In the five years since the last invited paper on model stellar atmosphere applications there have been many significant advances made on all fronts. The five aspects which I will cover in this paper are:

• (1) the results of white dwarf model atmosphere investigations;

• (2) the results of the inclusion of non LTE phenomena in the atmosphere computations of hot (T > 15,000 K) stars;

• (3) the probable understanding of the cause of peculiar abundance patterns in the Ap and Bp and Am stars;

• (4) the advances in theory and observations of cool star atmospheres; and

• (5) the use of synthetic spectra and colours.

Like the strong non-thermal radio bursts from the solar corona and from the earth’s magnetosphere, the Jupiter radio bursts are characterized by their duration which may be from milliseconds to seconds, and by their complex structure on the frequency-time plane. In addition, they exhibit a variety of periodicities in their rate of occurrence, the primary one of which is associated with the rotation of Jupiter. The smaller physical scale of Jupiter compared with the Sun naturally leads to a much shorter time scale in the various radio phenomena and it is only recently that suitable equipment has become available to permit the detailed investigation of the dynamic spectra of the bursts with the necessary high time resolution of the order of 10–3 sec (Ellis 1973a,b,c). As in the case of the solar radio bursts, a number of distinct types of dynamic spectra are observed and they provide a convenient basis for classification.

Introduction

My interest in the construction and costs of telescopes began with the design and construction of the building for the 48 inch Schmidt Telescope (on behalf of the Science Research Council of the United Kingdom) and was stimulated by a visit to Mt Palomar and Kitt Peak Observatories. With regard to costs I would like to quote Meinel (1969).

It is well known that a molecular cloud complex with a radial velocity of 40 km s–1 is located in front of and near Sgr A, the non-thermal source at the centre of our galaxy. The motion of this cloud is generally interpreted as a contraction towards the centre. In terms of the general kinematics of our galaxy the existence of contraction is not firmly established – the main spiral features are either stationary with respect to the local standard of rest or expanding outwards from the centre (e.g., the 4 kpc expanding arm). However, as a result of a high-resolution study of the H2CO absorption arising in the molecular cloud, an alternative interpretation not involving contraction is suggested.

The detection of formaldehyde absorption of the microwave background radiation in a region of the Southern Coalsack (Sinclair and Brooks 1972) was made at about the same time as an optical study of this region (Tapia 1972) in which six globule-like clouds of interstellar dust were found. The size (0.3 pc) and position of the formaldehyde cloud were similar to the corresponding parameters of one of the globules (No. 2 on Tapia’s list).

Preliminary mapping of the thioformaldehyde distribution in the direction of Sgr B2 followed the detection of the 211←212 transition of interstellar thioformaldehyde by Sinclair et al. (1973). Observations were made with the Parkes 64-m telescope in conjunction with a 9 cm parametric amplifier.

Following the detection of the 9 cm ground-state triplet of CH by Rydbeck et al. (1973), preliminary observations at the CH frequencies were made with the Parkes 64-m telescope (beamwidth ∼6′ arc) in December 1973 and January 1974. We have detected the CH lines in many galactic HII regions and in the direction of the galactic centre.

Large solar radio outbursts at metre wavelengths often consist of a group of type III bursts followed a few minutes later by a type II burst; in both spectral types the intense burst radiation drifts towards lower frequencies with time (Figure 1).

Any theory dealing with type IV solar radio bursts must explain the observed evolution of their brightness temperature and polarization. The behaviour of isolated, moving type IV sources is characterized by a long period of constant and low polarization during the major part of their lifetime, followed by a substantial increase in the degree of circular polarization during the declining phase. This behaviour puts severe constraints on and gives valuable clues as to the evolution of the physical conditions within the source.

The purpose of this paper is to advance an alternative hypothesis for the underlying distinction between solar radio bursts of spectral type V and ‘inverted U’ bursts, and to explore its implications.

Some while ago (Prentice 1973a) I demonstrated that if the present Sun were to possess a small burnt out central core of mass 2-3% M⊙ its neutrino flux would be a factor 10 or so smaller than that of conventional homogeneously evolved solar models. It was not, however, conclusively shown what conditions might lead to the formation of such a core or indeed whether a burnt out core could be attained after 4.7 × 109 yr. of nuclear evolution. In this connection Demarque et al. (1973) has considered the evolution of an initially inhomogeneous model consisting of a hydrogen free core surrounded by a homogeneous envelope.

In this paper we obtain an approximate expression for the temperature structure of a stellar atmosphere heated by an external flux of soft X-rays. Allowance is made for reradiation in both the soft X-ray and optical regions. As may be expected our results indicate that the temperature structure will be drastically changed from the grey T-T relation, particularly at low optical depths. The stronger lines should appear in emission with the possibility of a central absorption in certain circumstances. The present results are in general agreement with those of Strittmatter (1974).

The darkness of sunspots has been attributed by many authors (Biermann 1941; Danielson 1961) to the inhibition of the normal solar convective processes by the presence of strong magnetic fields. Observations of the solar photospheric granulation pattern have also shown that a weak longitudinal field exists outside the activity regions. Although these observations have not revealed any close association between the magnetic field and individual granules, nor the exact reasons for the darker cell boundaries, it must be accepted that, overall, the role of the magnetic field must be such as to influence the cell structure and reduce the normal heat transfer by convection.

It appears that the relative number of β–Cephei variables among the early B stars is comparable to the fraction of the evolutionary lifetime spent in the secondary contraction and early shell-burning phases (Lesh and Aizenman 1973a) and it may therefore be of some interest to investigate possible instability mechanisms associated with hydrogen shell-burning in stars within the mass range of 10 to 15 M⊙ (Lesh and Aizenman 1973b).

During recent years it has become rather clear that the β-Cephei variables are in a hydrogen-buring stage of their evolution. The 8 (β-Cephei variables which have been found in the Scorpio-Centaurus association highlights this. The 6 of these variables for which Glaspey (1971) provides (β, (u – b)0) photometry all fall at the tip of the sequence in the Hertzsprung-Russell diagram. This indicates that (β-Cephei stars are in the final stages of core hydrogen burning. This is in good agreement with the position of the β-Cephei strip of Watson (1972) in the H-R diagram.

The object EX Hya is a dwarf nova with a binary period of 98.3 min (Mumford 1964, 1967). Warner (1972, 1973a) has observed two complete cycles of this star with a photoelectric time resolution of 5 sec. These observations suggested that EX Hya can be understood in terms of the model proposed by Warner and Nather (1971) in their discussion of U Gem. In this model, a white dwarf primary of a semi-detached binary system is surrounded by a disk of gas formed from matter transfered from the secondary, which is a cool dwarf star filling its Roche lobe.

A considerable amount of observational interest in short timescale phenomena has developed in optical astronomy in recent years. This interest has stemmed primarily from search efforts directed towards optical pulsars and the study of rapid fluctuations in cataclysmic variable and flare stars. Such studies have been made feasible by the development of the necessary photon pulse-counting technology required for rapid data sampling.

The Schmidt camera is conceptually elegant and has a remarkable performance, but it suffers from several practical disadvantages. Two of these concern us here. First, there are very few craftsmen who are competent to make the aspheric correcting plate of the Schmidt camera compared with the number of those competent to make spherical surfaces of high quality.

Several methods are available for sensitizing photographic plates, each with its own disadvantage. Pre-flashing greatly increases the plate background and is useful only for enhancing the plate’s response to light levels near the plate’s threshold. Baking and (or) evacuating enhance the plate’s response at all light levels and leave a slightly higher background. Unfortunately all of these methods produce plates with a short shelf life after sensitization.

Important observations of X-ray sources and searches for the optical counterparts of X-ray and radio pulsars require a capability of detecting and analysing light variations with a time scale of milliseconds. X-ray sources in binary star systems are expected to be collapsed objects – neutron stars or black holes (Peterson 1973) – and are expected to produce light variations. In the case of a neutron star, pulses with the same period as the rotation period of the neutron star would be produced, and such have been observed from Cen X-3 (schreier et al. 1972) in the X-ray, and from Her X-1 (Middleditch and Nelson 1973) and the Crab Nebula pulsar (Cocke et al. 1969) in the X-ray optical.