Second harmonic plasma emission is attributed to the coalescence of two Langmuir waves into a transverse wave.

Perhaps the most direct evidence to date for shock wave acceleration of electrons in the solar corona is provided by radio observations of Type II bursts containing herringbone structure (Roberts 1959). On spectral records the herringbones appear to resemble miniature forward and reverse drift Type III bursts extending above and below the Type II backbone.

Sources of the Type I storms are thought to lie on closed magnetic loop systems above bipolar sunspots. Usually one sunspot is stronger than the other in the bipolar magnetic structure, and accordingly only one radio source associated with the stronger sunspot is usually observed. Occasionally however a double radio source is observed in conjunction with the bipolar sunspots. In such a case the double radio source should be intrinsically bipolar, viz. the two components should emit opposite senses of circular polarization (in this paper we use the word polarity for simplicity).

Distributions in longitude of solar radio bursts often are compiled by identifiying the position of the burst with the position of the associated H-Alpha flare. Early work on the longitudinal distribution of type II bursts compiled in this way (eg. Maxwell and Thompson 1962) indicated an approximately uniform distribution. Subsequently Svestka and Fritzova — Svestkova (1974) and Svestka (1976) published a distribution of 244 H-Alpha flares with which type II bursts were associated (hereafter called type II flares) that showed marked deficits near central meridian and the limb (Fig. 1). They suggested that the distribution was a product of propagational selection, being in some way dependant on the nature of the type II source mechanism as well as the manner by which the radiation reached the observer. On this basis they argued that the number of type II bursts that occurred near central meridian and near the limb was underestimated.

Solar type III radio bursts often occur for prolonged periods (≳ 1 hour) with small time intervals (≲ 1 minute) between bursts. Such phenomena usually are confined to decametre wavelengths (frequency ≲ 100 MHz) and are called ‘type III storms’ (e.g. Stewart and Labrum 1972).

As part of an investigation of solar radio bursts at high time (≈ 1 msec) and frequency (≈ 10 kHz) resolution, observations have been made between 30 MHz with the Llanherne broadband radio telescope (Ferris 1980). The radio frequency signals in the six frequency ranges 30-33, 33-36, 38-41, 54-57 and 79-82 MHz were recorded on six video tape recorders for about 10 minutes each day near solar transit.

The term ‘moving Type IV’ denotes a spectacular class of solar event in which the radio source, beginning low in the corona, moves outward, typically at hundreds of kilometres per second (Wild et al. 1963).

Flux density changes at low frequency in extragalactic sources were discovered in 1969 by Hunstead (1972a) and this low frequency variability has since been confirmed in the range 318 to 962 MHz at five other observatories. The changes are usually small and near the stability limit of most telescopes, but occasionally large changes ~ 30% are found. Many sources show changes at both low and high frequencies, but these appear uncorrelated even with delay lags of several years, and the likelihood of low frequency variability in a source cannot be predicted from high frequency behaviour. This paper reports statistics on the proportion of sources that showed changes of different amplitudes during a monitoring program with the Molonglo Cross in 1975/76.

Radio measurements of the outer planets at wavelengths longer than ~ 10 cm are difficult: the emission is weak (generally following an optically thick thermal spectrum — i.e. S ∝ λ-2) and the confusion due to background sources may be large.

Since the discovery of the first ‘head-tail’ radio source (Ryle and Windram, 1968) extensive radio surveys of clusters of galaxies have revealed a surprisingly diverse range of source morphology. In additions to the familiar symmetric double-lobed shape, several other morphological types are now recognised; these are presumed to be due to interaction between the source and a relatively dense, hot plasma pervading rich clusters. In particular, rapid motion of a radio galaxy with respect to this inter-galactic medium appears to produce a ‘bent-double’ shape, or in more severe cases, a ‘double-tail’ or ‘head-tail’ shape. In such cases, the source no longer has the simple symmetry of ‘ordinary’ double structure and it is clear that the attitude of the source with respect to the line of sight may markedly affect what is observed. Results and details are given below of a simple computer code used to simulate the appearance of a hypothetical bent-double source seen in projection. It is emphasised that the model has been chosen as one that can represent in a simple way many of the observed morphological forms and is not based on any physical theory of radiation or emission processes.

The origin of QSO redshifts is an important problem for which there is still no definite answer (Burbidge 1979). Recent observations by Walsh et al. (1979) of what is probably a gravitational lens associated with the twin QSO’s 0957+ 561 A,B offers the possibility of testing whether the redshift of the QSO involved is cosmological in origin. In general, if a gravitational lens is responsible for splitting the image of a QSO into two or more distinct components, it is possible to calculate the redshift of the QSO provided we have enough observational data involving the lens. The calculated redshift must then be consistent with the observed redshift if it is entirely cosmological in origin. The quantities that must be observed before the redshift can be calculated are the differences in the light travel times along the different paths from the QSO to us, the redshift of the lens, the angular separation of the images, and their relative intensities.

Most of the galaxy redshifts currently available in the literature have been measured from photographic spectra and are generally based on subjective estimates of the position of the narrow absorption features. In using only the few prominent absorption lines, most of the light from the galaxy is discarded. With the advent of digital linear-response detectors it is possible to obtain accurate and objective redshifts by cross-correlating all or part of the galaxy spectrum with a template galaxy of known redshift (Tonry and Davis 1979, Kelton 1980 and references therein).

Spectral line profiles in pulsating stars are affected by the interplay of a number of velocity fields. In addition to the basic velocities associated with the pulsation mode, the complications of stellar rotation, atmospheric velocity gradients, stellar winds and varying scales of turbulence may also be present. Initial modelling for line profiles in variables assumed a constant ‘intrinsic profile’ which was integrated over the limb-darkened stellar disk. This approach has been used even in recent work for nonradial pulsations (Stamford and Watson 1977; Kubiak 1978) because of computational ease. Employing an LTE analysis to predict centre-to-limb profile variations, which are then integrated over the disk, represents an improvement on this. This has been done, for example, by Parsons (1972) for radial pulsations in cepheids and by Smith (1978) for nonradial oscillations in B stars. Mihalas (1979) has recently made an even more detailed examination of profiles in expanding atmospheres which involved consideration of velocity gradients, departures from LTE and rotation.

Recent evidence suggests that radial pulsation can explain most of the variability in the majority of well-studied β Cephei stars (Stamford and Watson 1978; Campos and Smith 1980).

Observational tasks that require photometry of large numbers of objects, or over large fields, are still best approached using photographic photometry. Nevertheless, there are well-known problems in using photographic emulsions as two-dimensional detectors; these include their non-linearity and unknown photometric zero-points. The non-linearity can be calibrated using standard sensitometer techniques but the provision of photometric zero-points (and the verification of the validity of the intensity calibration) can be done properly only by providing a sequence of standards down to the plate limit. One approach to this latter problem, that has recently been revived by Racine (1969), is to use a sub-beam prism as originally suggested by Pickering (1891).

Print copies of the Palomar Sky Survey (PSS) are widely used for optical identifications of radio sources north of δ = -33°.

The 30 Doradus region offers an excellent opportunity to study cluster formation processes and recent star formation in the Large Magellanic Cloud.

The M supergiants are rare objects in the solar neighbourhood and consequently until recently their importance has been largely overlooked. It is now being realized that these stars hold the key to extragalactic distance determinations since they can be detected at large distances while the maximum luminosity they attain seems to be remarkably constant and independent of galaxy type (Sandage and Tammann 1974; Humphreys 1978, 1979a, 1979b; Humphreys and Davidson 1979). They are also important in their own right since they are massive stars in the late stages of their evolution and undoubtedly suffer from mass loss and chemical enrichment due to the mixing of processed materials to their surfaces. In this light they can be considered as precursors of the luminous carbon stars and supernovae.

As part of a general investigation of interstellar clouds associated with southern HII regions we have begun a high-resolution study of the sodium D-line absorption in the directions of early-type stars that are likely to be associated with or located behind the clouds.

Observations of CO emission (Kutner et al. 1977) have delineated a molecular cloud extending over several square degrees in the Orion region, and extensive surveys have been made of several other molecules (e.g. CN, CS, HCN, H2CO, H2H+) in the densest regions of this cloud.