It is more than 120 years since the discovery of spiral galaxies, and naturally there has been much speculation on their origin. The vital clue is the composition of the arms, and it is now known that the arms are patterns of newly formed, and hence very luminous, stars. Thus the problem of spiral arms is only part of the broader problem of the origin of stellar systems in general. The various galactic forms of the Hubble morphological classification are differentiated by their different stellar systems, and so the problem of spiral arms is part of the basic problem of the origin of the Hubble types. For this reason the potential significance of any theory of spiral arms is vastly extended.

One of the main reasons for pursuing a gravitational as opposed to a magnetic theory is that there exist galaxies which seem devoid of gas and yet show unmistakable structure. These galaxies are among those classified as SBO2/3 to SBa(r), and I shall, after Curtis, call them theta galaxies. Examples are: NGC 1512, 1291,4693, and 2859. The theta galaxies gradually merge into ringed bar galaxies, which in turn merge into ordinary spirals.

The observation of the X-ray sky from balloons, rockets, and satellites has led to the discovery of more than 100 X-ray sources. Optical and radio identifications have been made for about a dozen of these. A few X-ray sources are of extra-galactic origin, but the majority are objects in the Galaxy.

Phase coherent interferometers with intercontinental baselines became possible because of the development of stable frequency standards. With sufficiently stable frequency standards, no connection is necessary between the two ends of an interferometer. The first VLBI experiments were conducted by a group at the University of Florida who used an intensity interferometer with independent tape recorders for observations of Jupiter. Later, they changed to a coherent system using crystal-controlled oscillators. Since then, several interferometer systems have been developed. A Canadian group developed a system using video tape recorders at each end of the interferometer. They recorded the data in analogue form and managed to bring the two tapes together and to synchronize them to an accuracy of better than a microsecond. After synchronization, the outputs were combined and fringes extracted. Their system has a bandwidth of about 4 MHz. No-one else has attempted a wide-band analogue system.

Cen X-3 was first discovered by Chodil et al. Since then, observations using the UHURU satellite have shown it to be an X-ray pulsar, and it must now have become one of the most searched for objects in the sky. It is both an X-ray pulsar (P~4.8 secs) and an X-ray eclipsing binary (P~2.08712 days).

Zeldovich and Guseynov suggested that neutron stars and collapsed stars (‘Black holes’) may be found as highly evolved companions of single lined spectroscopic binaries. They pointed out that the detection of X-rays or γ-rays from such a system would constitute positive evidence for the highly evolved nature of the companion. Trimble and Thorne searched through the catalogue of orbital elements of spectroscopic binaries (Batten) and found a few systems which were single line spectroscopic binaries with companions of mass M2 ≥ (1.4 M⊙, M1), where M1 is the primary mass. None of these has, however been identified with an X-ray source and other explanations such as multiple systems could not be ruled out. The alternative procedure, namely the identification of galactic X-ray sources with spectroscopic binaries is clearly more rewarding from the view point of searching for black holes and neutron stars. In particular, a high mass secondary which could be identified with a black hole is more likely to be discovered as a companion of an X-ray binary with a massive primary such as a blue supergiant.

Observations of the Jupiter radio emissions have established that the radiation is normally received as bursts with two quite different time scales. The L bursts mostly have durations of 1-10 seconds, although extended bursts up to 100 seconds are sometimes observed. The S bursts on the other hand have time scales of milliseconds and fractions of a millisecond. Dynamic spectrographic observations have shown that both types are structured in the frequency-time plane although the frequency and time resolution used so far has been insufficient to investigate detail smaller than 50 KHz and 10 msec respectively.

Frequency dependent structure is not uncommon in observations of extended radio sources. Where a source consists of two components with different spectra at an angular separation considerably less than the resolution of the telescope, the centroid position will vary systematically with frequency. A well-known example is 3C 273 though the effect in this case is quite small. With beams of a few minutes of arc, larger position differences could occur between measurements made at widely separated frequencies without the source being noticeably resolved. In a preliminary study of a sample of 120 4C radio sources, Munro and Hoskins found 17 cases of discrepancies in the range 45 to 100 seconds of arc between positions measured at 408 MHz with the Molonglo cross (beam width ~3′ arc) and at 2700 MHz with the Parkes 64 m telescope (beam width ~7′ arc) by Wills and Bolton. These separations were considered to be significant in terms of the quoted errors which were typically 10 to 15″ arc in each coordinate for both sets of measurements. It was pointed out that to explain the results on the basis of a double source model it would be necessary for the two components to differ in spectral index by ~1.

Observations for the Parkes 2700 MHz catalogue are carried out in two stages: (i), a relatively fast finding survey, followed by (ii), accurate measurement (at 2700 MHz) of flux densities and positions of the sources on the survey scans. For the first six parts of the catalogue intervals of several weeks between the stages resulted from the necessity to reduce the (analogue) survey scans manually. This communication describes a computer program which records and reduces these scans, and which produces an on-line map and source listing for the survey region. Thus the measurements of the second stage may be carried out immediately after the survey; additional advantages provided by the technique are discussed later in the paper.

Free-free absorption by thermal electrons attenuates low frequency radio waves and such absorption will indicate the presence of ionized hydrogen. The Gum Nebula contains bright H-alpha emission regions and therefore it is not surprising that the region appears as an area of low brightness on low frequency continuum maps.

The spectrum of a symbiotic star consists of an M-type absorption spectrum, a B-type shell spectrum and nebula emission lines, the relative contributions of these three components varying with time. The light curves of the symbiotic stars vary with a semi-regular period typically 200-800 days while larger eruptions occur on a timescale of ~ 3.5 years. Some suggestions which have been advanced to explain the combination spectrum, variability and eruptive behaviour of the symbiotic stars are:

1. (a) the symbiotic stars are binaries consisting of a hot and cool component.

2. (b) the symbiotic stars consist of a single hot star surrounded by a large optically thick envelope giving the appearance of a hot continuum with the absorption spectrum of a cool star superimposed on it.

3. (c) the symbiotic stars are single stars surrounded by a shock wave heated chromosphere.

   Although some of the symbiotic stars are undoubtedly binaries (for example, T Coronae Borealis), observatienal evidence suggests that others may be explained by hypothesis (c) above. The calculations described below provide an explanation of the symbiotic stars in conjunction with hypothesis (c).

Coudé spectra were obtained for a selection of giant and supergiant stars having spectral types in the range A9 to F2 to determine their atmospheric characteristics. The 12 members of Table 1 were compiled from The Catalogue of Bright Stars. This group of stars includes all known supergiants south of declination −15° and brighter than mv = 5.0 with spectral type between F5 and A5. Delta Canis Majoris (F81a) which has been the subject of many detailed investigations, was also observed to check for possible systematic errors in our equivalent widths. The spectra were obtained using the Coudé spectrograph of the 40 cm reflector at the University of Tasmania Observatory. Linear dispersions of 1.8Å mm−1 (0.12Å resolution) in the range 3600 to 4000Å and 2.4Åmm−1 (0.18Å resolution) in the range 4000 to 5000Å were used for stars brighter than third magnitude; 9Å mm−1 (0.4Å resolution) was used for stars fainter than third magnitude. The classification of 196 Car. was observed to be later (F8III) than the listed value and the spectrum of this star was not analysed further.

The optical astronomy group at the University of Tasmania is engaged in a search for optical pulsations from pulsars.

It was considered that the PDP-8/F mini-computer, purchased with A.R.G.C. funds to control the 40 inch telescope and on-line experiments, would be extremely useful for this purpose, until completion of the 40 inch telescope. Interfacing the computer to the output of the Cassegrain photo-electric photometer required work at both the hardware and software level.

This paper suggests reasons why certain forms and frequencies are found in librations of the pole. The period is about 27 years, but there is also a shorter period which was found empirically to vary with the inverse of the amplitude of the Chandler nutation. It had been found also that the pattern of the motions would sometimes become reversed.

The Division of National Mapping has received, on long term loan from NASA and in co-operation with the Smithsonian Astrophysical Observatory, a Lunar Laser Ranging instrument consisting of a gigawatt pulsed ruby laser, a 150 cm aspheric Ritchey-Chretien telescope, and associated electronic equipment. The instrument was formerly operated by the Air Force Cambridge Research Laboratories at Mount Lemmon in Arizona. Its principal use by National Mapping will be direct determination of the distance between the telescope and any of the three retro-reflector arrays placed on the Moon at Hadley’s Rille, Fra Mauro and Mare Tranquillitatis by Apollo astronauts. Full scale operation in conjunction with similar instruments well separated in latitude and longitude, in particular at Mount McDonald in Texas and Mount Haleakala in Hawaii, will permit determination of Earth rotation and polar motion, lunar ephemeris and libration, and tectonic plate movement or continental drift, which justifies its use in a geodetic environment.

Although the maser hypothesis for anomalous OH emission encounters difficulties it has not been seriously questioned because, as pointed out by Turner, no alternative has been available. The following ideas might suggest that there is an alternative hypothesis for anomalous OH emission.

Bipolar structures are discernible in three categories of solar radio emission: the slowly varying component (S-component), microwave bursts and metre-wavelength type I storms. Piddington and Minnett, discussing the S-component, showed that when coupling between the magneto-ionic modes along the ray path is ignored the emission from a regular dipolar field should have two specific polarization properties. These properties (see Figure 1) are that both feet of the bipolar structure should be similarly polarized, i.e. both left-hand (LH) or both right-hand (RH), and that this handedness should reverse on central meridian passage (CMP). (Reversal of the handedness at CMP had been suggested earlier by Martyn.) These arguments should apply also to microwave bursts and to type I storms.

Type I storms generally occur in association with large sun-spots and the radiation is usually circularly polarized. Statistically it has been found that the sense of polarization, right-hand (RH) or left-hand (LH), usually corresponds to the ordinary magneto-ionic mode in the field of the dominant spot of the active region; when a following spot dominates, the polarization tends to be determined by this spot rather than by the leading field. One-dimensional position measurements show that the type I sources are usually not radially above the active region but are displaced by a few minutes of arc. The source sizes are about l′.2 to 4′.5 at 169 MHz and the sources frequently contain double, multiple or bipolar structure at 80 and 160 MHz.

Type I radio bursts, as distinct from the continuum component frequently associated with them in a solar storm, are short-lived (0.1-2 s), narrow-band (2-10 MHz) bursts with frequency drift rates from 0 to 20 MHz s−1. They come from coronal regions close to the corresponding plasma levels, i.e. the frequency of radiation ω is close to the local plasma frequency ωp. They occur more frequently at frequencies above ~100 MHz but at times extend to frequencies as low as 20 MHz. Their observed equivalent brightness temperatures are usually about 109 K but they can reach 1011 K or higher. Except for an average decrease in polarization towards the limb and except for initial stages of a storm, type I bursts are strongly circularly polarized (approaching 100 per cent) in the sense of the O-mode.

Type I storms are the most frequently observed solar phenomena at metre and decimetre wavelengths. Since the first identification of the emission of this type with a large sunspot group a great many type I storms have been recorded with radio-spectrographs, polarimeters and interferometers (see, for example, Wild, Smerd and Weiss, Kundu, Wild). Nevertheless, we can offer no satisfactory answers to the most fundamental questions about type I storms: ‘What kind of disturbances supply energy to a localized coronal region to maintain the storm activity for up to several days?’; ‘What is the emission mechanism responsible for the peculiar features observed in type I storms?’.