-matograph on the tower telescope can be operated simultaneously, the McMath-Hulbert observers can now obtain a simultaneous record of the motion of the prominence material in three dimensions and thus can obtain a remarkably accurate picture of these puzzling solar outbursts. To test the reliability of the methods used in analyzing these data, Mohler has constructed models of prominences based on his analysis and has then photographed the models and compared these photographs with those of the actual prominence. The similarity is so striking as to leave little doubt of the correctness of the methods used.

One of the important factors which must be considered in the analysis of light curves of eclipsing variables is the degree of darkening at the limb for these stars. This is difficult to determine for very distant stars, but should be relatively easy to determine for the sun. This has been done with high accuracy except for the region very near the limb. The solar image in this region is so distorted by the effects of the earth's atmosphere that only during the crescent phases of a total solar eclipse can accurate measures be made. Three Dutch astronomers, Messrs. Ferwerda, Uitterdijk, and Wesselink, have made such observations by means of a very ingenious photographic device, using aluminum-coated spectacle lenses and amateur movie cameras. From hundreds of observations they found that, whereas over most of the solar disk the brightness varies as the cube root of the distance from the limb, near the limb the brightness varies as the tenth root of the distance. They also found that the color remained constant, which suggests that the layer of the sun very near the apparent surface must be a nearly isothermal one.

The distance from the earth to the sun is a standard to which all other measurements in both the solar and stellar systems are referred. An accurate determination of this distance is therefore of interest and importance. One of the most accurate methods of determining this distance is the simultaneous observation from different stations of an asteroid which comes close to the earth. Eros, at the opposition of 1931, came within 16,000,000 miles of the earth. Since it was at that time a bright telescopic object it offered an excellent opportunity for a new determination of the solar parallax—a measure of the sun's mean distance in terms of the equatorial radius of the earth. Some thirty observatories throughout the world made series of observations of Eros at that time. Recently Jones, who has analyzed these thousands of observations, has announced the results of this great cooperative program. He finds, from all right ascension observations, the value 8'.7875 ± 0'.0009 for the solar parallax, and from all declination observations, the value 8'.7907 ± 0'.0011. The mean value is thus 8'.790 ± 0'.001, which is somewhat smaller than the mean of previous values and corresponds to a distance of 93,003,000 miles. Using this new mean value for the solar parallax Jones has obtained from the observations a new value of the ratio of the mass of the earth to that of the moon, namely, 81,271 ± 0.021.

Planets.

New determinations of the masses of Pluto and of Neptune's satellite have been made during the year. The new value for the mass of Pluto resulted from a comprehensive comparison of Newcomb's theory for the motion of Neptune with the observations of this planet from 1795 to 1938, made by Wylie at the United States Naval Observatory. He found the mass of Pluto to be (0.300 ± 0.028) 10^-5 sun's mass. Since the mass of the earth is 0.301 x 10^-5 sun's mass, it appears that the two planets have practically identical masses.

The mass of Neptune's satellite, Triton, has been determined by Alden from observations made at Johannesburg. He found that the ratio of the mass of Triton to the mass of the system is 0.0013 ± 0.0003. The mass of Neptune is roughly seventeen times the mass of the earth, so that the mass of Triton is approximately 0.022 the mass of the earth, or 1.8 times the mass of the moon.

Comets.

There were six comets discovered during the year. Two of these discoveries were actually rediscoveries of previously known objects, whereas the other four were new comets, one being easily visible to the naked eye for observers in the southern hemisphere.

Variable Stars.

The number of discoveries of variable stars in recent years has reached so large a figure that the need for a catalogue of these stars giving essential data and references to the literature has become urgent. This need has now been met by Prager in a supplementary volume to his History and Bibliography of the Light Variations of Variable Stars which has just been published. This volume, which contains data for 3,592 variable stars discovered between 1931 and 1938, will be of inestimable value as a reference volume for all observers of these stars.

Among the interesting and important variable stars are the so-called cluster variables. The light variations of these stars are somewhat similar to those of the Cepheids, though the periods are shorter. Recent observations of these stars by Fath and by Schwarzschild show that there is a secondary period of light variation superimposed on the primary period. Schwarzschild has analyzed this material from the standpoint of pulsation theory and finds that these secondary periods may be higher modes or overtones of the fundamental periods. He finds that the periods of the cluster variables in Messier 3 can be divided into two groups and that the group with shorter periods comprises stars which pulsate in the first overtone of the periods of stars in the other group. In other words, for one group the fundamental period predominates while for the other group the first overtone of the fundamental period is predominant in the pulsation. This result is of much importance in studying the pulsation mechanism.

The determination of the distances of these clusters is fraught with uncertainty because of the absorption of their light by interstellar material. In order to avoid this difficulty it is important to investigate clusters in regions where the obscuration by interstellar material is either absent or relatively small. Cuffey has done this for three clusters in the direction of the constellations Monoceros and Canis Major, where the Milky Way is fairly uniform and richly populated. In this direction he finds relatively little interstellar obscuration to a distance of approximately 3,300 parsees, and he also finds a marked absence of extra-galactic nebulae from these fields. This absence of spiral nebulae would seem to indicate that there is considerable absorption beyond these clusters. This must mean that either there is a concentration of material near the boundary of our galaxy, or that there is a spiral arm containing clouds of absorbing material and extending to a distance greater than the supposed average radius of our galactic system.

Probably no other star in the sky has received so much attention from astronomers as has the third magnitude eclipsing binary, Beta Lyrae. It has proved to be one of the most puzzling of these interesting stellar systems. Irregularities in the light curve and variations in the spectra have both defied adequate explanation. During the past year several astronomers have tackled this problem in search of a solution, the most profitable of these studies being the investigations of Struve and Kuiper. The latter has suggested a model for this system, based on dynamical calculations and on the spectrographic results of Struve, which seems to explain many of the puzzling features of the system. The two component stars of this binary are so close to each other, as they revolve about their common center of gravity, that their atmospheres partially merge. Since the two components are of quite different density, though of nearly the same mass, atoms of gas lying between the two will be attracted toward the denser star. There will thus be set up a current in the atmospheres. According to Kuiper's model, most of this circulating gas will continue to move about the two stars, but some of it having higher velocity will escape from behind the denser star and form a great spiral of gas about the pair. This escaping material will give a disc of gaseous matter which lies in the plane of the orbit, and if this orbit plane lies almost in our line of sight, this gas will come between us and the stars and thus account for most of the observed spectral peculiarities.

Nebulae.

The distribution of mass in extra-galactic nebulae is one of the important data needed for the study of the dynamics of these huge stellar systems. Of interest, therefore, is the recent investigation of the spiral nebulae Messier 31 and 33 by Wyse and Mayall. These astronomers have made numerous radial velocity measures at varying distances from the center of each of these nebulae, and find that in each case the mass is widely distributed throughout the nebula. The marked contrast between this result and the obviously strong concentration of light near the center of the nebula suggests that there is no apparent correlation between the distribution of mass and of luminosity. Possibly through this investigation a reinterpretation of observations of the rotation of our galaxy may lead to a similar result concerning the distribution of mass in our own stellar system.