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An evolutionary model for the M31 inner disk population is described, which at age 12 billion years agrees closely with the narrow-band colors and line indices recently measured by Spinradet al. (1971), and with the broad-band colors from 0.36 to 3.4 μ. Assuming that gE galaxies have the same stellar population as the M31 inner disk, this model is used to derive evolutionary effects in cosmology. Interpretation of the magnitude-redshift relation is substantially affected, since the evolutionary correction to V magnitudes at redshift 0.2 may be 0.09 mag. if H0=100 km s−1 Mpc−1, or ≥0.25 mag. ifH 0=50. Changes in some line strengths may be observable at redshift 0.46, showing the younger stellar population at earlier times. Our results differ from estimates by other authors, also based on the assumption that the gE and disk populations are identical, that the photometry of Spinradet al. (1971) precludes a significant rate of evolution. One reason for the difference is that our model has an unexpectedly shallow luminosity function just below the main sequence turnoff; another reason is that authors in the past have used the change in absolute V magnitude as the evolutionary correction, whereas the appropriate quantity is the change in absolute magnitude at theemitted wavelengths, and evolution is more rapid at these shorter wavelengths.

Before turning to the nearest stars, we shall first make another survey. From the observational point of view the very brightest stars offer obvious advantages. However, apparent brightness is not a good criteron of distance (Chapter 3). True, on the average the apparently brighter stars are nearer, but it is hopeless even to make a guess at the distance of individual stars on the basis of brightness alone. The reason is the tremendous range in stellar luminosities, illustrated by the limited sample of the brightest twenty-one stars, not including our Sun, arranged in order of their apparent visual brightness (Table 5.1). These stars are found in different parts of the sky. Several objects are double, multiple, faint companion stars have been ignored; Capella and Alpha Crucis are blended.

In connection with the spectrophotometric study of population-type characteristics of various kinds of stars, a statistical analysis of kinematical and distribution parameters of the same stars is performed at the Toruń Observatory. This has a twofold purpose: first, to provide a practical guide in selecting stars for observing programmes, second, to contribute to the understanding of relations existing between the physical and chemical properties of stars and their kinematics and distribution in the Galaxy.

The results of some 130 years of double star discovery and measurement are briefly reviewed. Knowledge of stellar masses is perhaps the most important of these results; this in turn gave rise to dynamical parallaxes. As shown by statistics of the near neighbourhood of the Su, binaries, and even multiple stars, are by no means exceptional occurrences. Probably not more than half the stellar population is formed by single stars, the other half occurring as binaries or more complex combinations.

Methods of construction composite models of galaxies are outlined. The available observational information renders it possible to distinguish the following populations in nearby galaxies: nucleus, core, bulge, halo, disk, young population. Parameters of models are presented on six galaxies (our Galaxy, M 31, M 32, M 87, and the dwarf galaxies in Fornax and Sculptor). New data indicate that the degree of mass concentration to the centre of the galaxies is much higher than adopted in previous models, and that giant galaxies maybe surrounded by massive coronae of very large dimensions.

Some supernovae have appeared at considerable radial distance, outside of the optical boundaries of their parent galaxies. This raises the important question from what kind of stellar population they have originated. This is a first, preliminary report on three typical cases.

A statistical analysis of the UHURU catalogue of X-ray sources leads to the following conclusions. If the weak sources are omitted, there is a strong concentration to low galactic latitude but the absence of a strong background and the presence of some strong sources at low galactic longitude indicate an appreciable number of sources of luminosity L ~ 104 L ⊙ in the vicinity of the ‘nuclear bulge’. This region generally suggests ‘stellar population II’ and therefore stars of small mass. However, there is some suggestion of a second class of sources, distributed in the galactic plane like ‘stellar population II’ and suggesting large stellar masses.

The apparently early evolutionary state of the Magellanic Clouds, while they are coeval with the Galaxy in their earliest star condensations, may be due to perturbation by the Galaxy as well as to their low mass and low density. Whereas classical barred spirals are probably characterised by large angular momentum, the Clouds may owe their ‘barred’ and asymmetrical distribution, including departure of the centre of rotation from the geometrical centre, to local fluctuations in H1 density and rate of star formation at different epochs. Comparison is made with some other asymmetrical galaxies.

Because the Magellanic Clouds provide a large number of variables at approximately the same distance and with little differential reddening, they provide a unique laboratory for the study of cepheid variability. Much more systematic observation of the variables will be necessary to fully exploit the possibilities they provide. It is already clear that the Magellanic Cloud variables differ from the galaxy variables in the number of variables at different periods, in the period amplitude relation, and, apparently, in the mean color. These differences must be due to intrinsic differences in the nature of the stellar populations of the Clouds and of the Galaxy. Finally, it has been found that certain non-linear features of cepheid variables can be correlated with stellar radius and luminosity. In the case of the Clouds, these features can be used to deduce distances in a new way.

Before discussing observational inputs and actual model calculations, I want to give a very elementary review of the relevant parts of stellar evolution theory. We shall only be dealing with stellar masses M below the Chandrasekhar limiting mass M ch(~1.2–1.45 M ⊙,depending on chemical composition). The inequality M

Extragalactic radio sources can be grouped spectroscopically into those that show absorption lines produced by a stellar population (e.g. the nearer radio galaxies, and some more distant ones, usually of D or E type) and those that show only lines (usually in emission) produced by low-density gas (e.g. QSO’s and many N-type galaxies). If absorption lines from stars are present, one can estimate a minimum age from the stellar population, and perhaps a mass for the system from the stellar velocity dispersion. If no spectroscopic evidence on the stellar population is available (and this includes evidence from narrow-band filter photometry), then we have much less hold on the problems of age and evolutionary state. Absence of stellar absorption lines does not necessarily mean that stars are absent, but simply that their light, if present, does not contribute appreciably to the overall optical spectrum.

The characteristics of the RR Lyrae stars in the Magellanic Clouds are discussed. The existence of numerous RR Lyrae variables in the Large Magellanic Cloud (LMC) cluster NGC 1835 is noted. The variables indicate that this cluster is of Oosterhoff type I. No cluster of Oosterhoff type II has yet been recognized in the Clouds. Some new results of a study of the RRLyrae variables in an LMC field 1° × 1.°3 are discussed. Periods have been determined for 50 out of 72 probable RR lyraes in this field. RR Lyrae ab stars with periods less than 0.d46 are absent. The time averaged \( \overline {\left\langle B \right\rangle } \) and \( \overline {\left\langle V \right\rangle } \) are 19.m56 and 19.m20 with surprisingly small dispersion. First impressions of results for a similar field in the Small Magellanic Cloud (SMC) suggest that the old stellar population of the SMC may have a slightly lower metal abundance than that of the LMC. The best available distance moduli for the Magellanic Clouds indicate a mean absolute visual magnitude \( {M_{\overline {\left\langle V \right\rangle } }}\) of +0.m5±0.2 for the RR Lyrae stars in these systems.

A star showing extreme features characteristic of the Ba n stars has been found in the old open cluster NGC 2420. A new colour-magnitude diagram has been constructed for the cluster, and a distance modulus of 11 . m 7 is obtained by fitting the unevolved main sequence to the zero-age main sequence. The location of the Ba star is consistent with its being a disc population star of absolute magnitude M v = −0 . m 3, mass 1.4 M ⊙ and age 2.4 × 109 yr.

Many people have attacked the problem of synthesizing the stellar population of the galaxies. We have performed such a synthesis, by using only the intensities of absorption lines in the nuclei of galaxies. It is then possible to obtain a synthesized continuous spectrum even if there are several possible solutions for the stellar composition, and the computed continuum does not vary much with the assumed model of stellar population.The method is to fit the equivalent widths of the absorption lines using different stellar compositions. In order to avoid any instrumental effect, we have observed different lines and bands from 3500 to 8500 å in stars of various well-known stellar types and luminosity classes, under the same conditions as the galaxies.We have applied this method to some ‘ordinary’ nuclei and to some Seyfert ones. As an example, we show the results for M81 and for NGC 1068.

IT is a well-established fact that energy production in stars is due to the thermonuclear transformation of hydrogen into helium. Since hydrogen forms about one half of normal stellar material, the rate of its consumption, that is, the luminosity of the star, determines the stellar life-span; in the case of our sun it can be calculated to be about fifty thousand million years, which is very long as compared with the estimated age of the stellar universe. Since, however, stellar luminosity (hydrogen consumption) increases very rapidly with stellar mass (hydrogen content), the life-spans of brighter stars are considerably shorter. Thus stars of about zero absolute magnitude would run through their hydrogen evolution in a few thousand million years, that is, in a period of time which is generally accepted as representing the age of our universe. Indeed, we notice that the stars of zero absolute magnitude seem to represent a demarcation line in the stellar population of our galaxy. Thus the main sequence of the Russell–Hertzsprung diagram shows a noticeable discontinuity at that particular point. The brighter stars are very scarce in space and are in a state of rapid axial rotation; the fainter stars are very abundant and scarcely rotate at all. Also, within stellar population II, as defined by Baade (the regions of the galaxy where no interstellar material is present), the upper part of the main sequence is completely absent.

ences, these differences are not related in a straightforward way to stellar population. A major difference between stellar populations may be found in their kinematical properties. It has been shown (Vyssotsky 1957) that younger stars have low velocity dispersions, and that the major axis of their velocity ellipsoid deviates from the direction of the galactic center. A study of the space motions of normal and metallic-line stars might reveal differences related to stellar population.

New observations in the low-absorption window that is centered on NGC 6522 yield a foreground reddening = 0.45. Combining this reddening value with the assumption that (Mv)RR = +0.5::::: 0.3 and that Av/ = 3.3 ::::: 0.6 yields a distance of 9.4 i 2.0 kpc to the center of the Galaxy. This is in excellent agreement with the value 9.5::::: 1.4 kpc that is obtained from radio observations of the product ARo. In a color-magnitude diagram the red giants of the nuclear bulge are concentrated in a zone that lies between the giant branch of M 3 and the lower envelope to the giant region occupied by field stars near the sun. The lower (metal-rich) part of this region is much more strongly populated than is the upper (metal-poor) part of this zone. It is concluded that the dominant stellar population in the nuclear bulge is metal rich. This conclusion is supported by measurements of the colors of "empty" sky patches near NGC 6522 which provide information on stars near the main-sequence turnoff point in the nuclear bulge of the Galaxy. Key words: stellar populations - galactic nuclear bulge - galactic distance scale

If massive stars evolve toward later spectral types as they move away from their initial positions on the main sequence, and if stars of higher mass evolve more rapidly than those of lower mass, then the brightest main sequence stars of a galactic cluster will be of progressively later spectral type as the cluster evolves. Thus, a stable cluster whose earliest stars are Ο stars (that is, an O-cluster) will become a cluster whose earliest stars are Β stars (a Bcluster), then an Α-cluster, and so on. If, at the same time, less massive stars are evolving toward the main sequence by gravitational contraction, the total stellar population of the cluster will appear to increase, because the number of stars contracting to the main sequence at any time is greater than the number leaving it. This apparent increase accounts for the observation that, in general, late type clusters contain more stars than early type clusters. Markarian1· 2 has discussed the possible evolution of O-clusters into Band Α-clusters, but he neglected the possibility that the total apparent number of stars in a cluster may increase with time. He therefore concluded that Oclusters do not evolve into clusters of later types and that Band Α-clusters are formed independently. If the apparent number of stars in a cluster can increase, the theory of independent origin is no longer necessary to explain the Band Α-clusters, and we are free to examine the possibility that all galactic clusters have a common origin. Since most, if not all, O-clusters are the nuclei of O-associations, O-associations may be the source of galactic clusters. The following evidence, both direct and indirect, is consistent with this possibility : 1. The distributions and motions of galactic clusters and Oassociations within the Galaxy are very similar. 2. The work of Sandage3 and of the Jascheks4 shows that the

A recent paper has given an outline of a revised system of classification of the forms of galaxies, together with types for 608 systems. That investigation will be designated Paper I, and the present one Paper II. The galaxies classified on the revised Yerkes system in Paper I include a large fraction of the northern entries in the ShapleyAmes Catalogue. It seemed advisable to make the classification approximately complete to some limiting apparent magnitude ; a program was therefore devised to classify a number of additional galaxies. Through the kindness of Dr. I. S. Bowen and the Committee of the Mount Wilson and Palomar Observatories, I was permitted to make use of the original negatives of the National Geographic Society-Palomar Observatory Sky Survey for the purpose. The present paper completes the classification of all galaxies brighter than magnitude 13.1 in the Shapley-Ames Catalogue and north of O = —25°, with two exceptions : the object NGC 2149 has been omitted from the catalogue, since it appears to be a diffuse nebulosity, rather than a galaxy; NGC 2798 was omitted inadvertently. The galaxies included in the present paper were classified on the 48-inch Sky Survey plates during a period as guest investigator at Pasadena in the autumn of 1958. A large number of galaxies in Paper I were reclassified without knowledge as to the earlier type assigned ; this overlap furnishes material for an accurate determination of the systematic characteristics of the two series. ii

ONE of the most interesting and most mysterious features of the stellar contents of our own and other known galaxies is the fact that there seem to exist two rather distinct types of stellar populations differing between themselves by mechanical as well as by physical properties of individual members. In fact, as was first indicated by Oort1, the so-called ‘high-velocity' stars which describe highly elongated elliptic trajectories around the galactic centre seem to possess rather different physical characteristics as compared with the ‘ordinary' stars predominating in the neighbourhood of our sun and moving along regular, almost circular, orbits. This distinction was amplified by the recent work of Baade2, who has shown that the ordinary type of stellar population (which he calls type I) is characteristic only for the region of spiral arms, whereas the other type (type II) forms the structure of the galactic nucleus along with the tenuous, almost spherical, ‘atmosphere' surrounding the entire system. All elliptical and spherical galaxies seem to be formed entirely from stars of type II.