Over the last two decades, the refinement of radio and infrared techniques, as well as work in space, have provided a remarkable amount of information about the center of our galaxy. Much of it fits into a more or less understandable and increasingly well-defined pattern. However, some of this information seems to represent phenomena we do not yet understand, and still other measurements appear to challenge us with inconsistencies. Much of the material to be presented at this conference represents new results which will add both to our information and our understanding. It is hoped this first discussion will properly introduce the overall subject, paying particular attention to those areas which are still puzzling or under debate. Because of the wealth of material, the complexity of the subject, and my own limitations, not all of the many important points and questions will be treated, nor treated equally. Fortunately, the many gaps which I must leave will be filled by others, since there are review talks on most of the various, more specific aspects of the field.

Viewed from outside the Local Group, the Galactic Center would be a paragon of ordinariness, both with regard to its stellar population and its nonstellar emission in the radio, infrared, and X-ray.

Nonetheless, at a very modest level it shows evidence for many of the processes that we believe dominate the properties of exotic galactic nuclei.

Great progress has been made toward measuring the size of the Milky Way. There are now several methods that employ independent calibrations to estimate the distance to the center of the Galaxy, Ro, and these methods have been applied to many types of astronomical objects. Ro estimates generally have been decreasing over the last 15 years. At this time a reasonable “best value” estimate for Ro is 7.7 ± 0.7 kpc.

A distance of R0 = 7.6 ± 1.6 kpc for the galactic center is given by comparison of expansion parallax of a cluster of H2O masers in W49(N) with the kinematic distance of that source. Data from additional epochs, now being calibrated, should further improve the accuracy of our determination of R0.

The phase-lag technique of estimating distance to OH-IR sources is briefly described. Preliminary results are presented from a study of nine OH-IR sources in the galactic centre direction.

Stars in Baade's Window at b = −4° are described. They are old, metal rich, and have a velocity dispersion greater than 100 km/sec. Their colors are unlike those of any other group of stars. Stars in other windows between b = −3 and −12° differ systematically from those in Baade's Window. The differences are best understood if they are caused by a metallicity gradient in the bulge. The distribution of surface brightness or surface density over the bulge has a much steeper falloff than that seen in other galaxies or in the very center of the Milky Way itself. It is now possible to link together stars in the immediate vicinity of the nucleus with those that are several hundred parsecs out. Understanding this linkage will help in understanding the history and evolution of stars throughout the bulge and that part of the inner disk with which it overlaps.

Data on abundances and ages of stars in the nuclear bulge of the Galaxy are in conflict with a scenario in which the bulge has been slowly accumulated by mergers of Magellanic Cloud-like systems taking place over a Hubble Time.

I review the abundance distribution of 88 K Giant stars in Baade's window, and show that the distribution is fit by the simple model of chemical evolution unmodified by gas infall. The metal rich stars in the bulge could not have formed in small galaxies of ≈ 1010 solar masses, which do not retain their metal-enriched gas.

Two large surveys of luminous evolved stars in the bulge–the M giant surveys of Blanco, and the carbon star surveys of Azzopardi, Lequeux, and Rebeirot, find no luminous carbon stars or M supergiants which would signify the presence of an intermediate-age population. I present infrared photometry of 33 bulge carbon stars which shows that they are intrinsically faint, confirming the lack of an intermediate age population.

The lack of young turnoff stars in the color-magnitude diagrams of bulge fields by Terndrup and Rich further strengthens the case against a substantial bulge population younger than 5 Gyr.

JHK photometry of 33 carbon stars we found in the direction of the galactic bulge shows that they most likely belong to the bulge population and that they are intrinsically faint, metal rich objects. The conventional mixing mechanism is excluded in their case and thus their formation is still a mystery.

We have produced images at 1.2, 1.6, and 2.2μm of a 1° × 2° region of the sky that includes the galactic centre using the drift scan techniques described by Catchpole et al. (1985) and Glass et al. (1987). These images were calibrated by observations of standard stars so that it is possible to make quantitative measurements of 3, H and K magnitudes for individual stars. Colour magnitude diagrams have been constructed, using K against H–K, for the stars within elliptical annuli centred on the galactic centre, with major axes along the galactic plane and an axial ratio of 2.2. The distribution of stars within each colour magnitude diagram is compared with a composite giant branch, based on 47 Tuc as seen at a distance of 8.2 Kpc, but extended to higher luminosity by including photometry of long period variables in the Baade windows. The assumption of a unique giant branch for the galactic centre stars is empirically justified by the small scatter in the colour magnitude diagrams beyond about 1.5° from the galactic plane. Fig. 1a & b give examples of colour magnitude diagrams for 400″ × 400″ areas of sky far from and near to the galactic centre. The observed distribution of stars relative to the 47 Tuc giant branch allows us to measure both the foreground absorption and the number distribution of stars within a given range of dereddened apparent magnitude. The number of stars counted must be corrected for the effect of crowding which is estimated by generating simulated star fields and comparing the number of stars found using our reduction programmes, with the number used to simulate the fields.

It is shown that the observed motion of neutral hydrogen in the inner 1000 pc of the Galaxy is, for the most part, consistent with flow on circular streamlines in the potential of the Galactic bulge as derived from the observed distribution of near infrared emission. The implied mass distribution is also consistent with recent kinematic determinations of the stellar mass in the inner few parsecs of the bulge. The non-circular gas motion seen between two and four kpc is most likely due to flow on elliptical streamlines in the presence of a weak bar distortion of the Galactic disk. Circular gas motion in the region of the bulge and elliptical streaming further out is an observed characteristic of flow in barred galaxies and is consistent with our present theoretical understanding of such systems. The implication is that non-circular motions of the molecular clouds in the inner 200 pc have a non-gravitational origin. A possible mechanism for exciting such motions is an accretion event resulting from an encounter of a molecular cloud with a massive black hole. A starburst leading to a high supernovae rate 107 years ago in the inner 50 pc is an alternative explanation. Observations of molecular cloud regions in the nuclei of external normal galaxies could distinguish between alternative mechanisms.

This review discusses the complex morphology and perturbed kinematics, as well as the outstanding physical characteristics of the interstellar medium within a few 100 pc of the galactic center. A total of ~107.9 Mo of dense molecular gas, representing ~10% of the Galaxy's neutral mass content, has settled in a thin layer [size: 450 × 40 pc], composed of giant molecular cloud complexes. The spatial distribution is highly asymmetric with respect to the center, and motions differ considerably from equilibrium conditions. The dynamical situation is still obscure (explosive event vs. response to distorted gravitational potential), but any disturbance must have occurred quite recently (τdy ~106 yr). Evidence for large–scale star–forming activity is reviewed, and for a standard IMF, a total star formation rate Φ ~0.5 M0 yr−1 is inferred.

The galactic center clouds differ considerably in their physical and chemical characteristics from GMC's in the outer Galaxy. Pervasive high bulk gas temperatures (Tkin ~70 K), mean H2 densities of n ~104 cm−3, and linewidths ≥10–20 kms−1 are probably a consequence of the clouds' location in the steep gravitational potential of the central bulge.

Abundance measurements in the Galactic Center (GC) probe material with a nucleosynthetic history unique in our Galaxy. The measurements are of two types: probing interstellar and stellar material. Measurements of gas-phase abundances are mostly toward SgrB2 and SgrA. They reflect the current state of nuclear evolution in the GC and include several important isotope abundance ratios. The isotope ratios provide the most accurate information and allow for comparison with results elsewhere in the interstellar medium. The second type of measurement is of abundances in (and around) stars, yielding chemical abundances in the bulge and disc populations, and reflecting the state of nuclear evolution when the stars were born.

When combined with our knowledge of evolution in the solar neighborhood, several of the results are consistent with the the greater nuclear “maturity” of the inner Galaxy. However, there are several important exceptions, which point to the fact that we really do not understand nuclear processing in the GC region. Some of the open issues may be resolved observationally, and with new infrared and millimeterwave techniques a clear opportunity exists to improve the observational record. Results should increase our understanding of stellar nucleosynthesis and of the history of star formation in the GC.

IRAS observations of the Galactic Center are presented. The maps at 12, 25, 60 and 100 μm have a typical resolution of a few arcmin and cover an area 6×2 deg2 centered on SgrA. All data have been corrected for the zodiacal light contamination and for the diffuse emission associated with the galactic disk. The infrared radiation originating in the Galactic Center arises from well-defined sources which can be identified with known radio sources and from an extended emission which accounts for most of the infrared luminosity. The total infrared luminosity associated with the Galactic Center (area 3°×2°) amounts to 109L⊙. The sources contribute 10% to this luminosity. A comparison with surveys in the radio continuum and in the near infrared is presented. Results are analyzed in terms of star formation history. In particular, it is suggested that the dominant heating source for the dust is the population of cool stars - K and M giants - comprising the galactic nucleus.

A decade of galactic center observations at the Crawford Hill 7 m antenna is summarized. The galactic center region contains several hundred high-mass, high-density molecular clouds with physical properties very different from clouds in the outer galactic disk. There is also a considerable amount of molecular gas not bound into clouds, but sheared by differential rotation into a molecular inter-cloud medium not seen elsewhere in the Galaxy. These observations can be explained by a combination of the tidal density limit and the virial theorem. The distribution of emission on the sky and in velocity suggests that most of the dense gas is confined to a 500 pc long ridge of emission which may be a dust lane along the central bar.

We have observed CS (J=1–0/2–1) lines in a 60′×30′(1×b) area of the Galactic center region. There are two large-scale features with elliptical shape in the position–velocity maps, which suggest shell–like structures in the region. One is the ring with a radius of ~40 pc in the positive galactic latitude region and another is the shell with a radius of ~20 pc in the negative galactic longitude region. These could be due to suggest star bursts occurred in the Galactic center region.

Preliminary results are reported from an OH mainline absorption study of the galactic centre. The molecular clouds form a well-defined structure 2 kpc in projected diameter, which is warped in its outer regions. The distribution of molecular clouds is highly organised. There are places where the line-widths are anomalously high. The possible nature of these broad-line clouds is briefly discussed.

Spectroscopic observations of CII line emission at 157.7 μm have been made of the Galactic Center region with a Fabry-Perot spectrometer onboard a balloon telescope. Strong emission has been detected ubiquitously in a wide area extending between ± 0.7° in galactic longitude. A ring-like structure is suggested from the double lobed distribution of the emission around the Galactic Center.

We have made 55″ resolution maps of the 158 μm [CII] emission line in the region of the curved, thermal filaments and the +20 / +50 kms−1 molecular clouds in Sgr A. The [CII] emission is spatially well correlated with the radio continuum in the filaments. The large intensity of the [CII] radiation excludes shocks as the origin of the ionization and we conclude that the curved filaments are most likely photo-ionized HII regions at the surface of dense molecular clouds. Our [CII] maps of the +20 / +50 kms−1 clouds indicate that the +50 kms−1 cloud is close to (<10pc) Sgr A west while the more massive +20 kms−1 cloud is at a greater distance from the center (>30pc).