A radio-astronomical observation in its most complete form is a determination of intensity and polarization as functions of frequencyf, time t, and position in the sky. An actual observation is usually much less complete.

Towards the end of February 1968 the astronomical world was staggered by a paper from the Milliard Radio Observatory at Cambridge announcing the discovery of an astonishing periodic phenomenon. The characteristics of the pulsating radio source—or pulsar as it came to be called—involved a fantastic multiplicity of time-scales. The duration of the individual events was measured in tens of milliseconds, the repetition rate was of the order of a second, the pulse amplitude showed drastic variations over times of seconds, minutes, hours and even months and, lastly, the stability of the basic periodicity indicated a time-scale of millions of years. A series of pulses from CP 1919, the first pulsar, is shown in Figure 1, and one notices here both the regularity of the pulses and the variation in their amplitude with time. When the individual pulses were observed on an expanded time-scale it was found that the pulses were made up of sub-pulses (Figure 2) and that there was considerable structure even down to a millisecond time-scale.

The internal constitutions of the terrestrial planets Mars, Venus and Mercury are investigated through the use of ‘equations of state’ empirically derived for particular internal zones of the Earth. The equations usually take the form of tabular relations between the pressure p and density p, temperature dependence being treated as of secondary consequence. In using p rather the full stress tensor in solid zones, i.e. in using a hydrostatic theory, the effects of strength and deviatoric stress are also treated as of secondary consequence. Except for the use of data on the ellipticity ∊ of figure of a planet to provide evidence on the moment of inertia I, the planets are treated as spherically symmetrical.

Until early in 1968 radio sources were believed to be relatively constant and slowly evolving objects. Sporadic intensity variations had been found at high frequencies in some quasars but, except for the Sun and some flare stars which exhibited occasional bursts of radiation, the time scale of changing emission appeared to be very long.

A typical stellar spectrogram shows a vast number of spectral lines. Each of these has its characteristic shape and strength which must, in some way, reflect the structure of the atmosphere in which the radiation arose. It seems reasonable that from all this profile data we should be able (and with considerable redundancy) to infer a good deal about the physical structure of the radiating gas, and a major effort has correspondingly been devoted to clarifying the physical basis of spectral line formation, i.e., exactly how the atmospheric structure and the atomic properties are reflected in the line profiles. This problem, however, is far from solved : Few, if any, of the profiles of strong lines can be predicted in detail from model atmospheres, nor have analyses of the profiles yielded unambiguous data on the atmospheric structure. Indeed, as recently as 1967, the participants at a conference in Bilderberg (Holland) concluded that no data at all which had been obtained from line profile analyses was worthy of inclusion in specifying the solar atmospheric model. Evidently, then, the problem of line formation is not trivial ; in this paper we discuss some of the difficulties and review the not inconsiderable progress which has been made in this area of astrophysical research.

At a previous meeting of the Society I well remember Paul Wild saying, in a humorous vein of course, that until recently he had always thought that an H-R diagram was a plot of H against R. It occurs to me that the same kind of vagueness, although perhaps not to the same degree, might be present with astronomers not directly concerned with the theory of stellar atmospheres in regard to ‘L.T.E.’ and ‘Non-L.T.E.’ analysis of spectral lines. In the present paper, I would like to point out some of the important features of non-L.T.E. analysis, to indicate how the basic theory may be developed, and to show what effect this has on the theoretically produced absorption lines. I will make particular reference to the solar atmosphere and here we take the ‘model atmosphere’ approach where all the physical parameters such as gas pressure, electron pressure, temperature etc., are all specified as functions of optical depth.

The radio installations at Culgoora Observatory evolved from the work carried out at Dapto field station between 1952 and 1965—which in turn was based on earlier observations. The basic instrument at Dapto was a radiospectrograph which produced two solar spectra per second over a frequency range originally of 40-210 MHz and finally of 5-2000 MHz. Until 1957 the Dapto radio spectrograph was the only one operating in the world and it fell upon this instrument to reveal many of the spectral phenomena which are now well known. The spectrograph observations referred to the total flux from the Sun observations with high directivity began at Dapto in 1958 with the introduction of a swept-frequency interferometer which measured the one-dimensional (east-west) positions of bursts and their approximate angular size over a continuous range of frequencies between 40 and 70 MHz. The results obtained from this combination of spectrograph and interferometer indicated that great advances would be made in our knowledge and understanding of the phenomena if two-dimensional metre-wavelength pictures of the Sun could somehow be recorded at short time intervals of about Is—again in combination with spectrographic observations. This requirement led to the start of the radioheliograph project. One requirement for this instrument was a site with linear dimensions of the order of 3x3 km. This was far too large for the Dapto site and a new site was selected at Culgoora in the north-west plains of New South Wales. The virtues of this site are its size, flatness, freedom from flooding, low radio noise level and accessibility from Sydney by air transport. Its sunshine and optical-seeing properties also made it a highly desirable site for optical observations, and developments assumed a new significance when Dr. Giovanelli and his optical colleagues decided to join us at the same observatory.

Since 1965 we have been unravelling the curious properties of the intense 18 cm emission lines observed from OH molecules. We are still without a quantitative theory of the emission that might tie the threads together and provide guidelines for the observational work. The most plausible hypothesis is that we are dealing with naturally occurring interstellar gas masers. I want to present some of the latest evidence we have in favour of the maser action, and to indicate where it is inadequate. I will concentrate on work done in 1967, as the earlier work has been discussed in detail elsewhere.

About 13% of bright galaxies are ellipticals (21% SO, 64% Spiral and Im, 2% IO and peculiar). Their obvious features are the smooth luminosity distribution and the near absence of the bright blue stars, HII regions and dust that appear in spiral systems; there is probably very little star formation going on at present in E galaxies. I plan to discuss some of the dynamical and physical properties of these systems; although many of them are known radio sources, I will not be able to discuss their radio properties.

One of the interesting questions in solar X-ray astronomy is the question as to whether the hard X-ray emission which occurs during major solar flares is a thermal or nonthermal phenomenon. The evidence for non-thermal emission has been based in large measure on a balloon-borne experiment by Peterson and Winckler, which constituted the first detection of high energy flare X-rays. In the Peterson-Winckler experiment, the incident solar X-rays were measured by both an ion chamber and a Geiger counter photometer, and from the ratio of responses, the hardness character of the incident X-rays was reduced. It was concluded that the observed result could have been explained in terms of the sudden non-thermal production of a group of electrons with energy of the order of 500 kilovolts.

Until comparatively recently solar activity was discussed in terms of independent centres around the Sun’s surface. These centres, which form a constantly but slowly changing pattern, were thought to be linked only beneath the photosphere by tangled magnetic fields, themselves dominated by the mechanical energy of mass motions at photospheric and lower levels. Recent discoveries now indicate that active centres interact with one another above photospheric level and especially through linkages within the corona. The linkages appear to be of at least two distinctive kinds: magneto-hydrodynamic shock waves capable of travelling vast distances through the corona at ∼103 km/s and triggering eruptions at distant centres; and magnetic field lines which loop high and wide in the corona and convey energy in the form of fast particles. These two kinds of interaction will be considered in turn.

This review is concerned mainly with the solar modulation of galactic cosmic rays and the progress that has been made since the Tenth International Conference on Cosmic Rays (Calgary, June 1967), My aim is to set out some of the advances in the basic theories; to indicate why certain disagreements between convection-diffusion theory and observations (noted in Webber’s invited review at Calgary) are to be expected; and to indicate some of the currently emerging areas of interest.

It has long been known that Fraunhofer lines show variations in intensity from place to place over the Sun’s surface, these being particularly noticeable in spectroheliograms obtained in the strong chromospheric lines. An early account of the weaker Unes was given by d’Azam-buja. McMath, Mohler, Pierce and Goldberg attributed intensity increases in (or decreases in depth of) metallic spectral lines to local temperature increases. Sheeley used high-resolution spectra to study these line weakenings further, finding them where, and only where, strong non-spot magnetic fields occurred. He also reported that in many cases the continuum in such regions was of reduced intensity, suggesting that fields often occur in the dark lanes and pores in the granulation. Spectroheliograms obtained in the cores of the weakened lines (e.g., Fel 6302.5Å) by Chapman and Sheeley showed that the bright network which these weakenings form appears as a sequence of sharp, bright points in the cores of the fainter lines and the wings of stronger lines but is more diffuse in the cores of stronger lines. They found the bright network in Zeeman-insensitive Unes (e.g., 5123.7A) also, indicating that it is due, at least in part, to variations in physical conditions. Lines of low ionization and excitation are weakened more than those of high ionization and excitation, and they attributed this to a temperature increase by 100-200 °K in the region of formation of the line cores ; a similar increase of 250 °K was found by Harvey and Livingston.

In April of this year agreement was reached between the British and the Australian Governments to build a 150-inch telescope for observations of the southern sky.

The agreement specified that:

• (i) the telescope would be built at the Australian National University’s present site at Siding Spring Observatory;

• (ii) the expected cost was $11 million and the telescope would be built and operated on a 50-50 basis as between the two Governments;

• (iii) the telescope would be virtually a copy of the existing Kitt Peak 150-inch design. This was intended to save the time usually needed for the design of a new telescope and, as we shall see, it might lead to considerable savings in cost.

The Sun being a highly ionized gas, the basic physical quantities most needed for understanding solar phenomena are temperature, pressure (or density), velocity and magnetic field, together with their variations in space and time. The general aim at Culgoora is to secure observations from which these quantities may be derived (insofar as is practicable) simultaneously over extended solar regions. To do this has involved the development and use of filters of high spectral and spatial resolution, the study of atmospheric seeing and ways of securing high-resolution observations, and the study of methods of analysing observations to yield the physical quantities needed.

The atmosphere of a star presents a real challenge to the astronomer or physicist. We can observe but the thinnest skin of the star; yet from the information obtained therefrom we must deduce the star’s temperature, chemical composition, its rotation (if any), the state of agitation in its outer layers, and eventually the surface gravity, total luminosity, mass, and evolutionary stage.

This paper describes a preliminary series of observations of the Sun made at a frequency of 80 MHz with the 3 km radioheliograph of the Culgoora Observatory. The instrument records, at one-second intervals, pictures of the solar image in the form of 60 (E-W) × 48 (N-S) points, each separated in angle by half the Rayleigh limit (2’ arc in the zenith). At the time of the present observations the instrument was incomplete in three main respects : (a) the facilities for recording opposite senses of circular polarization were not available; (b) the automatic image compensation for zenith-angle foreshortening was not available—hence the optical disk of the Sun appears elliptical; and (c) the phase and amplitude calibration procedures had not been fully established, resulting in a higher sidelobe level than that specified in the design—the effects are sometimes evident in the pictures as spoke-like brightenings.

It is known that in the radio spertrum the limb of the quiet sun is brighter in the equatorial regions than near the pole. But most of the available theoretical calculations of the brightness distribution over the quiet sun have been made with the assumption of spherical symmetry. We have therefore calculated two-dimensional distributions at several decimetre and metre wavelengths, taking account of the observed asymmetry in the north-south direction. Newkirk’s method of ray-tracing was used, the calculations being made with a CDC 3600 computer. Some of the preliminary results (particularly for a sunspot minimum period) are presented here; they indicate that the electron temperature of the solar corona has a value of about 1 to 1.5 x 106 °K.

The first meaningful observation with the 80 MHz Cul-goora radioheliograph was made on 2 September 1967, a few weeks before the instrument was officially commissioned. Systematic observations, normally for 4 to 6 hours per day, began late in February 1968 and the Sun has since been observed on 438 days. The remaining days and many nights have been used for investigations of radio sources and the interplanetary medium and for maintenance and instrumental extension. The standard of reliability thus achieved has been due to the sustained efforts of K. V. Sheridan, W. J. Payten, K. R. McAlister, M. Beard and their colleagues. These first two years have been a period mainly of exploration—a phase of great interest to those engaged in observing, though possibly bewildering to those not. The present review is a first attempt to collate and summarize what has so far been found and where possible to draw preliminary conclusions. The observations to be described and their discussion have been due to a joint effort on the part of a number of workers, particularly G. A. Dulk (on leave from the University of Colorado), K. Kai (on leave from the University of Tokyo), N. R. Labrum, D. J. McLean, A. C. Riddle, K. V. Sheridan, S. F. Smerd, R. T. Stewart, and the present writer. The period has also seen considerable development in the evolution of intricate computer programmes for analysing the data, particularly on the part of L. H. Heisler and D. J. McLean.

The term white dwarf was originally introduced, some 50 years ago, to describe intrinsically faint A-type stars. Spectroscopic studies and theoretical considerations confirm the suggestion that these are degenerate objects. Photographic colorimetry of faint proper motion stars can readily distinguish the blue objects and in this way several hundred white dwarf candidates have been discovered, mainly by Luyten. Accurate photometry of about 1000 of these stars has led to accurate estimates for the space density of 1.5 = 10−3 pc−3. However, subluminous late-type stars are more difficult to detect among the multitude of main sequence stars and some additional criteria are needed.