The motion of P/Borrelly has been investigated using all precise published positions and taking into account the perturbations by Venus to Pluto. Nongravitational forces have been found to influence the motion of the comet considerably, formerly as a secular deceleration, more recently as a secular acceleration. By linking the seven apparitions 1904–1960, with allowance for the nongravitational effects, we have obtained a set of elements which represents 802 observations with a mean error of 1′.89.

Close-binary central stars of planetary nebulae are of interest to participants in this Colloquium because of recent suggestions that the cataclysmic binaries, containing a white dwarf and a lower-main-sequence star, may be descended from such objects (e.g. Paczynski 1976; Ritter 1976; Webbink 1978; Meyer and Meyer-Hofmeister 1978; Livio, Salzman, and Shaviv 1979). The proposed scenario is. that a binary system of initially large separation (P.= 1-10 yr) forms a “common-envelope” binary after the primary has evolved to the red-giant stage and developed a degenerate core. The secondary star spirals inward inside the red-giant envelope, eventually transferring enough angular momentum to the envelope to eject it. The result is a close binary containing the hot degenerate core of the red giant and a cool main-sequence companion, surrounded by the ejected envelope, which is ionized by the hotter star. Much later, when the cool companion begins to evolve, it will start to transfer matter to the hot star (by now a white dwarf), and cataclysmic activity ensues.

At the General Assembly of the I. A. U. in Prague in 1967 I became involved in a discussion on the merit of using stars with positions taken from the Astrographic Catalogue as reference stars in finding positions of faint members of the solar system. The answer was that little choice exists if the positions are being found from photographs of small field taken with large telescopes.

A brief review is given of the several ways in which stellar motions may be used statistically. In the case where the motions of all stars shown on plates of certain regions are measured these are generally used to determine kinematical relations in the Galaxy and only secondarily for the estimation of luminosities; when motions are measured for special groups of stars the solar motion is first determined and then used for the calculation of distances and luminosities.

The Mariner 5 ultraviolet measurements obtained while the spacecraft was in interplanetary flight are interpreted as Lyman-α radiation. This radiation may arise from (1) the scattering of solar Lyman-α radiation by interplanetary atomic hydrogen, (2) the scattering of solar Lyman-α by atomic hydrogen that is present at the edge of the solar system, or (3) diffusely scattered Lyman-α radiation from the Galaxy. The Mariner 5 measurements show a symmetry with respect to the galactic equator which suggests that the major source of the observed emission is the diffuse galactic radiation. In the Münch model for the diffuse Lyman-α radiation from an HI region, the sources of the Lyman-α photons are stellar chromospheres, the photons are scattered a large number of times by interstellar atomic hydrogen, and are absorbed by interstellar dust. Some of the Mariner 5 measurements may be attributed to Lyman-α emission from an HII region since the field of observation scanned the edge of the Gum nebula.

A review is given of our current knowledge of the gas phase abundance of interstellar molecules. A comparison is made between cold dust clouds, where presumably gas phase processes dominate the chemistry, and regions of high mass star formation such as Orion where both shocks and evaporation of dust grain mantles may play a role. In dust clouds, carbon rich molecules often show enhanced abundances, whereas in hot turbulent star formation regions, saturated species such as methanol and ammonia are often favored. In both types of source, there is evidence for abundance gradients and inhomogeneities. A probable cause is that the chemistry is far from a steady state situation. A useful diagnostic in such cases is the relative abundance of deuterated species and our current knowledge concerning deuterium fractionation is discussed. Recent results show that the observed degree of deuterium enhancement in species such as water, methanol, and ammonia in the Orion-KL region is much too great to be explained on the basis of normal gas phase chemistry. It is argued that a plausible explanation of the observations is that one is observing material which has recently been evaporated from dust grain mantles. One important consequence of this hypothesis is that molecular line observations may yield important information on the composition of the solid phase.

Using the first 12 months of HIPPARCOS mission data already reduced, a synthesis solution was obtained by both CERGA and ARI teams. The reduction includes two steps. In the first one the origins of individual reference circles are determined, thus constituting the HIPPARCOS celestial reference frame. This solution uses a subset of some 30 000 bright and evidently single stars in order to minimize possible corruptive effects on the system. In the second step, the abscissa measurements of all observed stars are substituted in the system and then processed star by star in a least squares adjustment providing the astrometric parameters (positions and parallaxes).

The results of sphere solution and astrometric parameter determination are discussed. In this context error ellipses and internal errors are calculated. External errors are derived by comparison with suitable astrometric catalogues. Especially, for estimating the external errors of the HIPPARCOS measured FK5 stars the instrumental system was tied to the FK5 system by rigid rotations followed by comparisons of the positions with those of FK5. The mean internal errors of the HIPPARCOS positions of FK5 stars is 1.5 milliarcseconds (mas) while the standard deviation of the differences HIPPARCOS minus FK5 reaches 90 mas. For the 5022 measured parallaxes the mean internal and external errors are 3.8 mas and 16.7 mas, respectively. — The results of precision and comparisons with external data show that the accuracy predicted for the final catalogue can be reached.

QUASAT was proposed to ESA as a joint ESA-NASA medium scale mission for space VLBI in December 1982. At the time of writing, four and a half years later, QUASAT has passed through an Assessment Study in ESA, a pre-Phase A study in NASA and has just entered an ESA Phase A study in European industry, supported by JPL and Australian and Canadian space interests. If QUASAT is selected to fly at the end of the Phase A study, it will be launched in 1995–1996. The scientific goals, mission concept and current status are reviewed this paper.

We discuss the results of a systematic search for multi-periodic pulsations in Galactic Bulge δ Scuti and RR Lyrae stars. Six “normal” double-mode variables pulsating in two radial modes have been identified (5 δ Scuti-type and 1 RR Lyrae-type). In 37 RR Lyrae stars secondary periodicities very close to the primary pulsation frequency have been detected. These periodicities correspond to nonradial modes of oscillation. They are found in ∼ 23% of RRab and in ∼ 3% of RRc variables of our sample.

The subject of interstellar matter is one of the most active fields of astronomical research to-day. Observational information spans the full electromagnetic spectrum from long radio waves to gamma rays and includes cosmic-ray particles. Results of chemical research find as much application now as mathematical methods. Interstellar matter plays a leading role in galactic and extragalactic research, and contributes increasingly to stellar astronomy and in solar system studies.

For the past two years, the authors have been carrying out a program of CCD photometry of planetary-nebula nuclei (PNNs), using the 0.9-m telescopes at Kitt Peak National and Cerro Tololo Inter-American Observatories. The aim of this program is to investigate the variability of PNNs on timescales of minutes to days, in order to search for close binaries and pulsators, as well as unexpected new classes of variable stars.

The program represents an extension of the photoelectric photometry of PNNs carried out by Bond and Grauer (1987). Use of the two-dimensional CCD detector allows us to model and subtract the nebulosity surrounding a PNN, in addition to providing exactly simultaneous observations of the PNN and several nearby comparison stars. The latter allow us to compensate for variable atmospheric transparency, permitting accurate differential photometry to be obtained even when the observing conditions are not photometric.

RR Lyrae variables play a prominent role in many of the problems of globular clusters, and from several points of view. In the first place they can be considered as pretty good indicators of population and distance; although they do not form a completely homogeneous set of stars, the knowledge of their mean absolute magnitude gives a powerful means of establishing distances within and outside the Galaxy, and hence of determining the form and size of the Galaxy itself. Moreover, the number of RR Lyrae stars in clusters, the relative frequency of RRc and RRab, types, the length of the transition periods, the array of colors, when correctly interpreted, give important information on the degree of evolution, age and chemical composition of the clusters. Placed as they are in a peculiar region of the H — R diagram of Population II, the RR Lyr variables can be used as a good test of the theories of advanced evolution or the models of pulsating stars.

By integrating the truncated complex scalar gravitational motion equations for an anelastic, rotating, slightly elliptical Earth, the complex frequency dependent Earth transfer functions are computed directly. Unlike the conventional method, the effects of both oceanic loads and tidal currents are included via outer surface boundary conditions, all of which are expanded to second order in ellipticity. A modified ellipticity profile in second order accuracy for the non-hydrostatic Earth is obtained from Clairaut’s equation and the PREM Earth model by adjusting both the ellipticity of the core-mantle boundary and the global dynamical ellipticity to modern observations. The effects of different Earth models, anelastic models, and ocean models are computed and compared. The atmospheric contributions to prograde annual, retrograde annual and retrograde semiannual nutation are also included as oceanic effects. Finally, a complete new nutation series of more than 340 periods, including in-phase and out-of-phase parts of longitude and obliquity terms, for a more realistic Earth, is obtained and compared with other available nutation series and observations.

A new sample of some 900 faint IRAS galaxies will be constructed after optical identifications of all IRAS point sources from PSC and FSC in an area with 1500 deg2 at high galactic latitudes. The identifications are being made on the basis of FBS low-dispersion spectra, DSS images and infrared colours. Some 320 IRAS sources have been identified and 180 galaxies have been found. Spectral observations of these objects revealed new AGN and luminous infrared galaxies.

Magnetic fields are anchored in gas clouds. Field lines are tangled in spiral arms, but highly regular between the arms. The similarity of pitch angles between gaseous and magnetic arms suggests a coupling between the density wave and the magnetic wave. Observations of large-scale patterns in Faraday rotation favour a dynamo origin of the regular fields. Fields in barred galaxies do not reveal the strong shearing shocks observed in the cold gas, but swing smoothly from the upstream region into the bar. Magnetic fields are important for the dynamics of gas clouds, for the formation of spiral structures, bars and halos, and for mass and angular momentum transport in central regions.

Young stars on their way to the ZAMS evolve in significantly different ways depending on their mass. While the theoretical and observational properties of low- and intermediate-mass stars are rather well understood and/or empirically tested, the situation for massive stars ([gsim ]10–15 M$_\odot$) is, to say the least, still elusive. On theoretical grounds, the PMS evolution of these objects should be extremely short, or nonexistent at all. Observationally, despite a great deal of effort, the simple (or bold) predictions of simplified models of massive star formation/evolution have proved more difficult to be checked. After a brief review of the theoretical expectations, I will highlight some critical test on young stars of various masses.

The luminous S Dor variables or Hubble-Sandage variables have gained considerable interest during the past few years for their exceptional (if compared to normal luminous hot stars) stellar wind properties. They seem to represent a short intermezzo in the evolution of the very massive stars prior to becoming WR stars. More recent results on S Dor variables (mainly of the LMC) are briefly summarized. Since most of the data have been or are going to be published elsewhere only an outline will be presented here.

The increase of cosmic radiation on 23 February 1956 by solar radiation exhibited in the first minutes a high peak at European stations that were lying in direct impact zones for particles coming from a narrow angle near the sun, whilst other stations received no radiation for a further time of 10 minutes and more. An hour later all stations in intermediate and high latitudes recorded solar radiation in a distribution as would be expected if this radiation fell into the geomagnetic field in a fairly isotropic distribution. The intensity of the solar component decreased at this time at all stations according to the same hyperbolic law (~t–2).

It is shown, that this decreasing law, as well as the increase of the impact zones on the earth, can be understood as the consequence of an interstellar magnetic field in which the particles were running and bent after their ejection from the sun.

Considering the bending in the earth's magnetic field, one can estimate the direction of this field from the times of the very beginning of the increase in Japan and at high latitudes. The lines of magnetic force come to the earth from a point with astronomical co-ordinates near 12·00, 30° N. This implies that within the low accuracy they have the direction of the galactic spiral arm in which we live. The field strength comes out to be about 0·7 × 10–6 gauss. There is a close agreement with the field, that Fermi and Chandrasekhar have derived from Hiltner's measurements of the polarization of starlight and the strength of which they had estimated to the same order of magnitude.

A brief review on globular cluster sub-populations in galaxies, and their constraints on galaxy formation and evolution is given. The metal-poor and metal-rich sub-populations are put in a historical context, and their properties, as known to date, are summarized. We review why the study of these sub-populations is extremely useful for the study of galaxy formation and evolution, but highlight a few caveats with the current interpretations. We re-visit the current globular cluster system formation scenarios and show how they boil down to a single scenario for the metal-poor clusters (namely the formation in “universal”, small fragments at high z) and that a hierarchical formation seems favored for the metal-rich clusters.