At a joint meeting of the American Astronomical Society and the American Physical Society held in June of 1940, the University of Michigan astronomer Dean McLaughlin (1940) gave a review of the current understanding of stellar evolution. At the end he somewhat facetiously remarked that, “For several years I have told students that I knew all about stellar evolution in 1923, less is 1925, and nothing at all since 1930.” I would like to suggest that those dates were not chosen randomly, and in the first part of my own survey of stellar evolution to 1950, I would like to explain the significance of those dates.

Walter Baade was born one hundred and one years ago; he died in 1960, a third of a century ago. In my opinion he was the second most important observational astronomer of this century, after Edwin Hubble, who changed our place in the universe and our perception of it. I will give some of the evidence in this paper.

This paper reviews the recognition that kinematic characteristics of stars are related to their spectroscopic appearance on low and intermediate dispersion spectra. These differences result from the differences in the abundances of metals as determined from high dispersion spectra. Presumably, these abundance differences reflect differences in age. The high-velocity carbon stars share the molecular peculiarities of the high-velocity oxygen giants but, because of the greater strength of the bands of carbon-containing molecules, exhibit them to a greater degree.

In the development of the concept of stellar populations the classification scheme adopted at the Vatican Conference of 1957 represents a major milestone. Thirteen years after Walter Baade's seminal papers, the conference reviewed progress in relevant fields of research and formulated a classification of population types that would remain the principal reference for the next decades. I shall review developments preceding the Conference, the initiatives that led to the Conference, the new scheme, and a few developments of later date.

“Although the evidence presented in the preceding discussion is still very fragmentary, there can be no doubt that, in dealing with galaxies, we have to distinguish two types of stellar populations, one which is represented by the ordinary H-R diagram (type I), the other by the H-R diagram of the globular clusters (type II). Characteristic of the first type are highly luminous O- and B-type stars and open clusters, of the second, globular clusters and short-period Cepheids…it should be pointed out that these same two types of stars were recognized by Oort as early as 1926. Oort showed that the high-velocity stars of our galaxy (our type II) are of a kind quite different from the slowmoving stars (type I) which predominate in the solar neighborhood.”

We discuss typicalities and atypicalities in the stellar population of a Galactic globular cluster.

The field star halo is comprised of those field stars that are like the stars found in the halo globular clusters. We discuss the halo properties — in particular whether Vrot is a function of z. An analysis of recent surveys for blue horizontal branch stars is described; the halo that is defined by these stars is composite and contains both a spherical and flat component.

A hypothesis or theory is clear, decisive, and positive, but it is believed by no one but the man who created it. Experimental findings, on the other hand, are messy, inexact things which are believed by everyone except the man who did that work.

Correlations between stellar kinematics and chemical abundances are fossil evidence for evolutionary connections between Galactic structural components. Extensive stellar surveys show that the only tolerably clear distinction between galactic components appears in the distributions of specific angular momentum. Here the stellar metal-poor halo and the metal-rich bulge are indistinguishable from each other, as are the thick disk and the old disk. Each pair is very distinct from the other. This leads to an evolutionary model in which the metal-poor stellar halo evolves into the inner bulge, while the thick disk is a precursor to the thin disk. These evolutionary sequences are distinct. The galaxy is made of two discrete “populations”, one of low and one of high angular momentum. Some (minor?) complexity is added to this picture by the debris of late and continuing mergers, which will be especially important in the outer stellar halo.

Observational studies of the relations between ages, metallicities and kinematics of disk stars in the solar neighbourhood are discussed with emphasis on the recent survey by Edvardsson et al. (1993), and galactic metallicity gradients inferred from these nearby stars are compared with gradients determined from distant B stars and open clusters.

Away from the young disk, several classes of early type stars are found. They include (i) the old, metal-poor blue horizontal branch stars of the halo and the metal-poor tail of the thick disk; (ii) metal-rich young A stars in a rapidly rotating subsystem but with a much higher velocity dispersion than the A stars of the young disk, and (iii) a newly discovered class of metal-poor young main sequence A stars in a subsystem of intermediate galactic rotation (Vrot ≈ 120 km s−1). The existence and kinematics of these various classes of early type stars provide insight into the formation of the metal-poor stellar halo of the Galaxy and into the continuing accretion events suffered by our Galaxy.

Early observers measuring 21 cm HI profiles away from the Galactic plane found not only the emission near zero velocity expected from gas in the immediate vicinity of the Sun, but also occasional emission at velocities reaching several hundred km s−1. It seemed unlikely that these spectral lines could come from gas in normal galactic rotation (they are sometimes found at |b| > 45°), and so began the puzzle of “high-velocity clouds” (HVCs). The early result that all HVCs had negative velocity implying that they were infalling was soon shown to be incorrect with the discovery of many positive velocity clouds in the southern hemisphere. Attempts to determine the distance to HVCs by searching for them in absorption against stars yielded only lower limits, typically > 1 kpc. By 1984 several large-scale surveys had established that a significant fraction of the sky was covered with high velocity HI (e.g., Oort, 1966; Giovanelli, 1980). A recent major review is by Wakker (1991a; see also van Woerden, 1993). For this brief presentation to a specialized audience, I will concentrate on issues that may be relevant to the topic of stellar populations.

The Milky Way Galaxy offers a unique opportunity for testing theories of galaxy formation and evolution. The study of the spatial distribution, kinematics and chemical abundances of stars in the Milky Way Galaxy allows one to address specific questions pertinent to this meeting such as

1. (i) When was the Galaxy assembled? Is this an ongoing process? What was the merging history of the Milky Way?

2. (ii) When did star formation occur in what is now “The Milky Way Galaxy”? Where did the star formation occur then? What was the stellar Initial Mass Function?

3. (iii) How much dissipation of energy was there before and during the formation of the different stellar components of the Galaxy?

4. (iv) What are the relationships among the different stellar components of the Galaxy?

5. (v) Was angular momentum conserved during formation of the disk(s) of the Galaxy?

6. (vi) What is the shape of the dark halo?

7. (vii) Is there dissipative (disk) dark matter?

There seems to be no strong evidence that the young globular clusters in the MC have metallicities differing significantly from the metallicities of MC field stars of the same age. The old globular clusters in the LMC are of the same age as, or slightly younger than, those in the outer halo of our Galaxy. It is suggested that the increase in the SFR in the LMC ~ 4 Gyr ago was related to the collapse of the system to a plane. Evidence for a spread in metallicities amongst young stars in either Cloud remains tentative. There is no strong evidence for bursts of star formation being triggered by LMC-SMC-Galaxy interactions but the possibility is raised that the SFR in the SMC has been strongly affected by this interaction.

The stellar populations in M31 and its companions are reviewed from both historical and more modern contexts. Recent data for nearby resolved galaxies suggest that a revision of Baade's original population concept, particularly as applied to elliptical galaxies, is required. In addition, these new data highlight the difficulties in interpreting the data for, and the lack of direct information on, the more distant giant elliptical galaxies being used for cosmological studies.

The aim of the study of the populations in a stellar system is to understand and be able to describe the stellar content of a system in terms of physical parameters such as the age, star formation history, chemical enrichment history, initial mass function (IMF), environment, and dynamical history of the system. This is done given an understanding of stellar evolution and the ability to express the outcome in “observer parameters”, particularly a color-magnitude diagram (CMD), kinematics, and metallicity. From this perspective, the simplest systems are the galactic clusters and the globular clusters, where all the component stars are coeval and of the same metallicity. The current state of knowledge for these are discussed by others in this conference. We proceed to the next level of complexity (where metallicities are not necessarily all the same, and nor are the stars all coeval), and try to decompose their stellar content, particularly in terms of star formation rate and metallicity. In this regard the two classes of objects that come to mind are the dwarf spheroidals, and the dwarf irregulars. Both these classes of objects are more massive than the open clusters and globular clusters, and show evidence of complexities in their star formation histories, without being so convolved as to make such a study intractable. As we shall see, recent studies along these lines have presented some puzzling problems. Moreover, these are the smallest independent galaxies, and the study of star formation in these is likely to shed light on the history and formation of larger and more complex galaxies.