Truncated modal expansions have provided a powerful, reasonably accurate and flexible approach to the problem of non-linear thermal convection. They permit tractable numerical solutions of three-dimensional cellular motions, as well as readily accommodating large density variations, time dependence, rotation and magnetic fields.

A standard procedure for the simulation of galaxies is to replace the ∼ 1011 stars by ∼ 104 particles. These particles are assumed to have equal mass and their distribution in space produces a mass density ϱ which mimics the average mass density of the stars. From the mass density of the particles the gravitational forces between the particles can be computed. In the literature two methods have been used to compute these forces.

Ten years ago I put forward the suggestion that the Sun may possess a small burnt-out helium-rich core covering some 2-3% of its mass (Prentice 1973a,b). The purpose of this chemically inhomogeneous solar model was to account for the anomalously low flux of neutrinos being emitted by the Sun, as observed by Davis et al. (1971).

The satellites of the outer solar system appear to be composed principally of water ice and silicates, with the presence of ammonia and methane hydrates (Lewis 1971). Although these bodies are small (with radii typically < 1000 km) they can exhibit very active evolutionary histories, due to the low melting point of water ice.

There have been extensive calculations of model atmospheres of stars over the past few decades. Almost all of these calculations have assumed radiative equilibrium. Except for convection, very little work has been done on the stability of the surface layers of stars at small optical depths. Although there has been a considerable amount of research concerning the stability of stars, this work has assumed the diffusion approximation for the transport of radiation and has been primarily concerned with the envelopes of stars rather than their atmospheres.

The Ca II K line serves as an important tool in determining the physics of the photosphere-chromosphere region of the solar atmosphere (Cram 1983). To date detailed analyses have centred on the study of line intensity profiles.

It has been suggested (Holman et al 1980; Melrose and Dulk 1982a) that solar microwave spike bursts are due to electron cyclotron maser action. These bursts have been observed in the range 1-3 Ghz, and occur in conjunction with flare-associated impulsive microwave and hard X-ray bursts. The bursts have rise times of a millisecond or less (e.g. Slottje 1978).

A common test for hydrodynamic calculations is the Boss and Bodenheimer (1979, 1981, hereafter BB) problem in which an isothermal, spherical cloud of 1 M⊙ is given a cos (2Φ) density perturbation and the ensuing evolution studied.

Individual pulses from a given pulsar can differ greatly in both shape and intensity, and are often made up of narrower ‘subpulses’, i.e. local emission peaks one tenth the width of the mean pulse. Properties such as shape, polarization and position of subpulses presumably reflect details of the emission processes and of the regions that produce the radiation.

The classical model of the interstellar medium consists of cool clouds (typical temperature 80 K, number density 20-40 cm-3) moving through a warmer interstellar medium (104 K, 0.15-0.3 cm-3) at a rms velocity of ~ 10 km s-1. More recent models have included the coronal gas (106 K) as part of this medium. We consider the collisions of these clouds in order to determine whether these collisions initiate star formation and/or change the state of the interstellar medium.

A common form of wave propagation in the highly structured magnetic fields and density profiles of the solar atmosphere should be magnetohydrodynamic surface waves, i.e. waves with magnetic and velocity fields concentrated at an interface of two regions of plasma of differing properties. For example, local concentrations of equilibrium magnetic fields forming flux tubes will support these waves on their surfaces, in addition to the more commonly considered ‘body waves’ that are supported by an isolated flux tube, waves with an oscillatory radial dependence inside the tube. Mhd surface waves have recently been identified in a laboratory plasma at Sydney University (Collins, Cramer and Donnelly 1983).

The evolution of asymptotic giant branch (AGB) stars as a function of mass and composition has been studied by Becker and Iben (1979, 1980) for M≥3 M⊙. The results from calculations like these may be used to provide input data for calculations of synthetic AGB star populations, as has been done by Wood and Cahn (1977), Iben (1981) and Renzini and Voli (1981).

The rate of change of magnetic flux Ф contained within a closed curve C moving with the plasma velocity u, is given by

where B is the magnetic induction and S is a surface bounded by the curve C. Using the hydromagnetic equation,

where η is the magnetic diffusivity, this becomes

In many astrophysical contexts η is small and thus approximately

Since this implies that the flux within any moving closed curve does not change, the field is said to be ‘frozen into’ the plasma and moving with it.

During recent years a firm relationship has been established between disappearing solar fragments and geomagnetic disturbances (Joselyn and McIntosh 1981; McNamara and Wright 1982; Wright and McNamara 1983). Wright and McNamara (1983) showed that the average magnitude and delay of the geomagnetic disturbance depended upon the size of the filament.

The close association between flare events having metric radio type II and type IV bursts and the subsequent occurrence of sudden commencement geomagnetic storms is well known. In such cases it has been amply demonstrated (e.g. Malitson et al. 1973; Cane et al. 1982) that the sudden commencement (SC) is caused by the same magnetohydrodynamic shock which originates near the Sun and which produces the type II burst. Recently, a study of geomagnetic storms thought to be associated with the disappearance of filaments from quiet regions of the Sun (Joselyn and McIntosh 1981) showed that several of these storms began with marked sudden commencements.

Clusters of galaxies appear to contain much more mass than just that of the galaxies which provide most of their optical luminosity. There are two important ways to investigate the structure of their gravitational potential wells, firstly by an analysis of the radial velocities of the cluster galaxies, and secondly by a study of the X-ray emission from the hot gas which forms part of the intra cluster medium. We present new samples of radial velocity data for three clusters, and discuss some simple types of models of the X-ray sources.

The distribution of faint galaxies in a deep sample, down to a limiting magnitude of B ~ 21.5 in a region of some 30 square degrees of sky at the South Galactic Pole, is investigated. The sample was obtained from objective measurements made with the COSMOS automatic plate-scanning machine. The two-point angular correlation function shows positive correlation on scales out to ~1.5° and thereafter positive correlations on scales ~ 2.5° and ~ 5° indicating the presence of clustering in this sample on linear scales out to ~ 8h-1 Mpc and at ~ 13h-1 and ~ 27h-1 Mpc. On application of an algorithm for the detection of filamentary structure in the distribution of the galaxies, a negative result is obtained.

Plasma loops are the dominant structures, in the higher levels of the Sun’s atmosphere above active regions. Much of our knowledge of their properties has come from space observations made over the past decade or so in the EUV and X-ray regions of the spectrum and, more recently, from high-resolution, two-dimensional images of microwave sources obtained with the Very Large Array radio interferometer (VLA). On the other hand, the spatial resolution of rocket and satellite observations has so far generally failed to match that of ground-level optical observations made in Hα and other strong chromospheric lines. The latter permit one to study in much greater detail the morphology of individual loops, as illustrated for example in the recent work of Loughhead, Wang and Blows (1983). These authors addressed themselves to the task of determining the true geometrical shapes of individual loops, a problem which had hitherto received little attention.

The alignment of galaxies within clusters and superclusters may be sought either in the major axis of the observed optical distribution or in any radio or X-ray structure from an active nucleus.

Radiation generated in the solar corona can be drastically altered by refraction and scattering as it escapes. This is especially true for sources having a frequency near the local plasma frequency, where the index of refraction approaches zero. Early investigations of the problem (e.g. Fokker 1965; Steinberg et al. 1971; Riddle 1972, 1974) have shown that the size, shape, location, intensity and time profile for the observed source can all be influenced.