While it has not yet been possible to give a detailed step-by-step treatment of the evolution of a single star from the main sequence to the white-dwarf stage, such a treatment is available for close binary systems. It has been shown that by calculating the evolution including mass exchange in a system of main-sequence stars of small mass and relatively large separation, one can follow the system to its final stage of a white dwarf and a more massive main-sequence star. This type of evolution arises when the original primary has exhausted its central hydrogen content when mass exchange starts, and the mass of its helium core is small enough so that electron degeneracy prevents the ignition of helium.

Roughly three types of evolution of spectroscopic binaries may occur:

Case (a): the primary fills its Roche limit already before the end of core hydrogen burning;

Case (b): the primary fills its Roche limit after core hydrogen exhaustion but before the onset of helium burning, and

Case (c): the primary does not fill its Roche limit before the end of core helium burning.

Since long-period variables of Population I as well as Population II may reach radii of 10 AU, case (c) will occur in systems with periods up to 100 years. Since about 50% of the A and B stars are spectroscopic binaries, many evolved spectroscopic binaries are expected to occur in the galactic system. Computations show that, using the mass-ratio distribution of unevolved systems derived by Kuiper, the number of systems with one evolved component is expected to be at least as large as the number of unevolved systems, in the spectral regions A and B.

This investigation was originally conceived as a follow-up on a work by Abt (1965), in which he finds the close binaries, with periods of about 1–100 days, and with spectral types of about A4-F2 (IV-V), to be all Am stars. The stars to be observed were chosen from the ‘Fifth Catalogue of the Orbital Elements of Spectroscopic Binary Stars’ (Moore and Neubauer, 1948). The criteria in making the selection were that the spectral types are A0-A2 and F5-F6 (IV-V) and that the periods are between about 1–100 days.

Spectra of 11 binary systems were obtained at the Coude focus of the 84-inch reflector of the Kitt Peak National Observatory during three nights in May 1967. In addition to the binary stars, 5 standard spectrum stars, i.e. α Lyr (A0V), θ Leo (A2V), σ Boo (F2 V), ι Peg (F5 V) and 110 Her (F6 V), were observed to provide comparisons. The dispersion used was 13·5 Å/mm. Typically 2 or 3 spectra were obtained for each object. A spot densitometer was employed to furnish calibrations for the plates.

In this paper, I shall be concerned with three related questions: (a) what is the evolutionary fate of close binary systems, (b) is this concept consistent with the number of evolved close binary systems, and (c) are these evolved systems the Am stars?

According to various studies, more than one half of stars exist as members of binary and multiple systems. Therefore any theory for star formation must account for this fact. In other words, the problem of the origin of binaries has a far broader implication than is implied as a part of binary-star study. For this reason one cannot overexaggerate its importance.

The U.S. Lunar Orbiter Spacecraft Program was conceived to search out potential Apollo landing sites, and to return detailed photographic coverage of the lunar surface for scientific study. The first flight was launched in August 1966, and the successful launch of the fifth orbiter was in August 1967. All five were successful and returned to Earth a large amount of photographic data.

Presented in this paper is a short description of the spacecraft system with particular emphasis on the photographic system. The intent is to supply information which will permit a better understanding of the photographic data to be presented by Dr. William Brunk in the following paper.

The principal goal of the Lunar Orbiter program was to obtain photographic coverage of selected areas of the lunar surface at resolutions of 1 and 8 m. As the success of the program was greater than originally anticipated, the photographic coverage was extended to include the entire front side of the Moon at a resolution one order of magnitude greater than possible from the Earth and almost all of the far side. In this paper, a selection of photographs obtained from all five missions will be presented.

The newly established Inter-Union Commission on Solar-Terrestrial Physics (IUCSTP) has inquired of selected experts about actions useful in the period of the next solar maximum, which need an organization and coordination on international basis. These inquiries have shown that one of the problems, in which a broad international cooperation might lead to substantial improvement of scientific results, is the coordination of ground-base and space-vehicle observations of the Sun and its active phenomena.

Therefore, in order to help the IUCSTP in its preparatory work, the chairmen of IAU Commissions 10 (Solar Activity) and 44 (Observations from outside the Atmosphere) agreed to hold a joint meeting of these two Commissions during the IAU Assembly in Prague, on ‘Coordination of solar observations made at ground-base Observatories and with space vehicles’. It was anticipated that other IAU members interested in this problem, particularly members of Commissions 12 and 40, would also take part in the discussion.

Solar Research during the last decade has definitely shown that almost all primary processes which build up the chromosphere and the inner corona, as well as the observable basic events leading to the optical manifestation of solar activity occur practically all in the subtelescopic range, as far as ground-based observation is concerned. I think here mainly of the great variety of phenomena occurring in the intergranular space (width <0″.3) and intersupergranular space (width <1″). Because of this situation we have only very few reliable information about the structural transition between photosphere and chromosphere, how the granulation goes over into the complex chromospheric structures as bright and dark mottles, spicules, threads, loops, or fibrils. In this region theory can still get along without being disturbed too much by observation. The situation is somewhat like looking into a human face with a resolving power of 1 or 2 cm!

Up to the present time the technical possibilities of space research were not quite adequate for obtaining the ultraviolet spectrum of flares that occupy only a small portion (less than 10″ of arc) of the solar disk. Up to now all observations have been made in integrated sunlight. The importance of the knowledge of UV spectrum of flares is hardly necessary to emphasize. For example we just could mention that if ultraviolet spectra were available for a flare say from 850 Å up to 2000 Å, we would be able to estimate such extremely important parameters as the number of hydrogen atoms N1 in the first quantum state (by using L-α, L-β, etc.), electronic temperature and density (from Lyman continuum) and other physical parameters of a flare which at the present time we try to derive by different indirect and inadequate methods. Several resonance (ultimate) lines are concentrated in this spectral interval and their careful examination in flares can bring additional important information about conditions prevailing in flares and in the underlying chromosphere. The same applies of course to the whole UV spectrum below 800 Å, and in particular to the resonance lines of HeI (λ 512 Å) and HeII (λ 304 Å), as well as to a number of lines of highly ionized atoms.

Extending from the present to the early part of 1969 there are three Orbiting Solar Observatories to be launched, and these will all be capable of constructing spectroheliograms of the Sun in solar emission lines of the EUV and X-ray region. The recently launched and highly successful OSO-III has obtained EUV and X-ray spectra with high-time resolution, but without spatial resolution on the solar disk. The later OSO satellites will provide spatial resolution of 1′ of arc to 30″ of arc, and will provide the basis for the extension to even higher spatial resolution in the future.

The comparatively short periods covered by these satellites, coupled with a real probability of only partial success, make it particularly important to obtain the fullest possible use of the data by implementing a complementary and simultaneous series of ground-based observations.

A tentative spectrum of the Sun in the X-ray region is shown in Figure 1 (De Jager, 1967). The quiet Sun emits a measurable spectrum at photon energies below about 3 keV. During the occurrence of solar flares an enhancement of the emitted energy, and a hardening of the spectrum is clearly visible. The soft X-ray bursts apparently show a maximum radiation flux at wavelengths of about 10 Å.

In the spectral region above about 10 keV hardly any quiet solar radiation is observable. During the occurrence of solar flares hard X-ray bursts are occasionally observed. From the point of view of observational techniques these bursts may be divided in the so-called deka-keV bursts, covering the range 10-200 keV, and the deci-MeV bursts in the range between 0·2 and 1 MeV. Many deka-keV bursts have been observed during the years 1966-67 by Winckler et al., by means of the OGO-I and OGO-III satellites (see Arnoldy et al., 1967). The existence of deci-MeV bursts has been doubted various times (see e.g. Chubb et al., 1966). Its reality seems now to be proved (see also De Jager, 1967). However, they may be much rarer than the deka-keV bursts, although it is not yet completely sure that the selection is not observational.

Thus far, only two experiments have detected solar γ-radiation with energy significantly greater than 200 keV. In both events the γ-ray emission occurred during a solar flare. The first observation was in 1958 by Peterson and Winckler (1959), who recorded a burst of radiation that occurred in less than 18 sec from a class-2 solar flare. The radiation spectrum peaked in the 200- to 500-keV region. Recently, Cline et al. (1967) recorded in the OGO-3 satellite three rapid γ-ray bursts in the 80-keV to 1-MeV energy range and measured the integral energy spectrum. The measurements were made on July 7, 1966, during the first high-intensity flare (importance 3) of the new solar cycle. Many attempts have been made to measure higher energy γ-radiation from the quiet Sun and from solar flares, but no flux has been detected; this is primarily due to the fact that no high-energy γ-ray detectors have viewed a major solar flare during the maximum of the optical or microwave burst. However, theoretical estimates of the flux of solar γ-rays, based on a simple flare model, indicate a readily detectable flux from a major flare even to photon energies of 100 MeV. It is therefore important that experiments be performed during the coming maximum of the solar cycle to investigate this region of the electromagnetic spectrum.

We briefly summarize the main contents of the papers presented by the contributors to the present Joint Discussion. Partly the remarks refer to the spacecraft payload, partly they aim at improving the ground observing facilities.