A Wide Field Planetary Camera 2 Study of the Resolved Stellar Population of the Pegasus Dwarf Irregular Galaxy (DDO 216)

We present V and I photometry of the resolved stars in the cometary blue compact dwarf galaxy UGC 4483 using Hubble Space Telescope Wide Field Planetary Camera 2 (WFPC2) images. The resulting I vs. (V-I) color-magnitude diagram (CMD) reaches limiting magnitudes V = 27.5 mag and I = 26.5 mag for photometric errors less than 0.2 mag. It reveals not only a young stellar population of blue main-sequence stars and blue and red supergiants, but also an older evolved population of red giant and asymptotic giant branch stars. The measured magnitude I = 23.65 +/- 0.10 mag of the red giant branch tip results in a distance modulus (m-M) = 27.63 +/- 0.12, corresponding to a distance of 3.4 +/- 0.2 Mpc. The youngest stars are associated with the bright H II region at the northern tip of the galaxy. The population of older stars is found throughout the low-surface-brightness body of the galaxy and is considerably more spread out than the young stellar population, suggesting stellar diffusion. The most striking characteristics of the CMD of UGC 4483 are the very blue colors of the red giant stars and the high luminosity of the asymptotic giant branch stars. Both of these characteristics are consistent with either: 1) a very low metallicity ([Fe/H] = -2.4 like the most metal-deficient globular clusters) and an old age of 10 Gyr, or 2) a higher metallicity ([Fe/H] = -1.4 as derived from the ionized gas emission lines) and a relatively young age of the oldest stellar population in UGC 4483, not exceeding ~ 2 Gyr. Thus our data do not exclude the possibility that UGC 4483 is a relatively young galaxy having formed its first stars only ~ 2 Gyr ago.

We present V and I photometry of the resolved stars in the cometary dwarf irregular galaxy NGC 2366, using Wide Field Planetary Camera 2 images obtained with the Hubble Space Telescope. The resulting color-magnitude diagram reaches down to I ~ 26.0 mag. It reveals not only a young population of blue main-sequence stars (age 30 Myr) but also an intermediate-age population of blue and red supergiants (20 Myr age 100 Myr) and older evolved populations of asymptotic giant branch (AGB) stars (age 100 Myr) and red giant branch (RGB) stars (age 1 Gyr). The measured magnitude I = 23.65 ± 0.10 mag of the RGB tip results in a distance modulus m - M = 27.67 ± 0.10, which corresponds to a distance of 3.42 ± 0.15 Mpc, in agreement with previous distance determinations. The youngest stars are associated with the bright complex of H II regions NGC 2363 (=Mrk 71) in the southwest extremity of the galaxy. As a consequence of the diffusion and relaxation processes of stellar ensembles, the older the stellar population is, the smoother and more extended is its spatial distribution. An underlying population of older stars is found throughout the body of NGC 2366. The most notable feature of this older population is the presence of numerous relatively bright AGB stars. The number ratio of AGB to RGB stars and the average absolute brightness of AGB stars in NGC 2366 are appreciably higher than in the BCD VII Zw 403, indicating a younger age of the AGB stars in NGC 2366. In addition to the present burst of age 100 Myr, there has been strong star formation activity in the past of NGC 2366, from ~100 Myr to 3 Gyr ago.

We infer the properties of massive star populations using the far-ultraviolet stellar continua of 61 star-forming galaxies: 42 at low redshift observed with the Hubble Space Telescope and 19 at z ∼ 2 from the MegaSaura sample. We fit each stellar continuum with a linear combination of up to 50 single-age and single-metallicity starburst99 models. From these fits, we derive light-weighted ages and metallicities, which agree with stellar wind and photospheric spectral features, and infer the spectral shapes and strengths of the ionizing continua. Inferred light-weighted stellar metallicities span 0.05–1.5 Z ⊙ and are similar to the measured nebular metallicities. We quantify the ionizing continua using the ratio of the ionizing flux at 900 Å to the non-ionizing flux at 1500 Å and demonstrate the evolution of this ratio with stellar age and metallicity using theoretical single-burst models. These single-burst models only match the inferred ionizing continua of half of the sample, while the other half are described by a mixture of stellar ages. Mixed-age populations produce stronger and harder ionizing spectra than continuous star formation histories, but, contrary to previous studies that assume constant star formation, have similar stellar and nebular metallicities. Stellar population age and metallicity affect the far-UV continua in different and distinguishable ways; assuming a constant star formation history diminishes the diagnostic power. Finally, we provide simple prescriptions to determine the ionizing photon production efficiency (ξ ion) from the stellar population properties. The ξ ion inferred from the observed star-forming galaxies has a range of log(ξ ion) = 24.4–25.7 Hz erg−1 that depends on the stellar population age, metallicity, star formation history, and contributions from binary star evolution. These stellar population properties must be observationally determined to accurately determine the number of ionizing photons generated by massive stars.

Using the Wide Field Planetary Camera 2 on the Hubble Space Telescope, we have obtained a deep color-magnitude diagram in V- and I-band equivalents for more than 2000 stars in a patch of the outer disk of the Large Magellanic Cloud LMC). Aperture photometry is feasible from these data with good signal-to-noise ratio for stars with V ≤ 25, which allows us for the first time to construct a color magnitude diagram for LMC disk stars on the lower main sequence, extending beyond the oldest main sequence turnoff point. We analyze the structure of the main-sequence band and overall morphology of the color-magnitude diagram to obtain a star formation history for the region. A comparison between the distribution of stars across the main-sequence band for M_v ≤ 4 and a stellar population model constrains historical star formation rates within the past 3 Gyr. The stellar populations in this region sample the outer LMC disk for stars with ages of 1 Gyr or older that have had time to spatially mix. The structure of the main-sequence band requires that star formation occurred at a roughly constant rate during most of the past ≈ 3 Gyr. However, the distribution of subgiant stars indicate that a pronounced peak in the star formation rate likely occurred about 2 Gyr ago, prior to which the star formation rate had not been enhanced for several Gyr. Studies over timescales of more than 3 Gyr require a separation of the effects of star formation history and the chemical evolution on the LMC color-magnitude diagrams, which is difficult to achieve without additional constraints. If lower main-sequence stars in the LMC have moderate metallicities, then the age for most LMC disk stars is less than about 8 Gyr.

We have used the Hubble Space Telescope and Wide Field Planetary Camera 2 to image the putative tidal dwarf galaxy located at the tip of the Southern tidal tail of NGC 4038/4039, the Antennae. We resolve individual stars and identify two stellar populations. Hundreds of massive stars are present, concentrated into tight OB associations on scales of 200 pc, with ages ranging from 2 to 100 Myr. An older stellar population is distributed roughly following the outer contours of the neutral hydrogen in the tidal tail; we associate these stars with material ejected from the outer disks of the two spirals. The older stellar population has a red giant branch tip at I = 26.5 ± 0.2 from which we derive a distance modulus (m - M)0 = 30.7 ± 0.25. The implied distance of 13.8 ± 1.7 Mpc is significantly smaller than commonly quoted distances for NGC 4038/4039. In contrast to the previously studied core of the merger, we find no super–star clusters (SSCs). One might conclude that SSCs require the higher pressures found in the central regions in order to form, while spontaneous star formation in the tail produces the kind of OB star associations seen in dwarf irregular galaxies. The youngest population in the putative tidal dwarf has a total stellar mass of ≈2 × 105 M⊙, while the old population has a stellar mass of ≈7 × 107 M⊙. If our smaller distance modulus is correct, it has far-reaching consequences for this prototypical merger. Specifically, the luminous to dynamical mass limits for the tidal dwarf candidates are significantly less than 1, the central SSCs have sizes typical of Galactic globular clusters, rather than being 1.5 times as large, and the unusually luminous X-ray population becomes both less luminous and less populous.

We have used the Hubble Space Telescope and Wide Field Planetary Camera 2 to image the putative tidal dwarf galaxy located at the tip of the Southern tidal tail of NGC 4038/9, the Antennae. We resolve individual stars, and identify two stellar populations. Hundreds of massive stars are present, concentrated into tight OB associations on scales of 200 pc, with ages ranging from 2-100 Myr. An older stellar population is distributed roughly following the outer contours of the neutral hydrogen in the tidal tail; we associate these stars with material ejected from the outer disks of the two spirals. The older stellar population has a red giant branch tip at I = 26.5 ± 0.2 from which we derive a distance modulus (m - M)0 = 30.7 ± 0.25. The implied distance of 13.8 ± 1.7 Mpc is nearly a factor of two closer than commonly quoted distances for NGC 4038/9. In contrast to the previously studied core of the merger, we find no super star clusters. One might conclude that SSCs require the higher pressures found in the central regions in order to form, while spontaneous star formation in the tail produces the kind of O-B star associations seen in dwarf irregular galaxies.

We present results on the analysis of background field stars found in Hubble Space Telescope Wide Field Planetary Camera 2 observations of six of the old globular clusters of the Large Magellanic Cloud. Treated as contaminants by the globular cluster analysts, we produce (V-I, V) color-magnitude diagrams (CMDs) of the field stars and use them to explore the LMC's star formation history. The photometry approaches V ~ 26, well below the turnoff of an ancient (~14 Gyr) LMC population of stars. The field star CMDs are generally characterized by an upper main sequence broadened by stellar evolution, an old red giant branch, a prominent red clump, and an unevolved lower main sequence. The CMDs also contain a few visual differences, the most obvious of which is the smeared appearance of the NGC 1916 field caused by heavy differential reddening. More subtly, the base of the subgiant branch near the old turnoff appears extended in V, and the red giant branch appears broad in V-I in four of the fields but not in the NGC 1754 field. We use a maximum likelihood technique to fit model CMDs drawn from Bertelli et al. isochrones to the observed CMDs. We constrain the models by the age-metallicity relation derived from LMC clusters, test four initial mass function (IMF) slopes, and fit for the reddening, distance modulus, and star formation rate. We find that we can just resolve structure in SFR(t) with time steps of ~0.15 in log (age), implying a resolution of ~4 × 108 yr at an age of 1 Gyr. For a Salpeter IMF, our derived star formation history for the NGC 1754 field is characterized by an enhanced star formation rate over the past 4 Gyr, qualitatively resembling that derived by others for a variety of LMC fields. The remaining four fields, however, appear to have had high levels of star formation activity as long as 5–8 Gyr ago; these fields lie in the LMC bar whereas the NGC 1754 field lies in the disk, suggesting that the inner regions of the LMC contain significantly more older stars than the outer regions. Examining the residuals of the models and observations, we find that the old red giant branches of the models provide a poor fit to the observations, which suggests an error in the model isochrones. The effect of the disagreement appears to be to underestimate the contribution of the old population.

We examine the reliability of the tip of the red giant branch (TRGB) as a distance indicator for stellar populations with different star formation histories (SFHs) when photometric errors and completeness corrections at the TRGB are small. In general, the TRGB-distance method is insensitive to the shape of the SFH except when it produces a stellar population with a significant component undergoing the red giant branch phase transition. The I-band absolute magnitude of the TRGB for the middle and late stages of this transition (~1.3-1.7 Gyr) is several tenths of a magnitude fainter than the canonical value of M_I ~ -4.0. If more than 30% of all stars formed over the lifetime of the Universe are formed at these ages, then the distance could be overestimated by 10-25%. Similarly, the TRGB-distance method is insensitive to the metallicity distribution of stars formed except when the average metallicity is greater than = -0.3. If more than ~70% of all stars formed have [Fe/H] > -0.3, the distance could be overestimated by ~10-45%. We find that two observable quantities, the height of the discontinuity in the luminosity function at the TRGB and the median (V-I)_0 at M_I = -3.5 can be used to test if the aforementioned age and metallicity conditions are met.

Photometry on the UVI system has been performed on the resolved stellar content of the blue compact dwarf galaxy UGC 6456 using Wide Field Planetary Camera 2 (WFPC2) images obtained with the Hubble Space Telescope. The resulting color-magnitude diagram (CMD) goes to about V = 27.5 and reveals not only a young population of blue main-sequence stars and blue and red supergiants, but also an older evolved population of red giants and a fairly well represented asymptotic giant branch. The distance to the galaxy is estimated from the tip of the red giant branch to be 4.5 Mpc, placing it about 1.5 Mpc farther away than the major members of the M81 Group, with which it is usually associated. The youngest stars are generally associated with H II regions shown on our Hα image and are largely confined to the 745 pc field of our PC images. A comparison of their distribution in the CMD with theoretical isochrones suggests ages from 4 to 10 Myr. The population of older stars is found throughout all WFPC2 camera fields and seems to show an elliptical distribution with an aspect ratio of about 2.4 and an exponential falloff in surface density with distance from a center of symmetry that is not far from the centroid of the youngest stars. Theoretical modeling of the CMD at a metallicity of Z = 0.001 suggests star formation in the age interval 1–2 Gyr, a strong burst in the interval 600–800 Myr, and a lower rate of star formation up to the present. The evidence is compatible with a scenario beginning with the formation of a population of low-metallicity stars, enriching a major residual of prestellar material that subsequently fueled an active episode of star formation. That burst of star formation must have been particularly spectacular and may be related to the activity we now see in the distant blue dwarf galaxies revealed in deep imaging.

We have analyzed archival Hubble Space Telescope (HST) Wide Field Planetary Camera 2 (WFPC2) observations of the central region of the globular cluster M54. The 12 irregularly timed observations (6 F555W, 6 F814W) were reduced using the HSTphot stellar photometry package. Our statistical analysis of the HSTphot photometry yielded 50 variable stars that are likely RR Lyrae candidates. These candidates were missed by previous ground-based searches for RR Lyraes due to the extreme crowding in the core of M54. We performed two checks on our sample of RR Lyrae candidates to determine how different they were from the other variable candidates. The first test was a numerical analysis comparing the ratio of variable candidates to the total number of stars within the blue horizontal branch, the RR Lyrae region, and the red giant branch. We found ratios of 0.61%, 13%, and 1.1% in these regions, respectively. Our next test was a two-sample Kolmogorov-Smirnov (K-S) test, which allowed us to find out how different our 50 RR Lyrae candidates were from the 50 brightest other variable candidates. According to the K-S test, there is only a 0.0004% probability that these two populations are similar, indicating that our RR Lyrae candidates do indeed exhibit some real fluctuation in their brightness. We sampled the light curves of confirmed RR Lyrae light curves to simulate the HST observing conditions and found that a signature of variability in 2911 out of the 4000 (72%) generated stars. We estimate that there maybe as many as 19 more RR Lyrae candidates that were undetected in the WFPC2 field of view. The presence of additional RR Lyraes has implications into the current Oosterhoff classification for the cluster.

Using archival imaging from the Wide Field Planetary Camera 2 aboard the Hubble Space Telescope, we investigate the stellar populations of the Local Group dwarf spheroidal Andromeda V - a companion satellite galaxy of M31. The color-magnitude diagram (CMD) extends from above the first ascent red giant branch (RGB) tip to approximately one magnitude below the horizontal branch (HB). The steep well-defined RGB is indicative of a metal-poor system while the HB is populated predominantly redward of the RR Lyrae instability strip. Utilizing Galactic globular cluster fiducial sequences as a reference, we calculate a mean metallicity of [Fe/H] = -2.20 +/- 0.15 and a distance of (m-M)0 = 24.57 +/- 0.04 after adopting a reddening of E(B-V) = 0.16. This metal abundance places And V squarely in the absolute magnitude - metallicity diagram for dwarf spheroidal galaxies. In addition, if we attribute the entire error-corrected color spread of the RGB stars to an abundance spread, we estimate a range of ~0.5 dex in the metallicities of And V stars. Our analysis of the variable star population of And V reveals the presence of 28 potential variables. Of these, at least 10 are almost certainly RR Lyrae stars based on their time sequence photometry.

The stellar population of the blue compact dwarf galaxy SBS 1415+437 is investigated us- ing the archive database of the Hubble space telescope. The color index-magnitude diagram for stars reaches a magnitude of 29 m in the V and I bands. It comprises young main-sequence stars, blue and red supergiants, and the old population of red giant branch and asymptotic giant branch. The tip of the red giant branch (TRGB) was used to calculate the distance modulus, which turned out to be m-M = 30.65 0.08 m . The corresponding distance to the galaxy is D = 13.5 1.0 Mpc. The youngest stars are distributed irregularly near the bright H II region in the southwest part of SBS 1415+437. The old popu- lation occupies a larger area, it is distributed more evenly and forms the galactic halo. The spatial distri- bution of young stars shows that the star formation in the galaxy spread in the direction from northeast to southwest over the last 5 10 7 yr with an average rate of 60 km/s. The TRGB of SBS 1415+437 was found to be appreciably shifted to the blue range: (V - I)TRGB 1.30. The galaxy age turns out to be not smaller than the age of Galactic globular clusters (about 10 10 yr), provided that the galaxy originally had a very low metallicity (our photometric estimate is (Fe/H) = -2.4). If the metallicity of SBS 1415+437 changed almost not at all in the course of evolution and was equal to (Fe/H) = -1.3 (as estimated from the emission lines of ionized gas), the galaxy age is no more than 2 10 9 yr.

An analysis of Hubble Space Telescope (HST) Wide Field Planetary Camera (WFPC) and Wide Field Planetary Camera 2 (WFPC2) images of the gravitationally lensed Cloverleaf broad absorption line quasi-stellar object (QSO) H1413+1143 is presented. Astrometric and photometric measurements are derived for the four components of the lensed QSO for five different epochs over a baseline of 2.76 yr. Because of the replacement of WFPC with WFPC2 and the change in the purpose of the observation at the various epochs, the data were not always taken with the same filter. With the exception of the declination of component D, the relative positions of the four components are measured to within ≈ 3 mas; these results are consistent with but considerably more accurate than earlier measurements. The relative photometric measurements at any one epoch are typically accurate to ≈ 0.02-0.03 mag (1 σ). The initial HST WFPC images cover a baseline of 1.26 yr (1992.21-1993.47 [1992 March 16-1993 June 22]), and over this time interval there is little evidence for brightness variations of any of the components relative to one another at levels >0.06 mag (>2 σ). Photometric measurements of the more accurate WFPC2 data obtained with different filters extends this baseline an additional 1.50 yr (to 1994.97 [1994 December 22]). The WFPC2 data also fail to reveal significant brightness variations among the components. In addition, the WFPC2 data include both UV (F336W) and near-infrared (F814W) images. These color data indicate the presence of sight-line-dependent extinction, causing the F336W-F814W color index of component B (the most reddened component) to be 0.56 ± 0.04 mag redder than that of component C (the least reddened component). The lack of evidence for significant component brightness variations at all HST observation epochs suggests that the data could be reliably extinction-corrected to derive the relative amplifications of the four image components. This is done for several reasonable dust-extinction models. Thus, the derived astrometry along with the photometric analysis set clear constraints on models for the Cloverleaf. Since component D shows some evidence for microlensing, the results on its relative amplification should be used with caution. While existing models can successfully reproduce the relative positions, the relative amplifications have not yet been successfully modeled. The WFPC2 imaging data has also permitted a sensitive search for component structure and the gravitational lens itself. There is marginal evidence for elongated structure between components A and C that may be part of an Einstein ring. However, no significant evidence for the lensing object is found. The various measurements are quantified in ways useful for setting model constraints. Limits on the mass-to-light ratio and detectability of the lensing galaxy are also discussed.

Dwarf spheroidal (dSph) galaxies are the least massive and luminous objects known to exist. These galaxies are
\noften considered as the fossil building blocks of massive systems predicted by some cosmological models. Nonetheless,
\nevidence has been mounting that this idea of hierarchical assembly may be too simplistic a picture, since a number of
\nthe dSphs' characteristics, such as the [alpha]-element abundance patterns, stand in contradiction to the properties of stars
\nin the Galactic halo. Further, yet unsolved, puzzles include the missing satellite problem, the influence of feedback
\nand reionization, and the nature of dark matter. How then do the dSphs form, to what extent do they contribute to
\nthe build-up of massive galaxies, what can we say about their dark matter content and how can one characterize their
\nrole in cosmology and galactic evolution? These systems' intriguing properties, such as the omnipresence of old stellar
\npopulations, their gas deficiency, their high velocity dispersion and flat dispersion profiles provide stringent tests of
\nthe paradigm of galaxy formation and render these systems important benchmarks for studying galactic evolution
\nfrom the earliest epochs on. In particular, the proximity of the dSphs in the Local Group (LG) allows us to resolve
\ntheir stellar populations and to pursue near-field cosmology on the smallest scales.
\nIn this Thesis I address several evolutionary aspects of these galaxies by concentrating on three Galactic satellites
\nand additionally investigating the global satellite galaxy system of the Andromeda galaxy, M31.
\nIn this context, I spectroscopically analyzed the Carina dSph, which stands out among the LG dSphs because of
\nits unusual, episodic star formation (SF) history. Carina bears evidence of at least three prominent stellar populations.
\nHence, I aimed at studying the metallicity spread of such systems, investigating potential age-metallicity relations,
\nsearching for spatial gradients and exploring its evolutionary history accounting for chemical enrichment. This was
\nachieved by obtaining medium-resolution spectroscopy of ~1200 targets in Carina. Based on the near-infrared calcium
\ntriplet as a well established metallicity indicator, I was able to compile the metallicity distribution function (MDF)
\nof this galaxy from a large sample of stars. Despite the wide spread in stellar ages present in Carina, originating
\nfrom its episodic SF, it exhibits a remarkably narrow red giant branch (RGB). On the other hand, I found a wide
\nspread in the metallicities, reaching from -3 dex to near-solar. Hence, I could show that age and metallicity conspire
\nto produce old, metal poor stars at the same locus on the RGB as young, metal rich ones. This manifestation of
\nan age-metallicity degeneracy generically explains the observed narrowness of Carina's RGB. In addition, I could
\nalso derive the age of each single red giant from isochrone fits. The resultant age distribution indicates the presence
\nof three major peaks, with a prevailing intermediate-age population. These populations may in fact be related to
\nCarina's three well established SF episodes. By correlating metallicities and spatial information, I could confirm a
\nradial population gradient in the metallicities, in the sense that more metal rich stars are found towards the center of
\nthe galaxy. This phenomenon points to a deep central potential well in the dSphs, where gas is longer retained for SF
\nand enrichment. In order to analyze the shape of the observed MDF, several simple models of chemical evolution were
\ncalculated, which support the view that the galaxy's early SF must have occurred from pre-enriched gas. Moreover,
\nall the models in use tend to overestimate the number of metal poor stars, i.e., there is a persistent G-dwarf problem.
\nThis medium-resolution study was complemented by measurements of iron- and [alpha]-element abundance ratios
\nfrom high-resolution spectra of ten stars in Carina. These elements are important tracers of SF and thus reflect the
\nevolutionary status of any stellar system. It could be shown that the calibration of the metallicity [Fe/H] via the
\ncalcium triplet reproduces the \\true" stellar iron abundance well for moderately metal-poor stars, but the calibration
\ntends to fail towards the most metal-poor populations. Carina's [alpha/Fe]-ratios are well consistent with those measured
\nin other dSphs of the LG and confirm that they are systematically lower than those in Galactic halo stars of comparable
\nmetallicities. The overall abundance patterns are not inconsistent with an episodic SF, but the accuracy and small
\nnumber statistics of such measurements impedes quantifying the underlying evolution. It is, however, safe from the
\npresent data to say that also Carina inheres the typical characteristics of other dSphs in terms of a low SF efficiency
\nand the occurrence of strong galactic winds.
\nIn an analoguous manner I determined metallicities in the remote Galactic satellite Leo II. The resulting MDF
\nalso shows a deficiency in very metal-poor stars. Furthermore it turns out to be rather asymmetric, with a rapid
\ndecline towards higher metallicities. By comparing my measurements with model predictions of LG dSphs I illustrated
\nthat Leo II's MDF bears resemblence to the UMi and Scl dSphs, albeit none of the models succeed in reproducing all
\nfeatures of the MDF simultaneously. By additionally determining the ages of the RGB stars in Leo II, I showed that
\nthe age-metallicity relation in this galaxy is essentially flat over a long time interval, while there is evidence for an
\nenrichment during the last 2{4 Gyr. The overall wide spread in ages present in this dSph support earlier views that
\nLeo II is in fact a galaxy with a prominent old and a prevalent intermediate-age population. Contrary to Carina, I
\ncould not detect any radial metallicity nor age gradient in Leo II.
\nAnother important aspect of the nature and evolution of dSphs is the study of their kinematical properties. These
\nlow-mass galaxies are believed to be the smallest cosmological structures containing dark matter. By measuring radial
\nvelocities in the remote dSph Leo I I showed that the resulting velocity dispersion profile is essentially flat out to the
\nnominal tidal radius. The non-detection of any apparent velocity gradient across the galaxy supports the negligible
\nrole of Galactic tides in the course of its whole evolution. The application of dynamical modeling under the assumption
\nof an isotropic velocity distribution then yielded mass and density profiles. Moreover, the behaviour of the velocity
\nanisotropy was analysed. The resulting high mass to light ratio of Leo I is supportive of the idea that all dSphs share
\na common dark halo mass-scale of ~ 4 x 107M[sun], so that the pure velocity (dispersion) information of such a system
\n
\nis a direct proxy for mass. All this argues in favour of a general dark matter dominance in the dSphs and renders the
\nhypothesis that these systems are of tidal origin less likely.
\nLastly, information about the origin and evolution of the dSphs can be gleaned by examining their spatial distribution
\naround their host galaxies. It has often been reported that, in the Milky Way system, the dSphs are aligned
\nalong one or more great circles or polar planes. Hence, I reconstructed the three-dimensional distribution of the entire
\nM31 satellite sample. By applying detailed statistical methods I could demonstrate that seven out of 16 satellites are
\nlocated within a thin polar sheet. One reason for this planar alignment can be the break up of a common progenitor,
\nwhich was orbiting M31. Also plausible is that the dSphs fell in along the filamentary dark matter structure of the
\ncosmic web, which is underscored by the fact that the plane extends in the direction of nearby galaxy groups.
\nAll in all, my studies of the chemical and kinematical properties of a sample of nearby dSph galaxies do confirm
\nthat these are dark matter dominated systems, which are governed by highly complex chemical enrichment processes
\nand thus warrant detailed investigations. These turn out to be invaluable for drawing a global picture of galaxy
\nformation in a cosmological context.

Our Sun will expand enormously and lose substantial mass via a stellar wind during the red giant branch (RGB) phase; the rotational period will be prolonged by several orders of magnitude. It is difficult to predict how much mass the Sun will lose before it reaches the RGB tip. Therefore, the solar rotational period at the RGB tip is also quite indeterminate. In this work, the Sun is considered as a two-component system comprised of a core and a convective envelope, each being allowed to rotate freely. The angular momentum transfer from the inner planets to the solar envelope is taken into consideration. Using Eggleton’s stellar evolution code, we study how the solar rotational period at the RGB tip depends on the value of Reimers $\eta$ chosen. The solar envelope’s rotational period at the RGB tip varies from 1 792 to 736 934 years, as the Reimers $\eta$ is changed from 0.00 to 0.75. Recent observations show that the average Reimers $\eta$ of Sun-like stars is 0.477. Adopting this average value of the Reimers $\eta$ , the solar envelope’s rotational period at the RGB tip will be 24 868 years. We also show how the envelope’s rotational evolves with age and luminosity. Other Sun-like stars, with different planetary configurations, may prematurely eject mass and lead to planetary nebulae, if they engulf a brown-dwarf companion at the RGB tip. Swallowing a planet with 13 Jupiter masses and a 3-day orbit, a Sun-like star can become a rapidly rotating giant star.