The purpose of this review is to provide a convenient guide to the expected characteristics of the Space Telescope Observatory for astronomers and physicists. We have tried to provide enough detail so that a professional scientist, observer or theorist,can plan how the observatory may be used to further his observing program or to test theoretical models. Further detail is available in NASA documents that are referenced throughout this report.

The increases in resolution and sensitivity promised by ST over IUE, Copernicus, and OAO-2 will make it a facility of great significance to planetary astronomy, for each of these latter satellites have convincingly demonstrated the ability of earth-orbiting telescopes to contribute to the subject.

One of the prime motivations behind astronomical research ingeneral, and utilization of the Space Telescope in particular, is ourdesire to understand our ultimate origins. With the ST we expect to find answers to fundamental questions concerning the birth and deathof the universe. But another fundamental question of comparable significance deals with more local beginnings. The Sun and its planetary system were formed long after the initial creation, accreting in an obscure corner of our galaxy by processes perhaps less well understoodthan those of the universal Big Bang. In this paper, I consider ways in which the Space Telescope can contribute to studies of the birth and early evolution of the solar system, through studies of its smaller members.

The chief advantages of the Space Telescope over conventional ground-based telescopes, namely, ability to observe ultraviolet radiation and high angular resolution, make it particularly valuable for research on gaseous nebulae. These objects are interesting not only in themselves, but also because they provide an opportunity to measure the abundances of the common light elements in the first two rows of the periodic table. H II regions are samples of interstellar matter from which stars are now forming in our galaxy, and in other galaxies. Planetary nebulae are objects approaching the end of their evolution as luminous stars, in which an outer shell has been thrown off and is returning to interstellar space. They represent one of the most prolific sources of mass return to interstellar matter at the present time in our galaxy.

I am somewhat embarrassed to discuss ST observations of the inter-stellar medium (ISM) of our own Galaxy for two reasons. First of all itseems, at least at the moment, a less exciting subject for the ST thansome others. Secondly, I sadly found, while reading the literature inpreparation for this discussion, that I am not sufficiently familiarwith optical astronomy to be able to guess where the real advances willcome. As a consequence I shall merely summarize (a selection of) observations that have already been proposed, including some for the ISM inexternal galaxies. Unless otherwise stated, all observations for thehigh resolution spectograph (HRS) and the faint object spectograph (FOS)are described in more detail by Brandt et al (1977) and by Harms et al(1977), respectively. Observations for the wide field camera (WFC) andfaint object camera (FOC), on the other hand, were already discussed atlength at the ESA/ESO Workshop on “Astronomical Uses of the Space Telescope” and I shall refer to review papers in the workshop proceedings.

Why are globular clusters so important for astronomy? Here are some reasons. (i) Globular clusters are the oldest luminous objects in the Galaxy. They are samples from the early chemical evolution of the Galaxy, as the metal abundance [m/H] rose from about -2.5 to about -0.5. (ii) They are large enough to give a statistically significant data set for stellar evolution studies over this whole abundance range. (iii) Their stellar dynamics is very inviting; globular clusters are relatively simple structurally, so there seems to be a real opportunity for a fairly complete understanding of their internal dynamics. (iv) Some of their chemical and dynamical problems are similar to those of galaxies (eg the large clusters 47 Tuc and ω Cen show galaxy-like radial abundance gradients). These problems are much more tractable for globular clusters, because we can measure directly the chemical abundances and kinematics of individual cluster stars.

1. 1. A disc and halo population model is constructed to fit star counts and color data down to V ~ 23 at |b| = 90°. This model is used to predict star counts and colors down to V ~ 30. Deviations from these extrapolated relationships might provide constraints on the number of faint quasars and ”black dwarf” stars.

2. 2. The model shows that extra-galactic globular clusters start contributing significantly to ”star” counts at V ~ 25 and are more numerous than stars for V > 31.

3. 3. Morphological studies of galaxies with z ~ 0.5, which are feasible with the Space Telescope, could provide significant constraints on theoretical models that describe the evolution of clusters of galaxies.

4. 4. It is argued that the Space Telescope reduces the need for super-expensive earth-bound telescopes of heroic dimensions.Ground-based observations should, instead, exploit the advantage of low-cost photons that could be collected by mass produced ~3m thin mirror ”people's telescopes”.

The subject of normal galaxies is much too large to cover even superficially in this paper, and I shall necessarily be giving a very limited and incomplete review. I shall tend to emphasize elliptical and peculiar galaxies at the expense of spirals, partly from personal interest, but largely because the Space Telescope will break more new ground in our knowledge of the former. Because we live in one, we have quite detailed knowledge about the structure and contents of at least one spiral galaxy Therefore, the fact that the Space Telescope will allow us to study M31 in the same detail that the Magellanic Clouds can be studied from the ground, and to study spirals in the Virgo Cluster in the same detail that we have been able to study M31 is less important than, for example, the fact that we will be able to directly observe, for the first time, the stellar content of an elliptical galaxy.

Since almost every facet of astronomy is relevant to the evolution of galaxies, most of the topics discussed at this Colloquium are important to the field. For example, stellar populations and inter-stellar matter in galaxies are to be understood in the context of evolution, and when galaxies are used as probes in cosmological studies it is vital to know how their intrinsic properties vary with time. Some aspects of research on galactic evolution with the Space Telescope are discussed by several authors in the recent ESO/ESA Workshop (Macchetto et al., 1979). In this paper, I concentrate on “lookback” studies that compare present-day and younger galaxies directly. Many of the theoretical ideas mentioned here are quite speculative; the aim is to provide viewpoints from which ST (and related ground-based) studies can be planned, knowing that the real world billions of years ago will surely contain more surprises than verifications of any projections.

There is a growing body of evidence that the non-thermal extra-galactic sources in the Universe- -the Radio Galaxies, Seyfert nuclei, QSO's, BL Lacertae objects and the X-ray galaxies- -are all powered ultimately by collapsed objects at the centers of galaxies. Moreover, there is no reason at this stage to doubt that the central energy source is probably the same in all cases. At this point, five years before the Space Telescope is to be launched, the fundamental problem is to identify the nature of the central engine and then to understand in detail the variety of observed phenomena.

I shall discuss studies that can be carried out withthe Space Telescope (ST) of absorption-line systems which may be expected to occur in the spectra of distant objects.

Absorption-line systems with redshifts very different from the emission-line redshift have so far been observed only in the spectra of quasars and BL LAC objects. However, the fact that quasars exhibit rich absorption spectra and galaxies have not yet been observed to do so may be explained by two selection effects. Galaxies are by definition fainter than quasars and most of the strong absorption lines occur in the far ultraviolet. Thus galaxies at the redshifts sufficient to shift the absorption systems into the visible region are too faint to be studied at the high resolution necessary to detect narrow absorption lines.

My original brief was to cover all non-optical observations of all objects in the Universe but I have limited the scope of this presentation to the radio and optical wavebands and within them mostly to the study of extragalactic objects. The paper is in two parts. Sections 1, 2 and 3 contain some general remarks about the use of the Space Telescope and the study of objects in the radio and X-ray wavebands.The remaining sections outline the scientific objectives of a number of projects which involve observations with the Space Telescope.

The Space Telescope shall be useful to check the local extragalactic distance scale to within 10-15 percent. It opens up the opportunity to determine relative distances of cosmic standard candles, viz. brightest M-superqiants and SNe I in E galaxies. The M-supergiants shall map the velocity field out to vo ≈ 3000 km s-1 thus providing (1) a firm basis for the determination of Ho (global) from local distances; (2) the possibility to derive precise distances of all near by field galaxies from their recession velocities; and (3) in combination with the density profile of the Virgo complex an accurate value of the density parameter Ω/Ωcrit. Photometry of the SNe I out to z = 0.5 shall lead to a direct determination of qo via the Hubble diagram, and the form of their light curves offers a fundamental test on the nature of red shifts. The independent determination of Ω/Ωcrit and qo shall give anestimate of the smoothly distributed, invisible matter in the universe and test the assumption Λ = 0.

The business of modern cosmology is the search for the main constituents of the universe, the pattern of their distribution and motion, the relation to the geometry of space-time, and the way the whole arrangement is evolving with time. This leads us to a rich list of research problems. I describe here three examples that illustrate the expected role of space telescope in the endeavour.

There has been presented at this conference a bewildering variety of problems for the Space Telescope, so many and so broad in scope that it is clear that the instrument in its lifetime will not be used for all of them, however able it may be to solve them and however interesting they may be.

There are several conclusions to be drawn from this state of affairs, and I wish mostly in this summary to discuss the conference in the light of those conclusions.

It is a particular pleasure to be here tonight, the degree of pleasure having been heightened by the un-accustomed luxury of participating, albeit rather passively, in the scientific discussions of the past days. The net result of the listening is an increased conviction that astronomy offers, along with biology, the best prospects of all the sciences for major increases both in our comprehension of nature and, simultaneously, in our awe at natures complexities and energies. To the degree that the Space Telescope brings those prospects to fruition, it will have to rank as one of the premier astronomical tools of the 1980’s and 90’s. But I don’t need to preach its merits to this audience. What I’d rather do is to give you some of my views on a topic commonly thought to be an “issue” and which has risen to a state of high visibility within NASA in large part on account of the Space Telescope and its operation. That “issue” revolves around the scientist who happens to work in NASA.