Molecular hydrogen in spiral galaxies is distributed in clumps, i.e., molecular clouds, which have mass between 103M⊙ and 106M⊙ and a mass spectrum of n(m) ∝ m−1.6. Molecular clouds with masses greater than 105M⊙, are called giant molecular clouds (GMCs). It is generally accepted that GMCs are formed by the coalescence of molecular clouds through their collision. This process is studied by both numerical simulation and numerical integration. The observation with high resolution identified a great number of CO emission cores in galaxies. Based on this result, the aggregation or clustering formation of GMCs is numerically simulated. In the process of either coalescence or clustering, spiral perturbation plays an important role.

Temporal and spatial characteristics of solar flares are briefly reviewed in this paper. The global, temporal and spatial behaviours of flares are given first. Besides the 154-day periodicity, an 80-day periodicity of occurrence rate of large hard X-ray bursts for the period 1980 February – 1985 December, and the delay of the peak occurrence rate of large flares are pointed out, then the gregariousness of major flares is shown. In the third section, the time process and spatial structure of individual flares are shown and described according to space and ground-based observations. In the last section two problems on flare properties are discussed. (i) Previous classifications of solar flares are based generally on observations in a single spectral region. A new classification of flares based on observations in multi-spectral regions is given. (ii) Energy released in part of a loop seems to be not enough for a whole flare, and a qualitative model in which the energy is supplied by the untwisting of magnetic fields is proposed.

We present observational results obtained during the first three years following the explosion of Supernova 1987 A. We discuss aspects of the optical and near infrared spectra as well as results from spectropolarimetric observations. The observations of the circumstellar and interstellar medium are also briefly discussed.

The supernova rate in spirals of types Sc and Sd shows a sharp peak in face-on galaxies with inclination i ≤ 25°. This suggests that most supernovae in late-type galaxies occur in rich clusters or associations that are located at the base of chimney-like structures formed by a previous generation of supernovae. These chimneys are transparent because most of the dust contained in the rising column of gas within them had previously been destroyed in the hot bubble surrounding supernovae.

The disk stellar luminosity function has been redetermined by the mean absolute magnitude method, utilising the proper motion data of the LHS Catalog. The derived luminosity function shows a slightly deeper dip than that found by Wielen (1983) in the same magnitude range.

In this paper the author describes the instrumentation for the Infrared Space Observatory to be launched in 1993.

There has been a great deal of progress in our understanding of planetary nebulae and their central stars during the past decade and a half. Most of this has come about through progress in observational techniques covering almost the entire electromagnetic spectrum. Theories of planetary nebula evolution have been put to better and better tests as more and more discriminating data have become available.

This review describes some of the progress made in observations and their interpretation, particularly in the context of the evolution of the nebulae and the central stars. It includes a discussion on the improved determinations of magnitudes and temperatures of the central stars, and of progress in the measurement of distances, and a reassessment of the observed mass-distribution of the central stars. The last topic has been at the centre of a lively debate for almost a decade now and has been responsible for a large number of studies of central star evolution, some this review briefly touchs upon.

We present a progress report on the dynamical analysis of the IRAS sample of OH/IR stars. This sample is complete within a distance of 8 kpc. We distinguish two groups of OH/IR stars, one with high and the other with low expansion velocity of the circumstellar shell.

Using a quadratic programming method we fit a database of Galactic orbits to the observed distribution of radial velocities, longitudes and latitudes. The dynamical model yields a distribution function, based on two integrals of motion, for each group. Integration gives the density, the mean velocity and the velocity dispersions as function of R and z. The distribution functions for the two groups differ enough to suggest that they represent two different stellar populations.

We estimate the stellar lifetime for each group by comparing the velocity dispersions and z scale-heights at the solar radius with those found for local stellar populations. We conclude that the group with the low expansion velocity is likely to be part of the thick disc, while the high expansion velocity group has dynamical characteristics resembling an old disc population; this in contrast with the findings for differently selected samples of OH/IR stars (e.g., Baud et al. 1981a; 1981b), which were found to be less than 1 Gyr old.

The most favourable methods of observing the phenomenon of gravitational lensing are through high-amplification events and the time delay between the images. These effects provide us with the information to determine the Hubble parameter and the matter distribution in the universe. The image properties due to micro-lensing can be used to find the size and structure of the source.

The recent intensive study of Comet Halley based on in situ measurements, observations carried out with rockets and satellites and supplemented with co-ordinated Earth based observations has not only confirmed pre-Halley results, but also has given new insight into the nature of the nucleus, dust, gas and the interaction of cometary plasma with the solar wind. These observations also have raised many new questions and problems. Several of these aspects will be discussed. For a better understanding of these problems, the planned future missions to comets will also be discussed.

Time-dependent convection theory and non-local convection theory are described. The difference between the convection theories results from the different treatment of the non-linear terms of the hydrodynamic equations. We have obtained a better understanding of the thermodynamic coupling between convection and oscillations in comparison to the dynamic coupling.

We consider the linear and non-linear evolution of a perturbed X-type neutral point. A semi-analytic treatment is given for the case of small disturbances of the equilibrium field. This problem admits well defined azimuthal modes which allow a formally exact determination of the magnetic annihilation rate. It is shown that the longest lived modes are purely radial and decay of the timescale τ ≈ |ln η| where η defines the resistivity of the coronal plasma. Higher azimuthal modes decay much faster, generally on a fraction (O(m−1)) of the Alfvén timescale for the outer field.

We go on to perform finite amplitude calculations that demonstrate the implosive current build-up that precedes the reconnective phase of the relaxation. In general both linear and non-linear studies support the idea of an initial implosive stage which rapidly releases the bulk of the energy associated with arbitrary field disturbances.

In view of the already available reviews on the subject, the progress after 1972 is summarised. Newly identified species and molecular transitions are listed. How the studies of solar molecules can be helpful in understanding the solar atmosphere, in structuring better models and in deriving molecular parameters with some degree of confidence, is stated.

Astronomical observations from space-borne instruments are carried out in India in the areas of infrared, X-ray and gamma-ray astronomy. This paper briefly describes the facilities available in India for conducting experiments in space astronomy using balloons, rockets and satellites. It briefly reviews the important results obtained by Indian astronomers from observations made in India with the balloon, rocket and satellite experiments. The present status of research in different disciplines of space astronomy is discussed.

Inhomogeneities in globular clusters are reviewed with the observational evidence for chemical abundance variations from star to star in individual clusters and the large-scale structural variation of clusters. The reality of the radial colour gradient is tested in 47 Tuc (NGC 104). The result shows that the observed radial colour gradient comes from the integration of the calculated colours of individual stars. The cause of this radial colour variation is the result of the concentration of evolved stars and the reddening of the main sequence in the central region. We propose that the CNO abundance gradient in the early stage of a cluster’s formation is the interpretation of the observed radial colour gradient.

The relative importance of the stellar sources contributing to the production matrix of the heavy elements up to iron is reviewed. Three main element groups may be distinguished: (a) oxygen and the alpha-process elements; (b) the iron-peak group; (c) helium, carbon and nitrogen. Each of these is produced in stars of a different mass range and in different ways, and it is shown that an examination of metallicity-metallicity relationships can be used to constrain models of the history of star formation, stellar evolution, and the initial mass function in galaxies.

We can conclude that in the case of our local solar neighbourhood the initial Fe/O ratio was set by Type II supernovae, but that Type I½ supernovae were never important. Iron is produced by the Type I deflagration supernovae on a time-scale comparable to the infall time-scale of the gas. Carbon is produced by dredge-up in low-mass stars, but nitrogen is shown to be produced both in the stellar winds of massive stars, and in higher mass stars which give rise to the Type I planetary nebulae.

We present a series of numerical models describing the dynamical evolution of globular clusters with a mass spectrum, based on integration of the Fokker-Planck equation. We include three-body binary heating and a steady galactic tidal field. A wide range of initial mass functions is adopted and the evolution of the mass function is examined. The mass function begins to change appreciably during the post-collapse expansion phase due to the selective evaporation of low mass stars through the tidal boundary. One signature of highly evolved clusters is thus the significant flattening of the mass function. The age (in units of the half-mass relaxation time) increases very rapidly beyond about 100 signifying the final stage of cluster disruption. This appears to be consistent with the sharp cut-off of half-mass relaxation times at near 108 years for the Galactic globular clusters.