1940: Astronomy

Observatory Equipment.

Two observatories have added to their equipment for solar investigations during the year. The Harvard College Observatory has established a new high-altitude station in the Colorado mountains. This observing station, under Menzel's direction, has been equipped with a coronagraph of design similar to that devised by Lyot. The location of the station at an elevation of over 11,000 feet should provide the Harvard observers with exceedingly fine atmospheric conditions.

The McMath-Hulbert Observatory has placed in operation two new instruments for solar observation. A 24-inch reflector replaces the original 10-inch and a new 70-foot tower telescope with attached laboratory has been built. The addition of these new instruments makes possible the simultaneous observation of solar prominences in three dimensions and of the attendant energy output. The 70-foot tower telescope is to be used primarily for physical measurements, the attached laboratory being especially equipped for this purpose, leaving the 50-foot tower telescope and the new 24-inch reflector for studies of prominence motion. This new equipment and the novel observational techniques already developed at the McMath-Hulbert Observatory place this institution in the front rank of the world's solar observatories.

One important datum needed for a true understanding of the physical conditions which exist in our sun is an accurate determination of the amount of energy radiated by the sun at various wave lengths throughout the observable range of spectrum. Pettit at the Mt. Wilson Observatory, has made a study of this spectral-energy curve in the ultraviolet region of the spectrum, using a double quartz monochromator for his observations. After correcting for the effects of the earth's atmosphere, his observations give energy curves both for integrated sunlight and for the light from the center of the solar disk. These curves must then be corrected for the amount of energy cut off by the normal absorption lines of the solar spectrum, the so-called Fraunhofer lines. This is exceedingly difficult, for the determination of the true intensity of the continuous spectrum on which these absorption lines are superimposed requires the highest possible spectroscopic resolution. After correcting for these lines as far as possible, Pettit finds that the resulting energy curve in the ultraviolet is still quite different from that of an ideal black body. It is possible, however, that much of this discrepancy will be removed when more reliable corrections for the absorption of the Fraunhofer lines is possible.

Comets.

Five comets were discovered in 1940. Of these, two were previously known periodic comets which were rediscovered, whereas the remaining three were new objects. The comet which attracted most attention was that found on Harvard Observatory plates by Cunningham. This comet was approaching the earth when discovered, though still at a considerable distance. During the last month of the year it was visible to the naked eye in the very early evening, but soon disappeared from the view of northern observers because of its rapid southward motion.

The earth's atmosphere is an imperfectly transparent medium whose behavior shows considerable variation. It is therefore of much importance to the interpretation of astronomical radiometric observations that the amount and extent of the absorption produced by the earth's atmosphere should be determined with all possible accuracy. Adel and Lampland, at the Lowell Observatory, have made an extensive and careful investigation of the nature and amount of absorption produced by our atmosphere at various wave lengths in the infrared solar spectrum, their study covering the region from 5.5µ to 14.0µ. Their results will be of especial importance to planetary observers.

Meteors.

One means of determining the height above the earth's surface to which the atmosphere extends is the determination of the heights at which meteors appear. Numerous observers have noted apparent seasonal variations in these observed heights. McIntosh has made a study of all available data from the published real paths of sporadic meteors and finds a marked seasonal variation in the height of these meteors. The amplitude of this seasonal variation proves to be greatest in the lower levels of the atmosphere. McIntosh also finds that there is a similar seasonal variation in the width of the meteor zone. The cause of this variation lies apparently in the atmosphere itself, the greater height occurring in the summer due to heating and expansion of the atmosphere.

An important and interesting problem connected with the study of meteors is the question whether they are regular members of our solar system or come in from interstellar space. This question is of particular interest in the case of those very bright meteors frequently called fireballs. In 1868 Galle, the discoverer of Neptune, made a rather thorough study of the Pultusk fall of meteorites which occurred in that year. The orbit which Galle determined for this meteorite indicated that it entered the earth's atmosphere with a hyperbolic velocity; in other words that it had come in from interstellar space. Recently Wylie has made a study of this meteor and concludes that with one exception the data used by Galle were correct. This exception is the height of appearance of the meteor. Investigations of numerous fireballs by Wylie and by Watson have shown that the high velocities obtained by astronomers who use reports of the general public are frequently not real. The reason for this is that the adopted heights of initial appearance of spectacular fireballs, determined in more recent years, is less than that for the faint meteors, and furthermore there is very little variation in the well determined heights of initial appearance of fireballs which vary greatly in size and brightness. Wylie finds that if he adopts for the height of initial appearance of the Pultusk meteor a value commensurate with well determined heights for recent bright fireballs, this meteor turns out to have been a regular member of our solar system, with a 2.37 years period of revolution about the sun. It is possible that a number of meteors previously thought to have come in from interstellar space may actually have been regular members of the sun's family before being swept up by the earth.

Studies of the Moon.

A number of determinations of the temperature of the surface of the moon have been made during the past few years. Of particular interest are measurements of the change in the temperature of the lunar surface during a total lunar eclipse. Recent measurements of this sort have been reported by Pettit. His observations were made at the Mt. Wilson Observatory with a 20-inch reflecting telescope of short focal length and a sensitive thermocouple. Both a cover glass and a water cell were used to determine the temperature. A point near the center of the lunar disk was chosen for measurement. Pettit found that for this point the temperature dropped from 371° K to 200° K during the partial phases and then slowly dropped to 175° K during totality. From these measurements he found that the rate of radiation from the moon's surface was nearly proportional to the energy received by it from the sun. Using familiar radiation data and the known physical constants of lava, Pettit was able to determine the equivalent thickness of the radiating layer. An equivalent thickness of approximately one inch satisfied the conditions. Since solid rock is a relatively efficient radiator, Pettit's result indicates that the surface of the moon cannot be solid rock, but must be covered with some agglomerate such as volcanic ash or pumice.

Planet Venus.

Speculation concerning the possible existence of life on the planet Venus has continued over many years, encouraged to a great extent by the similarity of the size of Venus to that of the earth. Some years ago detailed investigations of the spectrum of Venus by Adams and Dunham showed that the planet's atmosphere contained a large percentage of carbon dioxide and that there was no trace of the presence of water vapor. One important datum necessary for a complete picture of the physical conditions existing on Venus is the surface temperature. Recently reliable laboratory measurements of the total radiation of gaseous carbon dioxide have been made. Wildt has made use of these measurements to determine an upper limit to the 'greenhouse effect' which is produced by the carbon dioxide in the planet's atmosphere. He finds from his study that the temperature of the sub-solar point on Venus is somewhere between 366° K and 408°K, the true temperature being probably closer to the higher value. Wildt concludes from this that the surface temperature is at least somewhat higher than the terrestrial boiling point of water.

When a mixture of water vapor and gaseous carbon dioxide is exposed to sunlight, a photochemical process takes place, with the synthesis of formaldehyde as a result. Although the atmosphere of Venus is now nearly, if not entirely, devoid of water vapor it is possible that Venus during its earlier history may have had a tenuous atmosphere in which both carbon dioxide and water vapor were present. Wildt finds no trace of formaldehyde bands on recent ultraviolet spectrograms of Venus. However, he points out that in a moist atmosphere formaldehyde would rapidly polymerize into a mixture of polyoxymethylene hydrates. This mixture is a white crystalline solid, and Wildt believes that the dense clouds in the atmosphere of Venus, since they cannot be water vapor, may consist of these solid polyoxymethylene hydrates. Astronomers at the Lowell Observatory have suggested that the exceptional brightness of the halo observed about Venus when it is between us and the sun may be due to 'dust' particles. Wildt's hypothesis is therefore of particular interest.

Publications.

Two new astrophysical monographs, sponsored by the Astrophysical Journal, have been published during the year. One, Spectra of Long-Period Variable Stars by Merrill, presents in concise form the results of the author's long study of these interesting stars. From a detailed discussion of the various changes observed in the spectra of these stars during their period of variation, and a discussion of measurements of total radiation and of light variation in different wave length regions, Merrill concludes that pulsation, such as observed in the case of Cepheid variables, cannot account for the observed light variations. He points out that at the low surface temperatures of these stars a relatively small change of temperature will produce a considerable change in the observable light, though the observed temperature changes are not sufficient to account for all the light variation. Another source of light variation is found in the considerable absorption of light in the spectra of these stars by bands of titanium oxide, and this band absorption will account for some though not all of the observed variation. A third possible source of light variation, suggested by Merrill, is the formation of a cloud or veil of either solid or liquid particles which are condensed from the gases of the star's outer atmosphere during certain phases of the light variation. It has been shown by Wildt that such condensation of compounds of low volatility is probable in the atmospheres of the coolest stars. It seems likely, from Merrill's discussion, that all three possible causes may contribute to the large range of light variation observed.

The other astrophysical monograph, The Masses of the Stars by Russell and Miss Moore, presents the results of the most extensive study of stellar masses ever made. All of the available data on visual, spectroscopic and eclipsing double stars which have been adequately observed have been included in this investigation. Of particular interest among the results obtained is the authors' conclusion that the relation between the masses and the luminosities of the stars may be represented by a simple linear function. The determination of this simple relation between mass and luminosity from so large an amount of data is of very great importance for the solution of many astrophysical problems. The monograph also includes a catalogue of the newly determined dynamical parallaxes of 2,529 stars.

Recent Investigations.

It has long been known that the component stars of a close binary system have a more or less ellipsoidal figure. The light-curves of short-period eclipsing stars show this very clearly. Several investigators have pointed out that the force of gravity at different points on the surface of such a star is different due to the unequal distances of these points from the center of gravity of the star, and that this gravity-effect will cause an unequal distribution of light over the surface of the star. Recently Sitterly has made a preliminary investigation of the effect of this unequal distribution of light on the observed light-curves of these stars. He finds that for the very close pairs this effect can be easily observed and that it must be allowed for in the determination of the photometric elements. Sterne has investigated the effect of this unequal light-distribution on spectroscopic observations and finds that when the gravity-effect is not allowed for, the orbital eccentricity for close pairs determined from these observations is too high. However, he finds that if proper allowance is made the corrected eccentricities agree with photometric results for the same stars, thus solving a problem which has greatly puzzled astronomers for some time.

During recent years a large number of stellar parallaxes, both those determined from trigonometric measurements and those determined from relative intensities of carefully chosen pairs of lines in stellar spectra, have accumulated. Russell and Miss Moore have made a detailed study of the errors of spectroscopic parallaxes, both systematic and accidental, based on the parallaxes of 1,140 stars for which reliable determinations by both methods were available. Certain deviations from a linear correlation between the mean reduced spectroscopic and trigonometric parallaxes are apparent but these, they find, are no larger than can be accounted for by ordinary accidental and sampling errors. It is suggested that most of the apparent errors of spectroscopic parallaxes may be accounted for by differences in the relative abundance of hydrogen and the heavy elements, and by differences in the magnitude of the contribution made by negative hydrogen ions to the general opacity of stellar atmospheres. These investigators find that after appropriate corrections have been applied, the probable error of a corrected spectroscopic parallax is ±21 per cent for main-sequence stars and ±16 per cent for giants.

Numerous studies of the magnitudes, distances, velocities, and structure of extra-galactic nebulae have been made during recent years. Little attention, however, has been paid to the colors of these objects. Of considerable interest, therefore, is a recent study of the colors of seven of these galaxies by Seyfert. Using red- and blue-sensitive photographic plates he determined both red and blue magnitudes of various features in each of these nebulae. The observed differences between red and blue magnitudes determines so-called red indices which are effectively indications of color. Seyfert finds that these nebulae exhibit a general background intensity of light of the same color as the nucleus, which decreases uniformly outward. The spiral arms, however, and the nebulous condensations observed in them are found to be considerably bluer than the nucleus.