The existence, in the Galaxy, of small absorbing dust grains has been known for a long time, as they redden the light of stars located behind them, and obscure by amounts which can often even be several magnitudes… In the infrared, where absorption is smaller, one can see through the dust, while at the same time, one can observe the emission of cold dust clouds. After a brief review of technical developments, the reasons for studying the infrared, and the principles of diagnostics, are given. The analysis of the spectral features leads to identification of ice, graphite, and various silicates, as main constituents of the dust. In the fifth section, examples are given of the various sources that one meets when travelling in the Galaxy: protostars, cold and dilute, dense envelopes of young stars (cold or hot), dilute envelopes of not so young stars, and ejected clouds surrounding evolved objects… The simultaneous observation of dust, molecules and atoms, in the same regions, shows that the knowledge of the circumstellar dense regions might be of a great interest, in the study of the origin of life, and of the evolution of the Universe.

The metaphor of physical law - a range of orderly behavior within certain authoritative constraints - has not escaped the historians of ideas. That scheme, in all its forms, east and west, gained its earliest strength from astronomy itself. The search for measured order in heaven is old and widespread; two instances from ancient China and pre-Columbian Mexico will make the point.

For our modern science, two universals rule: that of particulate matter - from neutrons to C2 H5 OH - and the only unsaturable force, long-range gravitation. The phenomena of astronomical scale are mainly examples of kinetic motion resisting for a time that untiring attraction, under the virial theorem. Matter is everywhere familiar, apart from the deep and puzzling question of what seems to be the large-scale failure of matter-antimatter symmetry.

After urging you all to briefness, I should set a good example. I “welcome you all: participating colleagues and all IAU members and guests, especially those from our host commissions 25, 33 and 45. I thank you all: the contributors to today’s events, to the members of the Organizing Committee, to the Presidents of the sponsoring Commissions, IAU officials and all those who are not here today but have helped to make the arrangements for today’s discussion.

To assist with the direction of our work I have asked Prof. K. Aa Strand, Prof. T. Elvius and Prof. B. Bok to preside over Parts One, Two and Three. Questions and comments may be made to the invited speakers and to the discussion contributors at the end of each Part of our program.

Twelve years ago, during the International Summer Course at Lagonissi, Bart Bok (1966) presented a paper “Desiderata for Future Galactic Research”. He pointed to the importance of studies in the galactic polar caps which would allow us, as he hoped, to determine some basic data for galactic structure.

The significance of his statements was demonstrated already in the following years when the gradual increase of our knowledge of the objects in the solar neighbourhood was suddenly interrupted by new observational data from which diverging conclusions have been drawn. Never before have such controversial opinions on problems in the vicinity of the sun been so eagerly discussed: A formerly unknown numerous population of low-velocity red dwarfs, the mass density resulting from stars three times that formerly thought, a remarkably increased luminosity function - yes or no? Obviously, the controversies have smoothed down again during the last months. But what is the situation after this troubled period? Which problems are still unsolved? Which promising programmes will be proposed? I think this Joint Discussion is justified in 1976!

Until recently the entire astronomical endeavor was restricted to passive observations from the surface of the Earth. Natural experiments — such as stellar occultations, eclipsing binaries, or supernova explosions — might fortuitously arrange themselves, and systematic inventories were performed of information carried by reflected or emitted electromagnetic radiation. But it is only in the last twenty years that our species has been able to carry out in situ experiments on astronomical objects.

Radar astronomy permits a kind of experimentation on solar system objects not too distant from Earth, and has made a number of major discoveries, including the determination of the 3:2 spin/orbit coupling of Mercury, the Earth-locked retrograde rotation of Venus, and major elevation differences on Mars. Radar is the tool of choice for geomorphological mapping of cloud-enveloped Venus, and efforts now underway at the Arecibo Observatory should lead to complete mapping of one hemisphere of Venus down to 10 km resolution. The radar reflectivity of the rings of Saturn has forced a rethinking of the nature of the constituent particles in the rings; and a determination of the radar cross-section of Titan is an experiment which can resolve much of the uncertainty about the atmospheric structure and surface pressure for that enigmatic and fascinating object.

One of the aims of the Strasbourg Stellar Data Centre is to collect and distribute the different stellar data available, so that they can be used for statistical work. As an example of the statistics which can be made, we present in the table some results for the galactic pole areas. The photometric data were obtained from a general file (Magnenat 1976) which lists the stars observed photoelectrically and the system in which observations were made. The results show clearly how fragmentary our knowledge still is; completeness exists at most up to m = 9.

Known periodicities in X-ray sources cover a wide range of values - a factor of 108 or 109 from the 33 mS of the Crab pulsar up to 35 days for the Her X-1 repeat of its up state, or even years if one believes in the not inconceivable idea that at least one “transient” (perhaps not yet even discovered!) is exhibiting some form of repetitive behaviour. These periodicities are not just astronomical curiosities to keep theoreticians busy speculating. They do have several solid uses which the mere experimenter can take advantage of. These uses do, inevitably overlap, and vary in importance according to whom one is speaking. In the short space available I will concentrate on recent measurements on X-ray sources showing Doppler modulated periodicities, and on sources which may well show such changes when they are studied more closely.

The notion of preparing a bibliography on the topic of today’s Joint Discussion originated at about the same time in Heidelberg, Strasbourg and Castel Gandolfo. Letters and lists were exchanged as we prepared for our work here today; you must judge the results of this draft and help us with your suggestions. New IAU policy prefers that bibliographical materials be limited to Reports on Astronomy of the various commissions; hence there is no place in Highlights of Astronomy for the full Bibliography. Both the Director of the Vatican Observatory and the Director of the Strasbourg Centre de Données Stellaires have offered to publish the final listing. We are grateful to both and shall accept the Strasbourg offer; it is planned that it will appear in the January 1977 issue of the Information Bulletin.

After listing the author, year of publication, an abbreviated publication reference together with Volume and page references and the title of the paper, we have listed, wherever possible, three additional sets of numbers. The first gives the approximate number of stars discussed in the paper; the second, the approximate magnitude range; and the third, the lower limit of the (absolute) latitude of stars contained in the paper. It should be noted that it was not always possible to furnish all these data for each article listed. Still to be added to the present compilation are the Reference Numbers to the Astronomy and Astrophysics Numeration in preparation by Gliese, a full list of bibliographical abbreviations used prepared by Jaschek and an Author Index and a preface written by McCarthy.

This is a short report on our study of the M dwarfs in the South Galactic Pole region in connection with the problem of the missing mass and the luminosity function of intrinsically faint stars. The M-type stars found in our objective prism survey with the Lembang Schmidt telescope have been studied in collaboration with Dr. D.H.P. Jones for the discrimination between dwarfs and giants. Furthermore, these stars are all observed photo-electrically in Kron’s R, I-system at La Silla, Chile. Results of the work will be published soon. A blink survey has been initiated at Nymegen using copies of red and blue Mt. Palomar Sky survey plates to find faint red stars in the S.G.P.-region.

Kumar has put forward the idea that Oort’s missing mass is to be found as a large number of very low mass stars (0.01-0.07 Mθ) in the solar neighbourhood. His idea is backed up by the fact that stars within this mass range have been found by double star observers. Staller suggested that the low velocity M dwarfs found in the directions of the galactic poles represent Kumar’s very low mass stars. This proposal is ment not only for giving an answer to the problem of the missing mass but also for those problem connected with the observed small velocity dispersion (10-15 km/sec) of the low velocity M dwarfs. Staller estimated the contracting and cooling times of these stars to be about 5 x 108 to 109 years. In the lifetime of our galaxy (about 10 years) 10 to 20 generations of these low mass stars have been created and evolve to very faint degenerated black dwarfs. The mass density of these stars, in the solar neighbourhood, will be large enough to solve the problem of the missing mass. The stars we observe at present as low velocity M stars are genuinely young red stars, and their small velocity dispersion will therefore not contradict Spitzer and Schwarzschild’s mechanism for the creation of “velocity dispersion by encounters with interstellar clouds. The age of the older very faint black dwarfs is comparable to that of our galaxy. These stars should therefore have a large velocity dispersion. Staller has shown that on the average the low mass stars have a velocity dispersion larger than 21 km/sec, and therefore the whole group of low mass stars is satisfying Toomre’s stability criterion for the galactic disk, such as newly derived by Biermann.

Prior to the launch of the Ariel V satellite in 1974, four transient X-ray sources had been observed. We placed the study of sources of this type amongst the major objectives of the satellite, and so ensured that the spacecraft ground centre was capable of quick reaction, and that experimenters had rapid access to their data, so that new transients could be rapidly recognised, and an observing programme for them produced and carried out. This has been very successful - in its first 18 months of operation the satellite made observations of at least 14 transients.

Of these transients, five form a rather well defined class whose properties are rapidly becoming clearer. This class I shall call the “classical” transient. The remainder form a much more miscellaneous collection which cannot be clearly distinguished from the normal X-ray sources. There is some indication that the classical transients include two species, so to start with I shall describe one of each which has been particularly well-observed by Ariel V and SAS-3.

The R.G.O. has a continuing programme to investigate both the density and motions of the stellar content of the solar neighbourhood. Gaps in current knowledge are wider for the fainter stars which are found in proportionally greater numbers near the galactic poles. Our approach divides naturally into two questions: (i) What are the proportions of red dwarfs to giants at each apparent magnitude? and (ii) What are the corresponding total numbers of red stars?

Question (i) was first tackled by a narrow-band photometric search for M dwarfs (Jones 1973). This approach is continuing and a progress report has been delivered (Jones 1976). To push the search deeper 41 stars in the South Galactic Cap (mostly fainter than V = 15) selected by P.S. Thé from a survey with the Lembang Schmidt as being M2 or later were observed with the Robinson-Wampler Scanner on the 150-inch A.A.T. The Na D lines, CaH and the band at λ5500 tentatively identified by Pesch with CaOH have been measured. Column density calculations of CaOH for giants and dwarfs of different temperatures support Pesch’s identification. Nearly all the stars are dwarfs but only seven appear in proper motion catalogues, (Thé and Staller 1974). At the galactic caps most stars brighter than V = 12 are giants and fainter dwarfs. The transition zone is only 10-14.

The OSO-8 satellite contains a focusing mosaic graphite crystal X-ray polarimeter that is oriented along the spin axis of the wheel section of the satellite. The polarimeter operates at 2.6 and 5.2 keV. Polarization in a source appears as a modulation of the counting rate at twice the satellite spin frequency. The amplitude and phase of the modulation are simply related to the polarization and position angle, respectively. Two independent polarimeters are employed, and their axes are offset by 128°. Focusing is achieved by mounting the crystals on a parabolic sector, which reduces the background without reducing the sensitivity. The low background that results from the focusing not only improves the statistical quality of the data but also substantially reduces the danger that an asymmetry in the charged particle background may produce a spurious polarization result. This is particularly important in the case of weak sources. The instrument has been described in detail elsewhere (Novick 1975); here we will briefly discuss the results obtained on the Crab Nebula, Cyg X-l, and Cyg X-2.

In spite of extensive proper motion surveys, there remains some uncertainty about the space density and the kinematics of late-type dwarf stars. This uncertainty is primarily due to the unknown kinematic bias introduced by the proper motion discovery technique.

A spectrographic search, especially one using an objective prism on a Schmidt telescope, can identify significant numbers of intrinsically (and actually) faint stars, free from any kinematic bias. To find faint and cool stars, one chooses an objective prism with low dispersion and emulsions sensitive to the red and near photographic infrared. At low dispersions, in this wavelength region, there are no reliable luminosity criteria. Thus Sanduleak conducted his survey in the direction of the north galactic pole, with the expectation that very few remote giants would contaminate his results. A catalog of 273 probable dwarf stars of type M3 and later (Sanduleak, N. 1976, A.J., 81, 350) based on his objective prism survey in the direction of the north galactic pole is now available. Thanks to W.J. Luyten (1976, Proper Motion Survey with the Forty-Eight Inch Schmidt Telescope XLVI (Univ. Minnesota, Minneapolis)) proper motions have been measured for all of these stars.

Cygnus XR-1 was observed Nov. 11 to 18, 1975, in the energy range 20 to 400 keV with a Cgl scintillation crystal spectrometer on board the OSO-8 satellite. A drop in the count rate of ˜45% between 23 and 153 keV was observed to occur gradually over a full day on Nov. 16. Comparison with 2 to 7 keV proportional counter data taken simultaneously on OSO-8 by P. Serlemitsos et al. (private communication), the count rate of which rose ˜40% on Nov. 16, shows the 23 to 153 count rate to be generally anti-correlated with the 2 to 7 keV count rate over the entire 8 days of observation (Fig. 1). The observations are in agreement with the predictions of the two-temperature accretion disk model of Shapiro, Lightman and Eardley (Astrophys. J., 204, 187, 1976).

As a result of the recent discussion concerning the luminosity function of late-type main-sequence stars (Weistrop 1976 and references therein), a program of photoelectric photometry of all red stars in a field near the North Galactic Pole was undertaken. The sample is complete for stars redder than (B - V) = 1.40 magnitude for the following apparent magnitude and area limits: V = 12.0-14.0 magnitudes, 13.5 square degrees; V = 14.0-15.0 magnitudes, 3.0 square degrees; V = 15.0-17.5 magnitudes, 1.0 square degree. Observations in BVRI have been obtained for the 44 stars in the sample. Giants and dwarfs are distinguished by their location in the (B - V)-(V - I) diagram or from published proper motion data, where available. The absolute magnitudes of the dwarfs are determined from the MR - (R - I) relation.

The density distribution perpendicular to the galactic plane of the dwarfs is consistent with the distribution for K giants found by Oort (1960). The derived luminosity function does not differ significantly from that determined by Wielen (1974) for stars close to the Sun. The local space density for stars in the interval MV = 8.5-14.0 magnitudes is 0.099 stars pc−3. The corresponding stellar density derived from Wielen’s luminosity function is >0.066 stars pc−3. Sixty-six percent of the density derived here is contributed by two stars with absolute magnitude in the range MV = 13.0−14.0 magnitudes.

Most of the variable phenomena of high-luminosity (≳1036 erg s−1) stellar X-ray sources can be explained, at least qualitatively, within the general framework of binary accretion models in which thermal X-rays are emitted in the vicinity of a neutron star or blackhole by plasma that has flowed downhill from the surface of a nuclear burning companion and been heated by conversion of its gravitational potential energy. The yield of X-ray energy in this process is so high, exceeding in some cases 0.1c2 per unit mass, that X-ray luminosities in excess of 104L⊙ can be generated with accretion rates of only ˜10−B M⊙ per year. Since the transfer process depends strongly on many parameters that specify the relevant properties of two stars and their interaction, one finds a remarkable variety and range of X-ray phenomena. If the compact object is a magnetized neutron star, rotation will cause its X-ray emission pattern to sweep over a distant observer and thereby produce regular pulsations like those observed with periods in the range from 1 to 103 seconds. Orbital motions can cause regular eclipses and absorption dips like those observed with periods in the range from hours to days. Changes in the rate of mass loss by the nuclear burning star or in the transfer efficiency can account for the variations in intrinsic X-ray luminosities that appear as flares, novae and on-off transitions. Irregularities in the flow of plasma near the compact star can also affect the intrinsic luminosity and appear as erratic fluctuations, spikes and shot-noise in the observed intensity.