It is likely that in most stars certain layers, either in the interior or outer regions, will be unstable against convection and the study of energy transport by convection has raised considerable interest since the early days of this century when Karl Schwarzschild derived his criterion for deciding whether a certain region is in convective or radiative equilibrium.

The visual spectra of some hot stars, including P Cygni, have emission with associated absorption troughs ˜ 102 km s-1 on the short-wavelength side (Beals 1929, 1951). These P Cygni profiles are easily understood in terms of mass flowing away from the star. Later, rocket observations of the far-ultraviolet resonance lines (Morton 1967) showed that the phenomenon is rather common among hot stars and the velocity shifts could be from 1000 to 3000 km s-1, demonstrating that the mass must be escaping from the star. Resonance lines provide the strongest absorption in the shell where neither the density nor the radiation field is high enough to leave many ions in excited states. Since the ion stages likely to be present around a hot star have their resonance lines shortward of the atmospheric cutoff, space observations are essential in this investigation. Figure 1 shows the P Cygni profile of O VI in ς Pup obtained with Copernicus satellite spectrometer.

Infrared astronomical observations using ground-based equipment are confined to the few ‘windows’ or observation ports allowed by our absorbing atmosphere, the chief absorbing molecules being water vapour and CO2. The majority of such observations have been made using broadband filters defining the photometric J, H, K, L, M, N and O bands. Some narrow band and spectral work (essentially all photometric) has also been carried out, particularly in the 8-14 micron window. From a very few high altitude, cold, dry sites (where the integrated water vapour content of the atmosphere above the site is consistently less than 1mm), it is possible to make observations at 35 and 345 microns. In Australia, the integrated water vapour content above us is typically 15 mm, only rarely dropping below 7 mm, so that long wavelength IR measurements simply cannot be made using ground-based instruments in Australia. Water vapour is fortunately not well mixed in our atmosphere and by raising our observational platform to a high enough altitude we can in fact achieve quite high transmission for all wavelengths from 10 microns to 1 mm, where we link up with microwave observations using essentially radio techniques.

There have been a number of attempts made in the last decade or two to observe deuterium in parts of the universe other than here in Earth. It is of interest merely to detect deuterium elsewhere just as it is to detect the occurrence of any nuclide. However in the case of deuterium there is a special interest because in big-bang cosmologies the great majority of deuterium in the universe is considered to have been formed in the initial fireball (Wagoner, 1973). Any observation of the present abundance of deuterium thus might give information about the very early stages of the creation of the universe. Detailed studies of nucleosynthesis during the early expansion of hot big-bang universes have however indicated a particular feature of deuterium production. (Fig. 1) The mass fraction produced X(D) is a very sensitive function of the size of the universe, as measured say by the present baryon density ϱb. Other nuclides that are mainly produced in the early expansion, such as 4He, have mass fractions less dependent on ϱb. Thus if we adopt the big-bang model for our universe we can determine ϱb from observations of X(D). Apart from any intrinsic interest in the present density of the’universe, there is considerable interest in whether the value is great enough for the present expansion to halt and go over to a collapse — or so small that the expansion of the universe will go on forever.

The standard formula for the resolving power R(= λ/Δλ) of an astronomical grating spectrograph is Here L is the linear size of the grating, perpendicular to the direction of the grooves, θB is the blaze angle of the grating, θS is the angular size of the slit, projected back on to the sky, and D is the diameter of the telescope objective. The formula is valid for gratings used in the Littrow condition, when the angle of incidence α, and the angle of diffraction β, are both close to θB.

The proposal made to ASTEC for an Australian systhesis telescope (AST) is for a high-sensitivity, high-resolution synthesis array to be located at the Australian National Radio Astronomy Observatory, Parkes, and used in conjunction with the existing 64-m antenna at that site as a national facility (Wellington 1976). During the past 18 months a design study group consisting of representatives from the Australian National University, University of Sydney, University of Tasmania and CSIRO has been investigating the design of such an array. This paper reports on one aspect of this design, the array configuration.

Aperture synthesis is a two-step imaging process in which the final map or image is reconstructed from intermediate measurements. As an alternative to digital computers, optical computers can be used to perform the reconstruction step. Optical reconstruction can be a fast and cheap way to handle the large amount of data now being generated by earth rotation aperture synthesis arrays.

From early experiments in 1966 (Cole 1968) the acousto-optical radio spectrograph has been developed at CSIRO to a sensitive, multi-channel spectral-line back-end. The principles of the instrument are described in detail elsewhere (Lambert 1962; Hecht 1977; Milne and Cole 1977). By means of a Bragg interaction between a laser beam and an ultrasonic beam derived from the radio signal, light is diffracted into an order whose light intensity distribution is accurately related to the power spectrum of the radio signal. In the earlier spectrographs the spectrum was recorded photographically (Cole 1973a, b; Hecht 1973) but this was limited to strong spectral features. To study weak spectral features a stable, linear system was needed with large dynamic range. The combination of an array of photodiodes and computer would be capable of observing these weak spectral features (Cole and Abies 1974). Development since 1974 has been a progressive identification of sources of thermal and mechanical instability and of excess noise in the spectrograph system.

One of the factors determining system sensitivity in radio astronomy measurements of microwave emission and absorption spectra is the flatness of the spectrometer response in the absence of any spectral features. Ripple appears as a quasi-sinusoidal variation of this baseline and occurs whenever two or more components of the same signal reach the receiver by different paths and interfere.

A new surface installed recently over the central 16.7-m-diameter zone of the 64-m radio telescope at Parkes has extended the operating range of this instrument to millimetre wavelengths.

The reliable detection and identification of weak, small-diameter radio sources require an instrument with both high sensitivity and high positional accuracy.

The initial flight of the University of Tasmania balloon-borne X-ray telescope was made from Parkes on Dec. 2, 1976. During the flight, enhanced X-ray emission was observed from the directions of 3U0900-40 (Vela XR-1), GX301-2 and the Galactic Centre. In this paper we report on the performance of the payload during the 11 hour flight and describe the preliminary results thus far obtained.

A new survey of the southern sky for pulsars is being carried out jointly by the University of Sydney and the CSIRO Division of Radiophysics. J. M. Durdin, M. I. Large and A. G. Little from Sydney University have been working with R. N. Manchester, J. H. Taylor and myself from Radiophysics. This paper provides a brief description of the experiment and an account of progress to date.

Our purpose in this paper is to explore the properties of the natural wave modes of a relativistically streaming electron-positron gas and to apply the results to the interpretation of the polarization characteristics of pulsar radio emission.

Tananbaum et al. (1976) have recently added to the Uhuru catalogue 4U1608-52, a source varying by over an order of magnitude in a time span of some months. They identified the object with MXB1608-52, a highly variable X-ray source of flares and bursts. (Kaluzienski et al. 1976; Belian et al. 1976; Grindlay and Gursky 1976a, b; Li 1976). There have been a number of earlier X-ray observations of sources very close to 4U1608-52 (Cooke and Pounds 1971; Luyendyk et al. 1973; Thomas et al. 1975; Grindlay and Gursky 1976b; Ricker et al. 1976), and in this paper we propose the identification of these earlier sources with 4U1608-52. We thereby obtain additional spectral information and can now rule out a globular cluster as the optical counterpart of the object.

The double-mode or beat Cepheids form a group of some 10 variables at the short period end of the classical Cepheid period range, which exhibit two (or three) simultaneous pulsations with remarkably constant period ratio (see Table I). If, as seems likely, the two periods (P0 and P1,) are to be identified with fundamental and first-overtone radial oscillations, linear pulsation theory may be used to yield a mass and a radius estimate for each beat Cepheid (Mbear and Rbeat) based on period observations alone. The masses so obtained may be compared with two other Cepheid mass estimates: the pulsation mass, Mpuls, is derived by the application of pulsation theory to one observed stellar period (usually P0) and a radius calculated from the stellar luminosity and temperature; the evolutionary mass, Mevol, is inferred from the stellar luminosity by the application of stellar evolution results (without mass loss) for Population I stars in the core helium-burning phase. If a Cepheid is a member of a cluster or association whose distance and reddening have been determined, the Cepheid luminosity and temperature may be obtained observationally, yielding Mpuls, and Mevol estimates directly. For other Cepheids, only indirect estimates may be made, based on the period-luminosity-colour relationship calibrated using the Cepheids with cluster membership.

As part of an identification programme undertaken on the southern section of the Molonglo Deep Sky Survey of Radio Sources, the limiting magnitudes of some of the available ESO-B films and SRC-J films and plates in this area have been determined. The Molonglo Deep Sky Survey has been described by Robertson (1977). The southern section comprises a 45’ wide strip centred about declination -62° and stretching in an irregular fashion from 18h 25m to 00h 16m.

Most supernova remnants (SNRs) show a ring-shaped radio intensity distribution which is interpreted as the projection of an approximately spherical shell of emission. However, in many cases the ring is neither circular nor complete; in this empirical investigation a major factor responsible for asymmetry is identified and the implications of this discovery are explored.

In this paper we report an independent determination of the Location of the break (change in spectral index) in the spectrum of the diffuse X-ray background by applying a simple analysis technique to data already in the literature.