Bibliographical study of the principal experimental research of the last ten years on atomic and autoionization continua. We have restricted ourselves to the alkaline, alkaline-ferrous and rare gas groups. Moreover, with one exception, all the work cited is concerned with absorption spectra.

We indicate the connections with theory and give the most important of the theoretical references.

To assist in the interpretation of ultraviolet continuum observations, we present observational features of a grid of non-grey model atmosphères in strict radiative equilibrium which have effective temperatures in the range 10000 °K to 20000 °K and surface gravities 103 and 104. For three models, we add the blended wings of the higher Balmer and Lyman hydrogen lines and include in an approximate way further effects of line blanketing in the ultraviolet. Finally, we present a preliminary calculation of the effect of departures from L. T. E. on the continuous flux.

To examine the validity of the models, we compare the predicted continuous flux and predicted line profiles with the corresponding observed quantities for Vega and Sirius.

We consider the question of discrepancies between theory and observations in the ultraviolet from the viewpoint of these new calculations, and find generally good agreement with recent observations made by T. Stecher.

The blended profiles of 106 ultraviolet lines, including all those expected to have equivalent widths greater than 2 Å in the wavelength range 911,6 to 3000 Å, have been computed for Bz V and B0 V model atmospheres. Most of the lines are found shortward of 1410 Å, where they have deep cores and many have extensive wings. This blanketing amounts to 31% at Bz V and 20 % at B0 V.

A comparison is made between the ultraviolet stellar fluxes measured by three sets of observers. The comparison is extended to visible and near ultraviolet measurements. Absolute intensities are compared in all cases and an enhanced ultraviolet emission is noted in i Ori. The well known ultraviolet deficiency is found in other cases.

Ultraviolet fluxes at 2200 and 2600 Å have been measured for about 180 B and A stars using filter photometers in rockets. Forty-eight of these stars have been observed more than once, and an analysis of these data show the uncertainty of a single observation to be less than 20 %. A U-V color versus B-V plot of luminosity class III, IV and V stars with little reddening shows a degree of correlation consistent with the observational uncertainty. The data appears to be of sufficient quality for use in deriving B star temperatures and interstellar scattering curves.

Stellar brightness measurements in the far ultraviolet at wavelengths above Lyman α have been obtained from several experiments beginning in 1960. The 1960 data were obtained using mirror telescopes and gas gain ion chambers. They indicated far ultraviolet stellar brightness about 1 magnitude lower than predicted by theory. Subsequent Geiger counter data appear to support this conclusion. The reliability of rocket far ultraviolet stellar photometry, however, is still insufficient, to place great confidence in the above conclusions.

Stellar brightness measurements in the band 1150–1180 Å indicate that there may be a considerably greater deficiency in far ultraviolet stellar emission at wavelengths below Lyman α than above.

The ultraviolet radiation from normal elliptical, spiral, and irregular galaxies and from abnormal objects (radio sources, quasi-stellar objects, and Seyfert galaxies) is discussed. A list of expected emission lines is compiled from previously published lists for planetary nebulae. The problem of the passage of Lyman α radiation from the source to the observer is reviewed and possible Lyman α fluxes from various types of normal and abnormal galaxies are estimated for the case that most of the Lyman α generated in the source can escape from it. The abnormal galaxies should be the stronger Lyman α emitters. Finally, it is shown that quasi-stellar radio sources in the apparent visual magnitude range 17–19 can have redshifts in the range z = 1–2, and Lyman α can come into the wavelength region accessible from the ground. To facilitate line identifications in existing spectra of such objects, ultraviolet spectra of planetary nebulae and of the relatively bright object 3C273 are greatly needed.

A single photoelectric spectral scan of Jupiter in the ultraviolet is presented in the form of a geometric reflectivity. The reflectivity is then assumed to be due to Rayleigh scattering by molecular hydrogen. An upper limit to the amount of molecular hydrogen above the cloud layer of a 11 km atm. is derived.

This review concentrates primarily on the problem of interpreting the recent X-ray and γ-ray observations of celestial sources. The expected fluxes of hard radiation from various processes are estimated (when possible) and are compared with the observations. We compute the synchrotron, bremsstrahlung, and (inverse) Compton spectra originating from relativistic electrons produced (via meson production) in the galaxy and intergalactic medium by cosmic ray nuclear collisions; the spectra from π°-decay are also computed. Neutron stars, stellar coronae, and supernova remnants are reviewed as possible X-ray sources. Special consideration is given to the processes in the Crab Nebula. Extragalactic objects as discrete sources of energetic photons are considered on the basis of energy requirements; special emphasis is given to the strong radio sources and the possibility of the emission of hard radiation during their formation. The problem of the detection of cosmic neutrinos is reviewed.

As yet, no definite process can be identified with any of the observed fluxes of hard radiation, although a number of relevant conclusions can be drawn on the basis of the available preliminary observational results. In particular, some cosmogonical theories can be tested.

Ten discrete sources of X-rays have been identified from observations made with Geiger counters aboard unstabilized Aerobee rockets. The distribution of sources is flattened toward the galactic plane. Tau XR-1 is an X-ray source within one arc minute of the center of the Crab Nebula and of angular diameter one arc minute as determined from observation of a lunar occultation. The position of the strongest source, Sco XR-1, is known to about 0.5 degree and the remaining eight sources to about 1.5 degree. Three X-ray sources, Sco XR-1, Cyg XR-1, and Cyg XR-2, are not accompanied by any known optical or radio objects at their positions. Oph XR-1 matches the position of the Kepler SN 1604 and Sgr XR-1 is within 2.3 degrees of Sgr A. The remaining five sources are not sufficiently well resolved or positioned to permit identifications with optical or radio sources. The X-ray flux from the Crab Nebula is compatible with synchrotron spectrum with index — 1.1 and the flux of 1.8 × 10-8 erg cm-2 s-1 (1.5-8 Å) fits the extrapolated optical synchrotron spectrum with the same index.

Final results are presented for a March 20, 1963 search for night sky sources of soft X-rays. Although one sky region was found to be associated with an X-ray flux 2,5 to 3 standard deviations above background, the probability of observing one random fluctuation of this importance in the data for the night sky is estimated to be as high as 20%. The region is located at α= 23 h 40 m, δ = + 78°. Apparently negative results from a search of the data for variations of X-ray background with galactic latitude are presented and discussed.

A simple gray atmosphere has been fitted to a sequence of neutron star models with central temperatures ranging from 2 × 109 to 2 × 107 °K in order to estimate the X-ray emission and cooling times. A neutron star with a central temperature hotter than 5 × 107 °K would emit sufficient X-radiation to account for that observed in Scorpius. More detailed atmospheric models including the K absorption edges of carbon, nitrogen, oxygen, and neon were constructed for effective temperatures of 107 and 5 × 106 °K.

As yet no experiment, satellite-borne, balloon-borne or earth-based has provided compelling evidence for more than upper limits to the intensity of cosmic gamma rays of more than a few MeV energy. Even these upper limits have been useful in blocking in some of the large scale properties of energetic particles in interstellar and intergalactic space. Nevertheless, the smallest upper limit set on the intensity of diffuse gamma rays 3.10−4 cm−2 s−1 sterad−1 (from the satellite experiment in Explorer XI) is a factor of about 20 above the intensity prediction which can be made with rather good confidence for gamma rays made in cosmic ray collisions with interstellar atomic hydrogen.

Predictions of the gamma ray flux from the various discrete-source emitters of synchrotron radio noise are model-sensitive and in general appreciably smaller than existing upper limits. These upper limits are in the 3.10−4 cm−2 s−1 region for gamma rays of E > 5.107 eV and in the 5.10−11 cm−2 s−1 region for gamma rays of E > 5.1012 eV.

Upper flux limits at the earth for γ rays of energy greater than about 5 × 1012 eV, have been established for the quasi-stellar sources 3C 147, 3C 196, 3C 273, the Crab Nebula, and the magnetic variable 53 Cam. They are all of the order 10-10 photons cm-2. s-1. Corresponding upper energy limits at emission are of the order 5 × 1047 ergs s-1 for the quasi-stellar sources, and 5 × 1035 ergs s-1 for the Crab Nebula, if γ-ray absorption is assumed negligible. Possible absorption mechanisms are discussed.

Collisions of high energy cosmic rays with intergalactic gas produce various secondaries, including neutral pions that decay into high energy γ rays. The Landau-Milekhin hydrodynamical model for proton-proton collisions is used to calculate the pion production spectrum corresponding to cosmic γ rays of energy above 10 Gev. A source function for these high energy γ rays in space is found by combining the pion production and decay spectra with the primary cosmic ray proton flux. The resulting γ ray spectrum follows a different power law than spectra based upon the usual assumption of a line spectrum for the pions in the center of mass system of the colliding protons. The high energy γ ray intensity in space is calculated for a simple model universe. By comparison with previous estimates for the proton photoproduction process, it is found that proton-proton and proton-photon collisions appear to contribute about the same order of magnitude to the intergalactic γ ray intensity above ∼1016 eV.

1. 1) Visible region.

   When carried on board of space vehicles, telescopes can be used with their maximum theoretical resolving power, by avoiding the effects of atmosphere turbulence. As the atmospheric scattering is suppressed, observations of very low contrast phenomena are made easier. Closer observations of the solar system objects are made possible.

2. 2) Infra red region.

   Observations taken from 30 km high are freed of most of the atmospheric absorption. But some infra red observations require the use of satellites (for example scanning of the whole sky within spectral regions). Needed observations are reviewed.

Two flights of the balloon borne telescope Stratoscope II have been made for infra-red spectroscopic observations. The paper summarises the results of observing cool stars, Mars, Jupiter and the Moon.

Water vapor in the atmosphere of Venus was discovered in November 1959 by observations made at 80000-ft. altitude with a manned balloon-telescope system. This result was confirmed and measured far more precisely in February 1964 with an automatic unmanned system. The method of observing Venus water-vapor absorption in the 1.13-μ band, and the interpretation of that absorption, are described.

The limb-darkening laws predicted for several models of the atmosphere and clouds of Venus are compared with limb-darkening observations in the 8–13μ interval made from the ground and from Mariner II. Model A attributes the limb-darkening to pure molecular absorption by the atmosphere above the Gytherean cloud layer; Model B, to multiple scattering by cloud particles of arbitrary albedo and altitude distribution. In each model, the temperature regime is considered specified, alternatively by convective and by radiative equilibrium. In Model A, limb-darkening arises from the increase of temperature with atmospheric depth. In Model B, both the T — τ relation and the dependence of emissivity on the angle of emergence contribute to the limb-darkening. Each model can be made to agree with observation, with an appropriate specification of atmospheric and cloud parameters. With the angular resolution available from the vicinity of the Earth, a discrimination among these models is difficult to achieve. But a Venus flyby or orbiter which performs only somewhat better than Mariner II can potentially determine uniquely the cause of the limb-darkening, and provide significant new information on the structure and composition of the atmosphere and clouds of Venus.

It is shown that the limb-darkening observed in the radiometric measurements of Venus cannot be interpreted unambiguously in terms of cloud structure or vertical temperature distribution. In order to understand the nature of the Venus atmosphere in the vicinity of the clouds, these data have to be supplemented with accurate measurements in the absorption spectrum.