1942: Astronomy

Astrophysical Observatory at Tonanzintla.

Many an astronomer has wished that he might have an observatory so located that from it the entire Milky Way might be observed. Mexico's new astrophysical observatory, located at Tonanzintla, is at latitude 19 degrees north and alone among important modern observatories commands a view of the whole Milky Way. This observatory was formally dedicated on Feb. 17, by President Manuel Avila Camacho of Mexico. The dedication ceremony was indeed a notable occasion and was attended by many distinguished astronomers of Mexico, the United States, and Canada. Following the dedication ceremonies an Inter-American Scientific Conference was held, the appropriate central theme of which was the structure and constitution of the Galaxy and its relationship to other galactic systems. The new observatory is equipped with a 27-31-inch Schmidt telescope and several smaller instruments. A very ambitious program of research has been outlined for this new observatory and its able staff of Mexican astronomers.

Solar Corona.

Many observational methods and types of instruments have been applied in recent years to the study of the solar corona, that intriguing outer gaseous envelope of our sun. During the year two pieces of work have added further data to our knowledge of the corona. Both are the result of observations made of total eclipses of the sun. Menzel and Petrie, from a study of spectra made in Russia at the 1936 total eclipse, have determined the wave-lengths of seven new lines in the spectrum of the corona. The determination of accurate wave-lengths of these new lines is the first step in their identification. When these new lines are identified we shall know more about the physical conditions in the corona under which these lines are produced.

The other piece of research, carried out by two Chinese astronomers at the total eclipse of Sept. 21, 1941, is of particular interest because of the very simple equipment used. The total light emitted by the corona is an important datum and very difficult to measure. Furthermore, there is some evidence to indicate that the total amount of light emitted may vary. Any reasonably reliable measure of the total emitted light is therefore valuable. Chang and Li used an ordinary Weston exposure meter, a 9 cm. camera lens, and a sensitive galvanometer to make their measurements. After carefully calibrating the exposure meter for the range of intensities involved, they compared the total light received from the corona during total eclipse with the amount of light received from the full moon. They found that at this eclipse the total light received from the corona was 0.39 that received from the full moon.

Asteroid Eros.

The asteroid Eros, the most observed of all the asteroids because of its usefulness in determinations of the solar parallax, has long been an interesting object for another reason. Its light varies in a period of approximately 5^h 17^m. This has been attributed to its irregular, elongated shape as it rotates about its shortest axis. Recently Roach has made very accurate photoelectric observations of this asteroid on six nights. From these he has obtained a composite light curve which is the most accurate yet obtained. Roach then has examined the type of theoretical light curve which will best represent the observed curve. He finds that if he adopts Lambert's law of reflection, and assumes a geometrical figure with its three dimensions in the ratio of 1:4:13, he obtains a theoretical curve which is a good fit for the observed curve. If he assumes an idealized case of a three-axis ellipsoid, the ellipsoid would have the dimensions 35 km., 15.6 km., and 7.2 km. This theoretical curve fits the general run of the observations remarkably well. There still remain, however, small departures of the observed light from this theoretical curve. These departures can best be explained by assuming that the geometrical figure of the asteroid is irregular. This is a conclusion previously arrived at, but from much less accurate data than presented in Roach's extremely accurate light curve.

Comets.

Seven comets were discovered during the past year. After preliminary orbits had been computed it was found that five of the seven 'discoveries' were actually rediscoveries of previously observed comets. One of these comets was first discovered in 1867. The two comets which proved to be actually new to observers were both discovered photographically at the Harvard Observatory by Whipple.

Novae.

One of the most interesting discoveries of the year was made on Nov. 10, when a nova was discovered by Dawson in the constellation of Puppis. With rare good fortune, this nova was discovered before it reached maximum brightness. This made possible the detailed study of its spectrum before maximum light, which will greatly aid in the interpretation of this star's 'life history' during the time that expanding shells of gas were blown off. Nova Puppis reached maximum brightness on Nov. 12, at which time it was about 0.5 magnitude and a conspicuous object low in the southern sky for observers in mid-northern latitudes. By Nov. 30 it had faded to 5.5 magnitude and was near the limit of naked-eye brightness.

Records of Chinese and Japanese astronomers of the eleventh century describe the appearance of a 'guest star' in the constellation of Taurus in the year 1054 ad. Recently published translations of these records by Duyvendak aid in locating the position of this object in the sky. Furthermore, a study of these translations seems to clearly indicate that this 'guest star' was no ordinary nova but a supernova — as a matter of fact one of the brightest supernovae ever recorded. This new evidence is of particular interest because about twenty years ago Lundmark and Duncan suggested that this 'guest star' and the well known Crab Nebula in Taurus were probably the same object. The position is as nearly the same as can well be judged from the early records. It is known that bright novae do frequently show nebulosity around them some years after the outburst — Nova Aquilae 1918 and Nova Herculis 1934 are two such examples. Furthermore, from a series of photographs of the Crab Nebula made over a period of years, the rate of expansion can be approximately determined. Using this rate of expansion, and the present dimensions of the nebula, the approximate time of outburst which produced the nebula can be computed. This turns out to be about 1100 ad — not too great a discrepancy since the rate of expansion need not have been constant. Mayall and Oort have recently made a careful study of all of the available evidence. They conclude that the Crab Nebula was indeed produced by the outburst of the supernova observed by the Chinese in 1054 ad.

Nebulae.

Baade has recently photographed the Crab Nebula in several regions of wave-length. He finds that the nebulosity is composed of two parts. Photographs made in the Ha (red) region of the spectrum show an outer system of filaments of a most beautiful and varied structure, whereas photographs made in the blue show an inner mass of amorphous structure. He finds that it is these outer filaments which give rise to the line spectrum, the inner amorphous mass producing a continuous spectrum. It is clearly apparent that photographs of other nebulae in several wave-length bands will aid greatly in the interpretation of these objects.

It has long been known that hydrogen is the most abundant element in the atmospheres of most stars. Recently a very surprising discovery about the abundance of elements in a stellar atmosphere has been announced by Unsold. He has made a thorough study of line widths in spectra of the hot star Tau Scorpii. From this study he finds that hydrogen is the most abundant, and neon the second most abundant element in the atmosphere of this star. To find that neon, the gas used in the tubes of many advertising signs, is so abundant in a stellar atmosphere, though so rare on earth, is indeed surprising. This discovery will certainly have an effect on our interpretation of the conditions existing in stellar atmospheres. It will be most interesting to see if tests of the abundance of neon in the atmospheres of other stars bear out Unsold's findings in the case of Tau Scorpii.

It is known that many of the early type, hotter, stars are surrounded by gaseous shells. Struve has made a thorough study of the available observational data on these stars. He finds that in an ideal case there are three layers which are observable. The lowest layer is a stationary reversing layer which produces the ordinary absorption line spectrum. There is an intermediate layer which is stationary and produces an a Cygni type of spectrum with exceedingly sharp lines. Above these two layers is an expanding layer of varying optical thickness which produces a P Cygni type of spectrum with bright lines. Struve suggests that the two outer layers may originate from the rapid rotation of the lower reversing layer, and that the structure of these layers is a function of the radiation pressure due to the Lyman α flux of the shell. This investigation will serve as a guiding stimulus for much further work on the many stars with gaseous shells.

Wyse has made a detailed study of the spectra of ten gaseous nebulae. Nine of these are planetary nebulae, the other the great Orion nebula. This study involved the accurate measurement and identification of many spectral lines — about 270 spectral lines were measured and some sixty per cent of these were identified — all of the measured lines being produced in the nebulosity and not in the exciting stars. From this thorough study Wyse concluded that the chemical composition of all of the planetary nebulae is essentially the same. The relative abundance of elements is likewise similar and there appears to be no important difference in chemical composition between these nebulae and the diffuse nebulae such as the one in Orion. Such differences as appear in the lines measured can be attributed solely to differences in the physical conditions existing in the various nebulae. There is no evidence of a division of these nebulae into two sequences, carbon and nitrogen, as had previously been suggested.

An extensive investigation of the rotation of Messier 33, the spiral nebula in Triangulum, has been carried out by Mayall and Aller. This is the second great spiral to be so investigated, Babcock having made a similar study of the Andromeda Nebula, Messier 31, a few years ago. Mayall and Aller made radial velocity observations of twenty-five condensations which show bright-line spectra. These condensations were well distributed over the spiral. In addition, several long exposures were made which record the spectrum of the central unresolved nebulosity. As a preliminary to the determination of the velocity of rotation, the velocity of the system was determined to be —167 km/sec, and the inclination of the principal plane of the nebula to the line of sight was found to be about 33 degrees. Using these results, and making the assumption of simple circular motion in the plane of the spiral, rotational velocities for the individual condensations were obtained. These results were then combined to give a curve of rotational velocities. This curve shows that the main body of the spiral, which extends to 18 ft. from the center, apparently rotates very much like a solid body. In other words, in this part of the spiral the rotational velocity increases fairly uniformly with distance. On the other hand, the outer part of the spiral extending between 18 ft. and 30 ft. from the center shows the rotational velocity decreasing with distance from the center as would be characteristic of a planetary system. The transition from one type of rotation to the other occurs at a distance of approximately 1,000 parsecs from the center, the rotational velocity in this region reaching its maximum value of 120 km/sec. When these results are compared with similar ones for the neighborhood of the sun in our own Galaxy, we find encouragement for the current hypothesis that the sun is actually located at a very considerable distance from the center or main body of our own stellar system.