The luminosity function (LF) and present day mass function(PDMF) for main sequence (MS) stars in the Praesepe and Hyades clusters are derived, showing the Wielen Dip which occurs at Mv = 9m in the LF. This dip is about 2 mag fainter than the case for the Pleiades cluster whose Wielen Dip position is consistent with that for the solar neighborhood field stars. The Wielen Dips of these clusters are reproduced by using a bimodal initial mass function (IMF).

The BV photographic photometry was made for 1714 stars (V < 19.5m) in NGC 6752, using 83 photoelectric stars and many secondary photographic standard stars over the area of 3.3' - 11.3' from the cluster center. Their data and finding chart are presented.

Real time CCD differential photometry was performed for BT Cnc in Praesepe cluster from February to March, 1994. New 885 differential V magnitudes were obtained for thirteen nights. From the frequency analysis, we have detected two distinct pulsational frequencies of f1=9.7783c/d and f2=7.0153c/d. The first frequency is nearly equal to the previous result(Breger 1980), but the second one is much different. Our reanalysis of the previous data obtained by Guerrero el al.(1979) indicates that the previous result of f3=5.95c/d might be uncertain; it was not detected in the power spectrum. Also it turns out that our second frequency could not be fitted to the previous data and the reanalyzed frequency(f2=7.8813c/d) of the previous data was poor-fitted to our data. Therefore we suggest that the second frequency which might be newly excited in the nonradial mode, has been changed over the last eighteen years.

More than 22,300 stars in NGC 6752 were measured over the region of 5' < r < 23' in B and V AAT plates. The most of these are main sequence(MS) stars and about 130 blue horizontal branch(BHB) stars were detected. The C-M diagram of all measured stars shows gaps appearing at V≈15.m2 and 16.m2 along the red giant branch(RGB) and their appearance shown by Lee & Cannon(1980) is found to be independent of measured region. The bimodal distribution of BHB stars is confirmed again and a wide gap shown by Lee & Cannon(1980) at V≈16m is clearly seen for stars in the outer part (8' < r < 13') in the cluster. It is noted, however, that this gap is occupied by about a dozen of BHB staIs located in the inner region (5' < r < 8'). The number ratio of bright BHB star (V < 15m) to faint BHB stars (V > 15m) decreases with increasing radial distance from the cluster center. Three faintest BHB stars were found, and two stars (V≈18.m5) of there are located in the inner region of r≈6' and the other faintest one (V≈19.m3) located in the outer part of r≈13'. Also a bluest star of (B - V) ≈ -0.5 at V≈17.m2 is found but it is located at the outer part of r≈13' in NE region. Therefore, the membership of the faintest BHB star and bluest star is suspected. The luminosity function(LF) and mass function(MF) for NGC 6752 were derived for MS stars. The LF for stars of Mv< 6m in the outer part of r > 8' shows a consistency with that derived by Penny & Dickens(1986).

The UBVIKC CCD photometry was performed in the central region(~ 20' × 20') of the extremely young open cluster IC1805. Member stars were selected in the (B-V)-(U-B) and (V-I)-(B-V) color-color planes. Applying recent stellar evolutionary models, we derived the age, age-spread, and initial mass function(IMF) of the cluster. IC1805 was found to be an extremely young(tage ~ 1.5Myr) and has a flat IMF with the slope of Γ=-1.0±0.2.

For the well observed 16 globular clusters with known metal abundance (Z), the helium abundances (Y) and ages are determined by various methods, and the relations between Y, Z and age are examined. The luminosity L R R of RR Lyrae stars is known to be dependent of evolutionary models and pulsation theory in the sense that the pulsation theory and horizontal branch (HB) models yield the anticorrelation between L R R and Z whereas main sequence (MS) and red giant branch (RGB) models yield the direct correlation between them. Similarly the anticorrelation between Y and Z is obtained from the HB models and pulsation theory whereas the direct correlation between them is obtained when the RGB model is applied. The current evolutionary models yield the anticorrelation between Z and age of clusters whenever the direct correlation between Y and Z holds. However when the anticorrelation between Y and Z is applied for age determination, the similar age of clusters is obtained as shown by Sandage (1982b). The ages, which are determined by the fitting of C-M diagrams to isochrones in the ( M v , B-V)-plane, suggest the two different chemical enrichment processes, which could be accounted for by the disk-halo model for the chemical evolution of the Galaxy (Lee and Ann 1981). Also it is known that the R-method is very useful for Y-determination and the derived Y's show the increasing rate of Δ Y Δ Z ≃ 0.5 which is comparable to the observed value of Δ Y Δ Z ≃ 0.3 from HII regions and planetary nebulae by Peimbert and Torres-Peimbert (1976). In this case, the age-metallicity relation of globular clusters could be explained by the disk-halo model.

We examined the characteristics of the SNU uvby, Hβ Hβ filter system through the computation of theoretical photometric indices and the transformation of the observed instrumental system to the natural and standard systems.

The well observed 8 open clusters, NGC 6530, 2264, 654, 129, 2168, Pleiades, Praesepe and Hyades were selected on the basis of photometric observation and proper motion study. The luminosity functions (LF) and mass functions (MF) of these clusters are different with cluster age and they could be divided into three age groups (t< 10 7 yrs, 10 7 10 8 yrs, 10 8 10 9 yrs). From these LF's and MF's, the mean LF and MF of the open clusters are derived and these functions suggest the time-dependent initial mass function (IMF) and the variation of observed MF with cluster age.

We have observed 85 UBV standard stars(V<6) with SNU photoelectric photometer system and 24" Cassegrain reflector at Sobaeksan Observing Station during the period between January and May, 1980 in order to examine the photometer system and to determine atmospheric extinction coefficients. From five night observations, the following results were obtained: (1) The set of filters(V=2mm GG 495, B=2mm GG 385+1mm BG 12, U=2mm UG 2) is better than the other filter set used in our tests on the basis of linear instrumental calibrations for V and B filters. (2) The SNU detecting system is very stable within σ = 0.002 in magnitude in a night but the variation in its stability from night to night is not negligible. (3) The extinction coefficients were derived from two different methods and we have found no systematic seasonal variation in the extinction coefficients although the mean dispersion is considerable.

For ∼ 240 nearby stars their age and mass were determined and kinematic parameters determined for 362 stars, applying Woolley's three-dimensional potential. Metallicity and kinematic parameters of these stars were correlated with their age, suggesting the slow collapse ( t ≳ a few billion years) of the Galaxy and the initial rapid enrichment in metal abundance ( Δ Z ≈ 1 / 3 Z 1 (present) for ∼ 4 × 10 8 yrs). The late slow enrichment rate is given by d ( Z / Z ⊙ ) / d t = 5.9 ∼ 7.0 ± 3.4 per Gyr.