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.

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.

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.

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 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.

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).

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.

The discoveries of both steady and bursting X-ray sources in globular clusters have made them as interesting for high energy astrophysics and X-ray astronomy as they were for unlocking galactic structure in Shapley’ s day. Globular clusters are massive (˜ 105 M⊙) spherical clusters of primarily low mass (⋞0.8 M⊙) evolved stars and may be the oldest systems in the Galaxy. Thus when several globular clusters were tentatively identified as containing X-ray sources (Giacconi et al. 1974), it was apparent that either dynamical or evolutionary processes must be occurring among the ancient stars in globular clusters to produce the relatively short-lived X-ray sources. The existence of these X-ray sources has prompted considerable discussion as to their origin, since they exist with ˜100 times the probability per unit mass in globular clusters as in the Galaxy as a whole (Katz 1975; Clark 1975). They may be yet another example of X-ray binaries (e. g., Clark 1975; Fabian et al. 1975) or be due to accretion onto a single collapsed object (black hole) at the center of the cluster (Bahcall and Ostrlker 1975; Silk and Arons 1975). At present both of these classes of models are possible and generally consistent with the data, though each has its particular difficulties. We shall review the observational data and then the models in an effort to point out the present balance of evidence for the binary vs. black hole models as well as the most promising directions for future work. Given the possibly high incidence of X-ray bursters in globular clusters, our discussion will refer to both observations and recent models for bursters. While this review will primarily address the X-ray sources in globular clusters, it is obvious that their eventual understanding will contribute much to studies of globular clusters in general and will prompt future studies in particular areas.

A model given by Henriksen and Chia (1972) for short duty cycle X-ray flare sources has been extended by Henriksen (1976), and applied to the recently discovered X-ray bursters. We use their notation here unless otherwise indicated. The model envisages a rapidly rotating magnetic white dwarf or neutron star located in a dense stream or cloud of gas such as might be found in close binary systems or in dense interstellar clouds. The basic charging mechanism for the flare is the accretion of matter into the hydromagnetic wave zone of the rotator at the ‘Bondi’ or ‘tail-shock’ rate (corresponding to a thermal cloud or a stream,-respectively), AB.

We consider spherically symmetric accretion flow onto a strongly magnetized neutron star. We show that, under certain conditions, the flow is intermittent and that the resultant accretion luminosity (X-rays) from the stellar surface is akin to that observed in the bursting X-ray sources. We investigate the properties of such burst flows under a variety of conditions, in the hopes of providing a basic theoretical framework on which realistic models of the observed bursting sources can be built. This work was supported in part by NSF Grant PHY75-08790.

The fate of nuclear energy carried by matter accreted onto a neutron star was considered e.g. by Rosenbluth et al. (1973). They examined pycnonuclear reactions on an originally cold star, and found that the whole star is thereby heated up to 106−107 °K. The thermonuclear reactions that can be ignited then, have been studied by Hansen and Van Horn (1975), who computed stationary burning shells, finding, however, that in most cases the shells are thermally unstable.

We consider here the possibility that X-ray bursts are due to instabilities of this type. The observational constraints fix the parameters uniquely.

Recent progress in identifying the optical counterparts of X-ray sources has been slow, mostly because candidates are faint and X-ray data show no periodicities by which the identification can be confirmed. This report therefore deals with the investigation of some candidates to seek confirmation of their identity with the X-ray emitters, as well as details which are new or important concerning the few known binary sources. I shall also make some general remarks on the properties of the sources as they now appear, but leave a critical assessment of the masses to the next speaker.

The presence of compact x-ray sources in binary systems provides an opportunity to determine observationally the masses of neutron stars and possibly of black holes. These masses are important for equations of state at nuclear densities, for theories of gravitation, and for late stages of stellar evolution. A mass range for each source is determined by finding the sets of orbital elements that are consistent with the available x-ray and optical data. As such analyses are subject to various uncertainties, the resulting mass ranges are rather wide. Nevertheless, significant mass estimates are obtained, accompanied by interesting results on other system parameters.

The most important recent observational discoveries in the field of X-ray binaries are probably those of the slow pulsars and of the winds of normal early-type main-sequence stars. These facts yield key information on the evolutionary history of the X-ray binaries and on the rotational slow-down mechanism for a neutron star in a stellar wind, as will be pointed out in section 3. In the theoretical field, the X-ray binaries have triggered much fundamental work, notably on the detailed processes of mass transfer and on tidal evolution, which will be considered in sections 2, 4 and 5.

The subject of accretion flows in binary X-ray systems has been reviewed recently by Lightman et al. (1977), and the subject of accretion disks by Shu (1976). Here I shall emphasize developments that have occurred since those reviews were written, and concentrate on issues concerning the gas flows rather than the radiation mechanisms.

The coordinated campaign to observe x-ray binaries sponsored by the IAU Commission 42 (Close Binary Stars) and Commission 44 (Astronomical Observations from Outside the Terrestrial Atmosphere) was initiated in January 1974 with Y. Kondo as its coordinator.

The main objectives of the campaign have been: (a) dissemination of information on satellite x-ray experiments and ground-based observing programs; (b) transmission of suggestions and recommendations from campaign participants; (c) designation of campaign dates for observation of specific objects. These functions have, in part, been performed through Campaign Circulars and Special Bulletins; the most recent, Circular No. 23, was issued a few weeks ago.