1938: Astronomy

Stockholm Convention.

The International Astronomical Union, which ordinarily convenes every three years, met Aug. 3-10, 1938, in Stockholm under the presidency of Esclangon, director of the Paris Observatory. Eddington was elected to succeed Esclangon and the invitation to hold the next meeting in Switzerland in 1941 was accepted.

New Astronomical Equipment.

The 82-inch mirror for the McDonald Observatory on Mount Locke, Texas, will soon be shipped to Texas where the building and mounting are ready to receive it. A formal dedication with distinguished visiting speakers is scheduled for May of the coming year. This telescope will rank as second largest in the world until the completion of the 200-inch telescope at Mt. Palomar, California. In the latter, the final grinding of the mirror is nearly completed. Then there remains the polishing.

The University of Michigan has recently received a disk which should give a mirror over ninety inches in diameter. The grinding will not be started, however, until the project, including the moving of the Observatory to a more favorable site, is adequately financed.

The skeleton spectrograph for the study of diffuse nebulae, constructed at the McDonald Observatory, consists of an equatorial mounting carrying a rectangular plane mirror and adjustable wide slit, from which light is reflected to a stationary plane mirror located in the continuation of the polar axis, and thence back to a point on the opposite side of the mounting from the rectangular mirror, where it passes through quartz prisms into a Schmidt camera. The length of the camera is thus effectively about 150 feet. It works very fast and with it, large areas can be quickly examined for the existence of emission lines.

Those present at the meeting of the International Astronomical Union last summer were astounded at the results obtained by Lyot in the clear air of Pic du Midi with telescopes designed to eliminate all stray light and with very clean optical parts. Without the advantage of an eclipse he has been able to photograph the bright lines of the corona and, with the introduction of a filter transmitting only the red line of hydrogen, has secured motion pictures showing the rapidly changing eruptive prominences in wonderful detail. The spectro-heliokinematograph has been developed by McMath for a similar purpose.

In the slit spectrograph very little of the light from a star gets through the narrow slit and the spectrum builds up on the plate to observable intensity only with long exposures. Bowen has arranged a set of narrow mirrors in such a way that they cut the image of a star into narrow strips, place them end to end, and send them all through the slit. He calls the device an 'image slicer.'

A great step forward in the efficiency and accuracy of the photoelectric photometer has been made by Kron at the Lick Observatory. He mounts two prisms in such a way that they can be moved at right angles to the optical axis until they are in position to receive light from two stars, such as a variable star to be studied and a constant comparison star. A third prism, in the optical axis, can be rotated to reflect first the light of one star and then the light of the other to the photoelectric cell. The necessity of repeatedly moving the whole cumbersome telescope from one star to the other and back with the attendant adjustments is entirely avoided, and the danger of changes in atmospheric transmission is minimized by making the observations on the two stars nearly simultaneous. With this 'photo-comparator,' as Kron calls it, he secures observations with probable errors as small as ±.0015 magnitude in six minutes.

Massachusetts Institute of Technology has received a gift of $647,700 from Godfrey L. Cabot the income of which is to be devoted to search for direct methods of converting the sun's radiant energy into useful power or storing such power for future use.

Planets and Satellites.

The news of the addition to the solar family of a planet or moon is exciting news. The last addition was the planet Pluto. Twenty-four years ago Nicholson discovered the ninth satellite of Jupiter. Last July by careful search he added Jupiter X and Jupiter XI. This kind of search is very difficult, for these objects are of the nineteenth magnitude — about one twenty millionth as bright as a first magnitude star — and, of course, they are only to be found very close to the very brilliant disk of Jupiter. Assuming that they are about as good reflectors as other satellites, we conclude, that they are only about twenty-five miles in diameter. The calculated orbits show that Jupiter X revolves about Jupiter in about 260 days and also in other respects is remarkably similar to VI and VII. Jupiter XI, on the other hand, falls into a group with VIII and IX revolving on the outskirts of the Jupiter system with a retrograde motion — opposite to the direction of nearly all other motions of revolution or rotation in the solar system — in about 700 days. These two sets of triplets offer very interesting problems.

Asteroids.

The Reinmuth 'Object,' classed as an asteroid and designated 1937UB, took on such an access of velocity right after its discovery that no one knew where to look for it and it was not seen again. With the pre-discovery positions from photographic plates on which it was subsequently found it was possible to compute an orbit of probably fair accuracy. Its point of closest approach to the sun is well within the earth's orbit, the perihelion distance being something like six-tenths of an Astronomical Unit; the eccentricity has the extremely high value of about 0.6; and its period is a little over two years. Calculations differ as to how far from the earth it was when nearest, but apparently it was not far outside the moon's orbit. In all these respects it strongly resembles Apollo, discovered in 1932, and Adonis, discovered in 1936. Their orbits are all strongly perturbed in their near approaches to the earth. Apollo, although observed over an interval of 17 days, has never been seen since and is pretty certainly lost. Conditions were more favorable for Adonis and it was observed over a considerably longer interval so there is reason to hope that it may be kept track of. There is very little hope for Hermes (as Reinmuth calls his 'Object'). One difficulty is its tiny size, perhaps a mile in diameter, which makes it exceedingly faint in reflected sunlight unless it is very close.

Comets; Aurora Borealis; Eruptive Prominence on Sun.

Hope long deferred must sometimes be very chastening to those who search the sky with telescopes night after night, month after month, in the hope of picking up a comet. Last year eight were found, this year one. And this one had been found once before (Gale 1927VI), so that there was some idea of when and where to look for it. It was very faint and was picked up photographically by Cunningham at Harvard.

The display of the Aurora Borealis widely seen over the earth on Jan. 25, 1938, of the coronal type and beautifully tinged with color, is said to have been the most brilliant for over two centuries.

On March 20, 1938, at the start of the routine daily observation of the sun with the spectro-heliograph on Mt. Wilson, a conspicuous eruptive prominence was seen over the north pole of the sun and about 296,000 km. above the chromosphere. It was then photographed repeatedly over a period of 2 hrs. 34 min., when observations were stopped by clouds. In that time it reached a height of 1,550,000 km., or 1.12 solar diameters, above the chromosphere — the greatest height ever observed. The velocity was observed to increase from 65 km/sec in two rather sudden changes in 200 km/sec.

Stellar Motions and Positions.

Boss's General Catalogue of proper motions and stellar positions, planned and started thirty-three years ago by Lewis Boss and carried to completion by Benjamin Boss and his colleagues at Albany, has just appeared. There are five volumes with entries of 33,342 stars. Its forerunner, the Preliminary General Catalogue, published in 1910, has hitherto been the standard source of material for all studies of positions and motions of the stars.

The published notes about the star Wolf 424 this year form an interesting bit of astronomical history. Kuiper made a tentative prediction in May that this star would be found to have even a larger parallax and be even nearer than Wolf 359. The argument was that it was also a dwarf red star (hence of about the same low intrinsic brightness) but appeared considerably brighter than Wolf 359. Then Reuyl announced that he had measured the parallax and found it to be much smaller than Kuiper had predicted, adding that the photographic image of the star appeared elongated. Kuiper then examined it carefully with the 40-inch lens and found it to be a double star. This solved the puzzle: two stars were contributing to the brightness and they were therefore much farther away than was at first supposed.

Variable Stars.

A generous grant has made possible the establishment of a Bureau of Variable Star Research at the Harvard Observatory. An intensive study is planned of nearly all the known variable stars brighter than the eleventh magnitude, some 2,000 in number, on the half million photographic plates that have accumulated in the last fifty years.

Binaries.

The star e Aurigae has been a puzzle ever since the suggestion was first made that it is an eclipsing binary with the unusually long period of 27 years. Eclipse lasts for nearly two years and the star stays at practically constant minimum brightness for about 11 months. Only one spectrum is visible and this persists right through the minimum although with some variations in the lines. Apparently the eclipsing star is practically dark and is semi-transparent to the light of the bright star. Kuiper, Struve and Stromgren have joined mental forces and picture to us a system of two stars, the bright one with 190 times the sun's diameter, the dark one 2,700 times. The orbit is inclined so that the bright star passes behind only the outer part of the dark star. This outer part is practically transparent except for an ionized shell, or 'Heaviside layer' produced by the bright star. This layer causes a weakening of the light of the bright star without greatly affecting the spectrum. There are a few other eclipsing binaries, such as z Aurigae, and the recently recognized VV Cephei, which appear to be comparable systems.

By photographing the spectrum of one star, then moving the telescope and photographing the spectrum of another star of a different spectral type, Hynek has produced artificial composite spectra duplicating natural composite spectra and confirming the opinion that duplicity is the most frequent cause of this characteristic. By photographing suitable pairs it was possible to judge what difference in brightness of stars of different spectral type one can have and still be able to see that the spectrum is composite. This is a very important question for eclipsing binaries where the certainty of the solution for the orbital elements often depends on the estimate of relative brightness of the two components from the intensity of the lines of their combined or composite spectrum. Various estimates have been given as to how bright the fainter component has to be in order to make its lines show on the photograph. Hynek finds that under very favorable conditions the difference may be as great as 5 magnitudes. This is a surprisingly large figure. Certain spectral combinations are very hard to detect.

Supernova.

Zwicky pictures a 'supernova' as a star in which a sudden enormous outpouring of energy is rapidly followed by a settling into a 'collapsed neutron star.' The tremendously increased gravitational force would slow down the light from the interior so that it would take infinitely long to reach the surface. And the light — even if it did arrive — would be of zero energy and hence incapable of affecting our instruments of observation.

Necrology.

During the year 1938 Astronomy has suffered severe losses in the death of George Ellery Hale, William Wallace Campbell, one-time director of the Lick Observatory, Francis G. Pease, William H. Pickering and Ernest W. Brown.