CONGRESSES AND CONFERENCES

THE FIRST CONFERENCE ON ASTROSPECTROSCOPY

Astronomical spectroscopy—the principal method of modern astrophysics—has developed very intensively and vigorously over the last quarter century. It has become an extensive branch of astronomy, closely connected with physics. This connection has two sides. On the one hand, astrophysics poses before physics a number of problems, both theoretical and experimental in character, and uses the results obtained by physics for the interpretation of phenomena observed in stellar spectra, in turn often assisting physics in the development of the theory of spectra. On the other hand, astrophysics in general, and astrospectroscopy in particular, make ever broader practical use in astronomical observations of the achievements of modern physics and technology—automation, electronics, interference and polarization instruments, etc. These reasons make the closest working contact between physicists and astrophysicists highly timely.

One means of establishing such contact through the exchange of experience and joint discussion of problems and achieved results was the Conference on Astrospectroscopy, held on August 16–19, 1950, at the Crimean Astrophysical Observatory of the Academy of Sciences of the USSR in Simeiz.

About 60 delegates took part in the conference, representing 22 physical and astronomical institutions of our country. The delegates arrived from Moscow, Leningrad, Kiev, Odessa, Riga, Lvov, Kazan, Abastumani (Georgia), Byurakan (Armenia), and other places.

The conference was opened by the director of the Crimean Astrophysical Observatory, Academician G. A. Shain. In his introductory address he recalled the enormous services to Russian and world science of the greatest Russian astrophysicist Aristarkh Apollonovich Belopolsky, the centenary of whose birth will be marked in four years. Academician Belopolsky was not only an astrophysicist but also an experimental physicist, a striking example of which is his brilliant experiment that confirmed the validity of applying the Doppler–Fizeau principle to optical phenomena. Belopolsky was an ardent supporter of close cooperation in the work of astrophysicists and physicists. Unfortunately, this sound idea is still far from sufficiently being put into practice. “...One may hope that this conference will to some extent promote greater mutual understanding, and perhaps even closer cooperation between physicists and astrophysicists,” concluded Academician Shain in his introductory address.

At the conference 23 papers were presented, of which 11 were devoted to questions of studying the physics of stars, 6 to the study of the Sun, 3—

on the study of the stellar system and interstellar matter, and one each on the study of the spectrum of a comet, the glow of the night sky, and a new method of photographing infrared radiation. New methods of observation and new designs of instruments were also broadly reflected in many papers.

It does not seem expedient to set forth the content of the papers in chronological order, but rather according to their subject matter.

In the paper by G. A. Shain and V. F. Gaze (Crimean Astrophysical Observatory), “On the Ratio of the Concentrations of the Isotopes C¹³:C¹² in Stellar Atmospheres,” the results were presented of the authors’ extensive work on the study of spectra of stars of spectral class N—low-temperature stars whose atmospheres are exceptionally rich in carbon. The most important result of the investigation should be considered the establishment, in the atmospheres of these stars, of a relative abundance of the carbon isotope C¹³, as compared with the isotope C¹², many times exceeding its relative abundance on the Earth and on the Sun.

A major paper by G. A. Shain, “The Variation of the Central Intensities of Spectral Lines in the Spectra of Stars of Various Spectral Classes,” was devoted to one of the very important and difficult problems in the theory of absorption lines in stellar spectra.

The central intensities of lines in the spectrum of a star are determined by the mechanism of their formation. If pure scattering takes place in the atmosphere of a star (a light quantum absorbed by an atom is re-emitted at the same frequency), the central intensity of the line must be equal to zero. In the case of true absorption (radiation of the absorbed quantum at the same frequency does not occur; for example, in the case of photoionization of an atom from an excited level), the intensity at the center of the line will be equal to the radiation at the given wavelength at the temperature of the outer layers of the star. Obviously, with this kind of central intensity it should be approximately the same for all sufficiently strong lines. Thus, the study of the central intensities of lines can provide valuable information about the mechanism of interaction between matter and radiation in the atmosphere of a star.

It is believed that the central intensities of lines are very strongly distorted by instrumental effects, and they have not ordinarily been studied for spectra with small and moderate dispersion. But experience in processing Simeis spectrograms shows that even with a spectrograph dispersion of 30 Å/mm one can obtain interesting results by using strong lines (equivalent width not less than 1–2 Å), especially the hydrogen lines.

The observed central intensities of the lines of a series of stars were compared with those calculated on the basis of the theory of the mechanism of true absorption; the dependence of the central intensities on: a) the spectral class and temperature of the star and b) on the line wavelength was studied. In both cases good agreement was obtained if one takes \(\overline{k}/k_\nu = 2\), where \(k\) is the so-called “Rosseland mean” absorption coefficient over the whole spectrum, and \(k_\nu\) is the value of the coefficient of continuous absorption near the line. This is somewhat unexpected; from general theoretical considerations one should rather have expected: \(\overline{k}/k_\nu = 1\).

The deviation of the observed central intensities from the theoretical ones may be due to a number of causes. The intensity at the center of a line may be increased by fluorescence in the line, i.e., as a result of the predominance of the number of transitions of type \(1 \to 3 \to 2 \to 1\) (a) over the number of transitions of type \(1 \to 2 \to 3 \to 1\) (b), where \(1\) is the ground state of the atom,

2 — its excited state and 3 — the state of ionization. Processes of type (a) will predominate over processes of type (b) in the upper layers of a star’s atmosphere, since the transition \(1 \to 3\) (ionization) is caused by continuous radiation, while the transition \(1 \to 2\) (excitation) is caused by monochromatic radiation in the line, which has already been strongly weakened in the lower layers of the stellar gas. The predominance of the number of transitions \(2 \to 1\) over \(1 \to 2\) will also cause an increase in the central intensity of the lines.

A second factor increasing the central intensity of lines will be the scattering of light by free electrons, whose number must be especially large in the upper atmospheric layers of white supergiants. The processing of Simeiz observations gives grounds for believing that the influence of this factor is found in a number of stars.

Processes increasing the central intensity of lines must be closely connected with the processes of formation of emission lines in their spectra. Indeed, there is a number of stars in which only \(H_\alpha\) or \(H_\alpha\) and \(H_\beta\) are observed in emission, while the central intensities of the remaining hydrogen lines are considerably greater than is normal for stars of the given class. The study of such stars is of great interest.

With the same task of detailed study of stellar spectra, another report by G. A. Shain and P. F. Shain was connected: “Certain Remarks on the Laboratory System of Wavelengths.”

One of the very important tasks of modern astrophysics is to establish the presence in stellar atmospheres of heavy elements (uranium, thorium) and rare earths. The spectral lines of these elements are very numerous, but weak.

At present, stellar spectra, at least the brightest ones, can be obtained with a dispersion of up to \(2\ \text{Å}/\text{mm}\). However, astronomers have encountered a new difficulty: weak lines of heavy elements in a star’s spectrum can be singled out only when all the weak lines of iron and other abundant elements have been identified. Unfortunately, it turns out that laboratory tables of wavelengths are far from complete and cannot satisfy astrophysicists. For example, in one region of the spectrum of \(\alpha\) Cygni, obtained with a dispersion of \(2\ \text{Å}/\text{mm}\), of 377 measured lines, 45 could not be identified, i.e. more than 11%. It is very likely that these are, for the most part, ion lines, laboratory data on which are very incomplete.

Furthermore, for astrophysics it is very important to study the internal motions of gas in stellar atmospheres, which can be detected from the differential shifts of lines of various elements; these shifts are very small. Such work has been done, and the results of some of it give grounds for assuming the presence of small systematic errors in laboratory wavelengths for various elements, especially ions. These errors are of the same order of magnitude as the shifts sought.

Therefore, for astrophysicists it is urgently necessary to expand and refine the laboratory tables of wavelengths of spectral lines by means of:

a) a more complete account and measurement of the wavelengths of weak lines of neutral atoms; in doing so it is necessary to take into account the Rowland tables of lines in the spectrum of the Sun;

b) new careful measurements of the wavelengths of ion lines;

c) the inclusion in the tables of the wavelengths of lines of atoms and ions predicted from known terms, even if not observed in the laboratory;

d) the introduction of the wavelengths of forbidden lines.

In this connection it is also necessary to take into account the new results of spectrophotometry of stars with large dispersion.

The fulfillment of this work, very important also for physicists, will naturally require much labor and time.

A group of papers was devoted to the study of the continuous spectra of stars.

As is known, the spectra of “new” stars at an early stage of an outburst are very similar to the spectra of stars of spectral classes A and F, i.e., stars whose outer layers have temperatures of about 10 thousand degrees. But in the spectra of “new” stars there is a complete absence of continuous absorption beyond the limit of the Balmer series of hydrogen, which is characteristic of the spectra of A and F stars. The report by E. R. Mustel (Crimean Astrophysical Observatory) was devoted to explaining the cause of this. He comes to the conclusion that the outer layers of novae must be almost isothermal; the temperature gradient in them must be very small. This may be caused by exceptionally strong turbulent mixing of the matter in them immediately after the outburst, accompanied by the ejection of matter. In the case of an isothermal layer of stellar gas, the lines of the Balmer series will be observed, but absorption beyond the series limit will not be observed, owing to the different mechanism of absorption in the lines and beyond the series limit.

B. A. Vorontsov-Vel’yaminov (P. K. Sternberg State Astronomical Institute, Moscow), in his report, presented the results of a study of the distribution of energy in the continuous spectrum of Wolf-Rayet type stars, which have bright emission lines. From the continuous spectrum, the temperatures of the Wolf-Rayet stars he studied proved to be close to 10–12 thousand degrees, whereas their temperatures, determined from the intensities of emission lines by Zanstra’s method, are close to 50 thousand degrees. The result obtained is not unexpected and agrees well with the theory, developed by N. A. Kozyrev, of the transfer of radiation in stars that have extensive atmospheres whose height is comparable with the radius of the star.

Another type of star with emission lines in their spectra, the Be stars, was the subject of the report by V. G. Gorbatskii (Leningrad State University). Starting from the theory of the transfer of radiation through a moving stellar envelope, developed by V. A. Ambartsumian and V. V. Sobolev, the speaker considers the observed continuous spectrum of Be stars as the result of the superposition of the continuous spectra of the star and its envelope. From the observed distribution of energy in the continuous spectrum, he seeks the physical parameters of the star and of the envelope.

The spectra of low-temperature stars of class M, in which, along with atomic lines, molecular absorption bands—primarily those of titanium oxide—are very strong, present a very complex picture. The distribution of energy in them has until now practically not been studied. P. P. Dobronravin (Crimean Astrophysical Observatory), in his report, presented the results of studying the distribution of energy in the spectra of three such stars. It is shown that even at the points of greatest intensity in the spectra of these stars there is very strong absorption, caused mainly by absorption in the bands of titanium oxide. This must be taken into account in all work connected with the study of the intensity of bands and lines in the spectra of class M stars.

Perhaps, by taking into account such a general weakening of the star’s radiation, it will also be possible to explain the exceptionally large amplitudes of long-period variable stars in photographic and visual rays, with comparatively small oscillations of their total, bolometric radiation. Strong absorption in bands of molecules in the visible and photographic region of the spectrum will “transfer” radiation to the infrared region, thereby producing considerable total radiation at low visible brightness. Much more work is still required in this direction.

For numerous statistical works connected with the study of the stellar universe, knowledge of the absolute magnitudes, i.e. the actual luminosities, of as large a number of stars as possible, especially faint ones, is extremely important. Such determinations can be made from relative intensities of certain lines in their spectra. But faint stars are accessible for spectrography only with low dispersion, using an objective prism, i.e. a prism placed in front of the objective of an astrograph. Such spectra are usually poor in detail; the lines in them are not very distinct, and it is necessary to find some reliable criteria for estimating from them the luminosities of stars.

Reports by R. A. Bartaya (Abastumani Astrophysical Observatory of the Academy of Sciences of the Georgian SSR) and L. S. Galkin (Crimean Astrophysical Observatory) were devoted to methods for determining the absolute magnitudes of stars from photographs with an objective prism.

The question of using photographs of spectra with an objective prism for an approximate estimate of the intensities of some spectral lines was the subject of a report by N. M. Goldberg (Main Astronomical Observatory of the Academy of Sciences of the USSR).

The construction of a modern stellar spectrograph is a rather complex task. The spectrograph must possess great rigidity, sufficient to ensure that the position of the spectrum on the photographic plate changes insignificantly over the course of an exposure lasting up to 2–3 hours; the temperature of the prisms and other parts of the instrument must be kept constant to within 0.1–0.2°. Violation of these conditions may make the instrument unsuitable for determining the radial velocities of stars. The spectrograph must include devices for photometric standardization of negatives, etc. A design for a spectrograph intended for fabrication for work with the large reflector of the Crimean Astrophysical Observatory was reported by V. A. Albitskii. The spectrograph will be a two-prism one, with interchangeable cameras giving dispersions (at \(H_\gamma\)) of 35, 75, 140, 300, and 500 Å/mm. For the two last dispersions the use of mirror meniscus cameras is proposed.

A number of reports were devoted to questions of studying the Sun.

V. A. Krat (Main Astronomical Observatory), in his report, presented the results of processing a spectrogram obtained with a quartz spectrograph (from 3200 to 6500 Å) during the total eclipse of the Sun on July 9, 1945. The negative contained the spectrum of the solar corona, the chromosphere, and a faint prominence which happened by chance to lie in the slit of the spectrograph.

The continuous spectrum of the solar corona shows no traces of Fraunhofer lines; assuming, as is generally accepted, that the continuous spectrum of the corona arises as a result of the scattering of sunlight by free electrons, the absence in it of absorption lines can be explained by the enormous velocities of the electrons, corresponding to a kinetic temperature of several million degrees. Faint traces of the sodium \(D\) line are visible only in the upper part of the spectrum; this line belongs to the spectrum of the outer corona, where the temperature is lower. From the presence of only this one line against the background of the spectrum of the inner corona, Krat draws a conclusion about the reddish light of the outer corona.

On the negative, all the known emission lines of the coronal spectrum are clearly visible. From the change in the intensity of the lines with increasing height above the photosphere, it proved possible to divide them into three classes, corresponding to the value of the ionization potential of the ions producing them: I—230–260 volts; II—320–360 volts, and III—450–700 volts.

The study of the spectrum of the chromosphere led the speaker to the conclusion that its electron temperature is about 30 thousand degrees. The degree of ionization of the various atoms in the chromosphere indicates, according to Krat, that ionization in the chromosphere is determined exclusively by radiation traveling downward, while the role of collisions is negligibly small. Some interesting conclusions were also obtained from the study of the spectrum of a prominence.

A great deal of work on developing methods of solar observation, carried out at the Crimean Astrophysical Observatory, and on new results from observations of the Sun, was covered in several reports.

In the report by E. R. Mustel and A. B. Severny, the results of a study of the spectra of chromospheric flares were presented. A chromospheric flare (or eruption) is the name given to a briefly occurring bright formation on the visible surface of the Sun (lasting from several minutes to half an hour). The spectra of these rapidly occurring phenomena have so far been poorly studied, and their nature has not yet been deciphered.

In 1948 two photographic cameras were attached to the spectrohelioscope of the Crimean Astrophysical Observatory, making it possible, simultaneously with visual observations, to photograph the spectrum of details on the Sun in the regions around the \(H_\alpha\) line and the \(H\) and \(K\) lines of ionized calcium. In 1948, 1949, and 1950 the speakers obtained many highly interesting spectrograms, some of which have already been processed.

It was shown that at the moment a chromospheric flare appears, thin emission lines appear in the centers of the broad \(H\) and \(K\) lines; in the \(H\) line the emission line \(H_\varepsilon\) is also sometimes noticeable. At the same time the \(H_\alpha\) line practically disappears, “filling in with emission.” In this process there is an increase in the residual intensity in the wings of the hydrogen and calcium lines, which the speakers interpret as the result of ionization of hydrogen and of the second ionization of calcium in the absorbing layer of the Sun under the action of intense ultraviolet radiation traveling downward from the chromospheric flare. The decrease in the number of absorbing atoms \(H\) and \(Ca^+\) causes a weakening of the lines. The height of the chromospheric flare above the visible surface of the Sun is estimated by them at approximately 2500 km.

In the report by I. S. Shklovsky (Crimean Astrophysical Observatory), questions of the study of solar coronal radiation in the far ultraviolet region of the spectrum were considered. The speaker shows that both radiation in the continuous spectrum of the corona due to hyperbolic transitions of coronal electrons and monochromatic radiation must be intense. Especially strong should be the lines 776 Å (\(\mathrm{Ne\ VIII}\)) and 625 Å (\(\mathrm{Mg\ X}\)). According to Shklovsky’s calculations, the hard coronal radiation must be the principal factor ionizing hydrogen and helium in the chromosphere and prominences. The same radiation fully accounts for the ionization observed in the upper layers of the Earth’s atmosphere.

A new method of observing the Sun was described in the following three reports.

In the report by A. B. Severny and A. B. Gilyar (Institute of Crystallography of the Academy of Sciences of the USSR), “Interference-Polarization Light Filters for Studies of the Sun,” the construction of such light filters and the results of work with them were described.

An interference-polarization light filter consists of a set of quartz plates of strictly specified thicknesses, placed between Polaroids. As a result of the interference of the ordinary and extraordinary rays, the filter proves transparent only for a very narrow region of the spectrum, a few angstroms wide.

At present there are only 4–5 such light filters in the world, used for observations of the Sun.

The first interference-polarization light filter was made by the speakers in 1947–1948 from domestic quartz. It gives a passband 1.8 Å wide, centered on the line $H_{\alpha}$. The position of the passband depends on the temperature of the light filter; therefore it must be thermostated with an accuracy of not less than 0.1°. The passband also depends on the inclination of the rays to the optical axis of the instrument. The useful field of the Simeiz light filter is about 1°.

With the aid of this light filter, mounted in an optical arrangement of the Lyot coronagraph type, one can successfully observe prominences and the chromosphere, but details on the disk of the Sun are still of low contrast. A motion-picture camera is attached to the installation; during 1948, 1949, and 1950 many motion-picture recordings of phenomena on the Sun were obtained. Some of them have been arranged as the first Soviet motion picture showing, accelerated by a factor of 700–1000, the motion of solar

Fig. 1.

prominences. Two photographs obtained with the aid of the filter are shown in Fig. 1.

The same motion-picture frames served as rich material for very interesting investigations of the motion of individual knots of matter in prominences. All the photographs were photometrically standardized, and it was possible, simultaneously with the study of the motion, to investigate also changes in the brightness of the details. It has already been possible to draw the conclusion that there is a connection between oscillations in the velocity of motion of the knots and the intensity of their emission, and also to study the character of the damping of the emission.

A further narrowing of the passband of the light filter to 0.5 Å was achieved by attaching to the filter a Fabry–Perot etalon or a spar plate. Such a narrow band already makes it possible to see details on the disk of the Sun, but it is too narrow for observations of prominences, since differential motions of matter in prominences shift, by the Doppler principle, the radiation to a greater magnitude.

The second light filter, calculated and manufactured by the speakers, is intended for observation of radiation at six wavelengths: 6562.8 Å ($H_{\alpha}$); 6374.5 Å (red coronal line); 5875.6 Å (the $D_3$ helium line); 5302.8 Å (green coronal line); 5183.6 Å (the $b_1$ Mg line)

and 4861.3 Å ($H_\beta$). The transmission bands of this filter have a width of about 3 Å. Exact adjustment of each band to the corresponding wavelength is accomplished by changing the temperature of the filter.

This six-band light filter has also shown very high optical quality; photographs of protuberances in the helium line have already been obtained with it. Comparison of photographs of protuberances in the lines $H_\alpha$, $D_3$, and $H_\beta$ makes it possible to draw many valuable conclusions.

The third interference-polarization light filter is intended for observations in the infrared part of the spectrum. With it one can make observations in the coronal lines 10 746 and 10 798 Å, and with crossed polaroids in the helium line 10 830 Å. Trial photographs have already been obtained.

In the report by A. B. Severny and G. A. Monin, “The Spectroheliograph of the Crimean Astrophysical Observatory,” the design of this instrument, developed by the speakers, was described. The instrument was built in the observatory workshop. Results obtained with the aid of spectroheliograms were demonstrated (Fig. 2)—photographs of the surface of the Sun in the rays of ionized calcium (left) and hydrogen (right), quite good in their distinctness.

Fig. 2.

In the brief communication “Experience in Interference Spectrophotometry of Fraunhofer Lines,” A. B. Severny described work carried out at the Crimean Astrophysical Observatory on studying the contours of lines in the spectrum of the Sun from photographs obtained with a spectrograph crossed with a Fabry–Perot etalon. The etalon was placed in a parallel light beam, between the grating and the camera objective of the spectrograph. A system of interference fringes is thereby superimposed on the spectral lines. Such a combination of a spectrograph and an etalon, when the angle of the interference fringes is varied, is equivalent to a spectral instrument with very large dispersion. The preliminary results obtained indicate the great effectiveness of this method.

As an addition to Severny’s communication, V. B. Nikonov spoke about the photoelectric spectrophotometer constructed at Simeiz—an instrument for the automatic recording of the structure of the solar spectrum. Its receiving part is a photomultiplier which, together with a narrow slit placed before it, moves along the spectrum of the Sun. Changes arising

which in this case is registered by a mirror galvanometer on photographic paper. In the instrument, automatic monitoring of the constancy of the receiver’s sensitivity and of the transparency of the atmosphere is provided. With its aid it becomes possible to study the spectrum of the Sun and details on its surface directly, bypassing photographic photometry with all its shortcomings. The spectrograms obtained by Severny and Nikonov attest to the great resolving power of the instrument and to the stability of the record.

The conference also devoted considerable attention to the study of the structure of our stellar system, the Galaxy, and of interstellar matter.

The nature of bright and dark diffuse nebulae, their composition, the mechanism of their glow, and their connection with stars have not yet been studied sufficiently. A report by G. A. Shain and V. F. Gaze, “The Study of Bright Gaseous Nebulae at the Crimean Astrophysical Observatory,” was devoted to this important problem.

Photographs in monochromatic radiation can provide much for understanding the physical nature of a nebula. Such photography was carried out in Simeiz with two very fast mirror cameras (diameter 450 mm, aperture ratio 1:1.4) with glass and interference light filters transmitting a narrow region of the spectrum. One of the light filters used transmitted radiation in a narrow band centered on \(H_{\alpha}\); the other, a band of the same width shifted toward shorter wavelengths outside the \(H_{\alpha}\) line.

The glow of a nebula may be caused by ionization of atoms of hydrogen and other elements by the distant ultraviolet radiation of a hot star and their subsequent recombination; in this case the radiation of the nebula will consist of individual emission lines, among which the lines of the Balmer series will be strong. But the cause of the nebula’s radiation, in the case where it contains not only gas but also dust particles, may be the scattering of starlight, which gives a strong continuous spectrum. Nebulae of the first type, gaseous ones, should be clearly visible on photographs with the first light filter, which transmits \(H_{\alpha}\) but strongly attenuates the light of stars and the glow of the night sky; on photographs with the second filter they should not be visible. Nebulae containing dust particles will be visible on both photographs. Thus it proves possible to separate purely gaseous and gas-dust nebulae.

The method proved very effective and, although the work was begun only recently, very substantial results have already been obtained. Many nebulae turned out to be purely gaseous—outside the \(H_{\alpha}\) line their radiation is practically absent and they are not visible. If dust is present in such a nebula, it is present only in very small quantity.

Other nebulae, on the contrary, show rather strong radiation even outside \(H_{\alpha}\), which indicates a significant presence of dust particles in them. It has been possible to discover many previously unknown nebulae, as well as extensive fields of weakly glowing hydrogen in interstellar space. Photography of a number of regions has been begun with the aim of compiling an atlas of photographs of the entire region of the Milky Way.

The observations obtained already provide material for a number of important conclusions. Monochromatic images of nebulae show the filamentary structure of the nebulae, which gives grounds for thinking that braking occurs when a moving cloud collides with other similar masses of matter.

The same photographs give grounds for believing that the connection of nebulae with the stars situated near them, by which they are illuminated, is beyond doubt; and the genetic—stellar and nebular—connection is of common origin. A very remarkable fact is that purely gaseous nebulae are associated with very hot stars of classes O–B0, while gas-and-dust nebulae are associated with stars B2–B5. This provides a basis for conclusions about the evolution of nebulae and stars; moreover, nebulae and stars of the first type must be younger. In the gaseous envelope surrounding a star, the condensation of gas into solid particles must in time occur, a process somewhat analogous to the formation of smoke. At the same time the star will also cool, passing from classes O–B0 to classes B2–B5. There are grounds for considering this evolution comparatively rapid—on the order of \(10^6\) years.

The conclusions set forth are preliminary. Much of value should be provided by photographing nebulae in other lines, in particular in the forbidden lines of singly and doubly ionized oxygen, which has now been undertaken. Experiments have also been made in photographing clouds of luminous hydrogen in extragalactic nebulae.

Dark interstellar matter is a serious obstacle to the study of the central parts of our stellar system—in the direction of the galactic center there lies a powerful cloud, concealing it from us, opaque to visible and photographic rays. The study of the region of the galactic center, however, is very important for understanding the structure of our stellar system and its position among other stellar systems.

It has long been known that dark interstellar matter is more transparent to rays of greater wavelength. Hence it is natural to attempt to observe the region of the galactic center in infrared radiation.

In the report by V. B. Nikonov, V. I. Krasovsky (Crimean Astrophysical Observatory), and A. A. Kalinyak (Main Astronomical Observatory), “Observations of the Region of the Galactic Center in Radiation with a Wavelength of about One Micron,” the results were presented of photographing the region of the galactic center by means of an electron-optical converter. A description of the authors’ first work, carried out in 1948, had already been given in UFN (vol. 38, issue 3, p. 446). The observations were continued and expanded in 1949. A large luminous body, similar to the known star cloud in the constellation Sagittarius and invisible on ordinary photographs, was again obtained on photographs taken in infrared rays. In Fig. 3, on the left, an ordinary photograph of the region in the constellation Sagittarius is shown; in it only the “ordinary” star cloud A is visible. The right-hand photograph of the same region of the sky was taken in infrared rays; in it, besides cloud A, a second, “invisible” cloud B is clearly seen.

The photographs of 1949 covered a larger region of the sky than the photographs of 1948. The speakers made preliminary photometric measurements of the negatives, which proved to be in good agreement with the results obtained at the Mount Wilson Observatory by an entirely different route. Apparently, there are grounds for believing that both star clouds, A and B, are part of the galactic nucleus, whose middle is hidden from us by an even denser cloud of dark matter, opaque even to rays of the wavelength used. Taking the distance to the galactic nucleus to be 7.5 kiloparsecs (about 25,000 light-years), one may estimate its dimensions at approximately 1,500 parsecs (5,000 light-years). The question of whether the new star cloud belongs to the galactic nucleus can be finally resolved after a study of the stellar composition of the clouds, the establishment

Fig. 3.

of the presence in them of variable stars of certain types, characteristic of the nuclei of other stellar systems, etc.

In recent years the existence of radio emission reaching us from outer space has been definitely proved. It is attracting much attention from astrophysicists and physicists; its sources have not yet been found. One of the possible sources of the occurrence of radio emission was the subject of I. S. Shklovsky’s report, “Monochromatic Radio Emission of the Galaxy and the Possibility of Observing It.”

Monochromatic radio emission with a wavelength of 21 cm should be observed in forbidden transitions between the components of the hyperfine structure of the level of the hydrogen atom. The source of such emission may be clouds of interstellar hydrogen. Shklovsky’s calculations show that the emission should have an intensity quite sufficient for the possibility of its detection. Another source of monochromatic radio emission may be transitions between the components of the Λ-doubling of the ground level of the OH and NH molecules of interstellar gas. The discovery of such emission is a task of the very near future, in the solution of which physicists must also take part.

In the report by V. I. Krasovsky (Crimean Astrophysical Observatory), “On the Nature of the Radiation of the Night Sky,” an attempt was made to interpret the nature of the bright bands in the infrared radiation of the night sky in the region up to 11,000 Å. This radiation was first detected by Krasovsky on spectrograms obtained with the aid of an electro-optical converter. Having considered possible mechanisms for the origin of these bands, the speaker attributes them to a forbidden electronic transition between the states \({}^{1}\Delta - {}^{3}\Sigma\) of molecular oxygen.

I. S. Shklovsky, speaking on the report, pointed out the possibility of another interpretation of the radiation discovered by Krasovsky: it may arise as a result of rotational-vibrational transitions of the hydroxyl molecule (OH).

B. A. Vorontsov-Velyaminov (GAISh, Moscow), in the report “The Spectrum of Comet 1942 Tevzadze 2,” communicated the results of photometric processing of spectrograms of this comet, obtained with an objective prism at the Abastumani Astrophysical Observatory. The distribution in the comet’s envelope of various luminous molecules (CN, C\(_2\), CO\(^+\), CH\(_2\)) was studied.

The report by Z. L. Morgenshtern (FIAN, Moscow), “The Use of Flash Phosphors for Photographing the Infrared Region of the Spectrum,” contained a presentation of a new method of photographing infrared radiation, the further development of which will open new and interesting possibilities for astronomers.

Closing the conference, Academician G. A. Shajn noted that all the reports made at the conference had clearly demonstrated the connection of modern astrophysics and physics both in the sense of problems and in the sense of research methods. The number of physicists who took part in the work of the conference was quite significant, but, unfortunately, the problems of physical spectroscopy were still little reflected in its work. Academician Shajn expressed the hope that this first meeting of astrophysicists and physicists would be followed by other meetings that would promote closer mutual contact.

The conference adopted the following resolution:

“Astronomical spectroscopy, one of whose founders was the great Russian astronomer Academician Aristarkh Apollonovich Belopolsky, has made enormous successes over the last quarter century. With each year the close connection between astrophysics and physics becomes more and more urgent, both in the posing and the solving—

problems, and in the sense of creating new methods and apparatus for observations. The fruits of this mutual assistance are clearly visible in the work of Soviet astrophysicists.

Unfortunately, however, the connection between astrophysicists and physicists is still not sufficiently close at present, and it must be strengthened. The present conference should be the first step in this direction.

Noting the great successes of Soviet astronomer-spectroscopists, and for the more fruitful further development of the work, the conference considers it necessary to strengthen contact between astronomers and physicists by:

a) the repeated regular convening of conferences on astrospectroscopy;

b) more active participation by astronomers in conferences on spectroscopy;

c) the presentation of astronomical survey reports at the academic councils of physical institutions, and of reports by physicists at the academic councils of astronomical institutions;

d) the periodic publication of articles on the most important questions of astrophysics and astrospectroscopy in the journal Uspekhi fizicheskikh nauk.

The conference considers it desirable to establish, within the Commission on Spectroscopy, a Subcommission on Astrospectroscopy.

The conference emphasizes the need to expand work on astrospectroscopy at the observatories of the USSR.

“The Organizing Committee of the present conference is instructed to take measures to implement the wishes expressed.”

On Sunday, August 20, the participants in the conference traveled to the Bakhchisarai region, where they viewed the new construction site of the Crimean Astrophysical Observatory of the Academy of Sciences of the USSR.

P. P. Dobronravin