Differences in the distributions of v sin i for various samples of B-type stars may reflect different mechanisms of star formation. The overall distribution of v sin i for late B-type stars in clusters at low galactic latitudes is bimodal, but normal class V field stars may have a Maxwellian distribution of v. The spin axes of stars in tightly bound clusters may be preferentially aligned perpendicular to the galactic plane. Early B-type stars in the youngest subgroups of associations may have a Maxwellian distribution of v, but there is a prominent excess of slow rotators among stars in older subgroups and field stars. This excess cannot be entirely accounted for by tidal or magnetic braking during main-sequence evolution.

There are indications of the presence of numerous stellar clusterings in the Milky Way which are so poor and unconspicuous that they become detected merely by accident. The true frequency of such objects is probably considerable as there is reason to believe that only a minor fraction has been detected.

NGC 581 was studied using new plates taken by the Kottamia 74′ telescope and measured by the Helwan Iris photometer. A mean value of 3.13×107 years and 2334 pcs were found for the age and distance of the cluster respectively. The cluster has an evolved giant branch.

In a previous investigation of the Luminosity Functions (LF) it was found that some SMC clusters show significant characteristics like the very large number of stars at Mv = O. Omag and a gap just below the Horizontal Branch (HB) like feature of their colour-magnitude (c-m) diagram (Kontizas and Kontizas, 1982).

This review will attempt to do two things: (i) discuss some of the data which are available for testing the theory of evolution of low mass stars, and (ii) point out some problem areas where observations and theory do not seem to agree very well. This is of course too vast a field of research to be covered in one brief review, so I shall concentrate on one particular aspect, namely the study of star clusters and especially their colour-magnitude (CM) diagrams. Star clusters provide large samples of stars at the same distance and with the same age, and the CM diagram gives the easiest way of comparing theoretical predictions with observations, although crucial evidence is also provided by spectroscopic abundance analyses and studies of variable stars. Since this is primarily a review of observational data it is natural to divide it into two parts: (i) galactic globular clusters, and (ii) old and intermediate-age open clusters. Some additional evidence comes from Local Group galaxies, especially now that CM diagrams which reach the old main sequence are becoming available. For each class of cluster I shall consider successive stages of evolution from the main sequence, up the hydrogen-burning red giant branch, and through the helium-burning giant phase.

Since the mid-1970's it has been apparent that giant stars of similar V and B-V within a “normal” globular cluster [i.e., one with a narrow giant branch in its color-magnitude diagram (CMD)] exhibit a perplexing range of strengths for such spectral features as CH, CN and NH. This complex subject has been reviewed by Kraft (1979), McClure (1979) and Freeman and Norris (1981). DDO photometry first revealed star-to-star differences of CN strengths at MV>+1, where observational confusion between asymptotic and first-ascent giant stars is removed (Hesser, Hartwick and McClure 1976, 1977; Hesser 1978). Subsequently, we have sought to place observational constraints on possible mechanisms by studying such questions as: At what MV's do spectral differences first become observable? Do spectral features other than those from CNO-based molecules vary from star-to-star? Can small temperature or gravity differences produce the observed ranges?

VandenBerg (1983a; hereinafter V) has recently computed a large grid of stellar evolutionary sequences from the zero-age main sequence to the base of the red-giant branch (RGB) and carried out extensive comparisons of the associated isochrones with published photometry of globular clusters (GCs). These calculations were based on the latest Los Alamos opacities and, in addition, made use of available model atmospheres for improved surface boundary conditions as well as for the transformation of the stellar models from the (Mbol, log Teff)-to the (Mv, B-V)-plane. In general, very encouraging agreement of the predicted and observed morphologies of cluster C-M diagrams was obtained.

Globular cluster work touches on virtually all branches of astronomical research. The age of the globular system (or any variations in age among individual clusters) has important cosmological implications as well as relating to the formation time of the halo of our galaxy. Star to star chemical inhomogeneity within a cluster may set important constraints on either mixing within the stars themselves or on the chemical inhomogeneity of the early universe. Metallicity variations among clusters may provide the clue to galaxy-wide enrichment processes, while the cluster color-magnitude diagrams themselves are a testing ground for virtually every facet of stellar evolution.

The individual photometric study of the coeval stars in globular clusters presents one of the best observational tests of the stellar evolution theory. Our own globular cluster system provides fundamental clues to the dynamical and chemical evolutionary history of the galaxy, and the study of their ages give us a lower limit to the age of the galaxy as well as to that of the universe. We have been undertaking a systematic research program, and discuss herewith the ages deduced by fitting main sequence photometry to theoretical isochrones of six galactic globular clusters : M4, M22, M30, NGC 288, NGC 3201 and NGC 6397.

A globular cluster is an admirable object for the study of stellar evolution and theoretical investigations of the isochrones in the colour-magnitude diagram (CMD) have been numerous, with the most recent being that of Vandenberg (1983, Ap. J. Suppl. 51, 29), but there are still uncertainties such as rotation, abundance variations and gravitational settling. The matching observations are in fair agreement. However only a small number of clusters have been observed and the detailed matching is poor. Most of the CMDs are from photoelectrically calibrated photographic photometry which are affected by the variable background found in clusters. Modern detectors suffer less from this problem and are very sensitive. A programme has been underway on a number of clusters using electronographs and CCDs with the SAAO 1.Om telescope and the crowded-field Starlink Aspic software. This paper reports on preliminary results from four of the clusters. The CCD observations were done in collaboration with W.K. Griffiths of Leeds University.

Simulations of globular cluster luminosity functions have shown that many of the features observed in actual clusters are probably statistical fluctuations. We suggest one new way of looking at the data which may ultimately lead to some information about the physical parameters of clusters.

Deep electronographic photometry has been performed down to mv 21.7 with the 90 mm McMullan camera on the Danish 1.54 m telescope at La Silla. Three exposures per colour were obtained. The plates have been scanned with the Geneva microdensitometer. The selected stars are isolated and are located in the outer corona. The magnitudes are obtained by the profile fitting method, cf. Blecha (1982), which enhances the dynamical range of the plate and reduces the noise due to crowding. The random error appears magnitude independent down to mv = 21 with σ = .05 per colour. Below mv = 20 a number of galaxies were identified and rejected according to their image profiles.

In the log Te, log P' plot for RR Lyrae variables Sandage (1981) has noted that at a given color the period of the variables decreases as cluster metalicity increases. In particular the variables in M3 were shifted in log P' by 0.065 days compared to M15. The only explanation he could find for this shift was that Y in M3 was 0.05 less than in M15 and thus anticorrelated with Z. I have investigated the possibility that the shift is due to variations in [CNO/Fe]. At least for the range of [CNO/Fe] considered, it does not seem possible to explain the shift at constant Y.

Studies are underway concerning the masses of horizontal-branch A stars. Hayes and Philip (1979) discussed masses determined for seven field HB stars using gravities and temperatures obtained from spectrophotometric scans. They found a mean mass of 0.6 M⊙ ± 0.4 for these stars. However, there was a strong trend of mass with temperature. In the present study four-color measures of over 150 FHB and BHB stars have been dereddened and plotted in the grid relating (b-y), c1 and log g, Teff (Philip and Relyea 1979). If one calculates a mean mass, again the figure of 0.6 M⊙ is obtained. A more detailed look at the data, however, reveals structure in the distribution of points in the various diagrams. A discussion of this result follows.

To understand the evolution of carbon and oxygen rich asymptotic giant branch (AGB) stars it is important to observationally establish the relations between parameters such as luminosity, age, and metallicity. The existence of luminous (brighter than the top of the first giant branch) AGB stars in clusters in the Magellanic Clouds (Mould and Aaronson 1979; Frogel, et al. 1980) provides a nearly ideal situation in which these relations can be determined. Although metallicities and main sequence turn-off ages are known for only a fraction of the clusters, the ranking scheme devised by Searle, et al. (1980; SWB) indicates that age and metallicity are closely related to one another.

The evolution of planetary nebula (PN) nuclei has been studied at masses of 0.60, 0.70 and 0.76 M⊙, and for the ejection of the PN at various phases of a helium shell flash cycle. The evolution at high luminosities takes longer for nuclei resulting from PN ejection at shell flash peak than it does for those resulting from ejection in the inter-flash phase. Comparison of our calculations with various observational results does not allow us to reach any definite conclusions regarding the phase of the shell flash cycle at which PN ejection occurs.

Near the tip of the AGB we find the optical Mira variables and also the OH-IR stars. Observational data on the luminosities, periods, and population membership of the OH-IR sources are beginning to be available; in principle this information should constrain AGB models very strongly. In practice, current data are found to be insufficient to unambiguously determine the relation between Miras and these sources; in fact three very different interpretations are currently possible.

The Ba II, CH, and R0-R3 carbon stars are related in that they are giant stars having about the same position in the HR diagram, and having enhancements of carbon features. The Ba II and CH stars are also noted for enhancements of s process elements. The Ba II stars normally don't exhibit C2 bands that are characteristic of the CH and R stars, indicating that the Ba II stars have a C/O ratio close to but less than unity (Smith 1983). The Ba II and R stars are members of the old disk population (e.g. Eggen 1972), whereas the CH stars are high velocity, population II stars (Keenan 1942).

We show that Ba stars cannot originate from single stars, they can only be formed in binaries by mass transfer. The companion must then have been an evolved star with log L/L⊙ ≥ 3.2, requiring a radius larger than 0.5 a.u., explaining the long periods of the observed Ba star binaries. If the companion lost half of its mass to the present Ba star we expect to see a white dwarf companion now. Companions with Teff ≈ 12000K have been seen for the nearest Ba stars. For Ba stars at larger distances companions can only be detected if they have Teff ≥ 20000K.

By definition, the chemically peculiar stars have element abundances that are non-solar. Many of these stars show peculiarities in s-process elements. This paper discusses observable indicators of the neutron environment for the s-process in these stars.