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Stellar atmospheres constitute excellent environments to study the chemical evolution of our Galaxy. The chemical composition of these atmospheres reflects the composition of the gas from where these stars were born. As the Galaxy evolves, the composition of the gas changes from being primordial (Big-Bang nucleosynthesis) to being enriched in heavy elements (stellar and interstellar nucleosynthesis). The abundances of fragile chemical elements can be affected by stellar mixing processes. Precise lithium, beryllium and oxygen abundance determinations in old stars are presented in this thesis. These determinations are based on the analysis of the observed spectra of a sample of thirteen metal-poor subgiant stars. According to stellar mixing theories, these stars are in a stellar evolutionary stage in which mixing by convection is expected. Abundances of fragile elements like lithium and beryllium are thus expected to be affected by such mixing processes. As a consequence of this, the abundances of these elements are discussed in a dilution context. Lithium and beryllium abundances are compared with the abundances of stars with similar characteristics but in a less evolved stellar phase so that mixing processes have not acted yet. As expected, our abundances seem to be depleted following reasonably well the standard predictions. Stellar abundances of oxygen should give an estimate of the oxygen contribution of core-collapse supernovae to the interstellar medium. However, there is poor agreement among the abundances determined from different atomic or molecular indicators in general. Abundances coming from three different indicators are compared in this thesis. The abundances determined from the O I infrared triplet lines at 777.1-5 nm give the poorest agreement among the three indicators. The abundances based on OH ultraviolet lines around 310 nm are lower for the subgiants in comparison with previous studies of main-sequence stars, becoming even lower than values base

When modelling the collapse of massive stars leading to supernova explosions and the cooling of neutron stars, understanding the microphysical processes, such as the interaction of neutrinos within a dense medium are of vital importance. The interaction of neutrinos with nucleons (neutrons and protons) is altered by the presence of the medium, compared to the same process with free nucleons. Neutrino scattering and production processes may be characterized in terms of the excitations that are created or destroyed in the nuclear medium. One way to analyse the effects of the medium is by using Landau's theory of normal Fermi liquids. This theory gives simple relationships between physical quantities such as the spin susceptibility or the response to a weak interaction probe in terms of Landau parameters, that are measures of the interaction between quasiparticles. One problem when using Landau Fermi liquid theory for nucleon matter is that the interaction has a tensor component. The tensor interaction does not conserve the total spin and, as a consequence, there are generally contributions to long-wavelength response functions from states that have more than one quasiparticle-quasihole pair in the intermediate state. Such contributions cannot be calculated in terms of Landau parameters alone, since in the usual formulation of Landau theory, only singlepair excitations are considered. In this thesis three problems are addressed. First, we obtain bounds on the contributions from more than one quasiparticle-quasihole pair by using sum-rule arguments. Second, we derive expressions for static response functions allowing for the tensor components of the interaction. We analyse which the most important effects are on the static response of nucleon matter, and find that the major contributions comes from renormalization of coupling constants and transitions to states with more than one quasiparticle-quasihole pair. Third, we show how contributions to the dynamical response co

Quasars form one of the most energetic phenomena in the universe, and can be traced out to very large redshifts. By studying the galaxies which host the active nuclei, important insights can be gained into the processes that trigger and maintain the quasar powerhouse. The evolution rate of the quasar population is furthermore similar to that of ordinary galaxies, which implies a connection between black hole accretion and star formation in the host galaxies. While the properties of quasar host galaxies at low redshift have become better constrained in recent years, less is known about hosts at earlier cosmic epochs. In addition, though radio-quiet quasars are by far more common than their radio-loud counterparts their host galaxies have not been studied to the same extent, in particular not at higher redshifts. An imaging campaign of a large sample of quasars at intermediate redshift (0.4 < z < 0.8) was carried out at optical wavelengths using the Nordic Optical Telescope, and is studied in this thesis together with two smaller samples. The joint material forms more than half of the total number of observed sources in this redshift interval and increases the number of resolved radio-quiet hosts at z>0.4 considerably. The morphology and mean magnitudes are found to be similar for radio-loud and radio-quiet host galaxies. Both types of host are shown to have optical colours as blue as those of present-day late-type spirals and starburst galaxies, which is likely the result of ongoing star formation. With increasing redshift, observations of host galaxies become more difficult. High spatial resolution can be achieved with adaptive optics, but the variation of the point spread function in the near-infrared wavelength band which is most suited for detection is large and rapid. A statistical approach to the problem of characterizing the point spread function has been developed, making use of simulated objects which are matched to the different atmospheric conditi

Stellar chemical abundances provide astronomers with vital information about the production of chemical elements. Some stars preserve the composition of the environment in which they were born on their surfaces. By analysing the light from a star, the abundances of elements, its age and its path in space can be derived, and translated into the language of galactic history. The spallative history of boron in the early Galaxy was reinvestigated by observations of an ultraviolet spectral line in the old star HD 140283 with the Hubble Space Telescope. The line was barely detected and the upper limit abundance derived was lower than expected, which calls for further observations of this line in halo stars. Stars evolved into subgiants were observed with the ESO CAT, La Silla, and NOT, La Palma, to deduce their usefulness for galactic evolution studies. The high resolution spectroscopy study of the 26 objects showed that these stars are indeed useful for such studies. They are more luminous than dwarf stars and their ages can be accurately derived. They do not seem to have changed their surface abundances due to their evolution into giants. Subgiants can successfully be used to observationally reach regions further from the Earth, which can remove local biases that may appear when only observing nearby dwarf stars. A NLTE investigation of neutral Ca showed that cool metal rich dwarf stars did not deviate significantly from LTE, as had earlier been suggested. By an LTE analysis of a sample of 17 such dwarfs, using recent MARCS atmospheres, synthetic spectroscopy and modern atomic line data, cool metal rich dwarfs were shown not to deviate significantly from the expected abundance patterns in a number of elements. This increases the number of potential targets for studies of galactic chemical evolution in the metal rich regime since most stars are cool.

A study on the chemical abundances of dwarf irregular galaxies (dI's) is presented in this work. This type of galaxies have typically low metallicity. First of all, a definition of dI is proposed, in order to obtain a homogeneous group of targets. Due to the small size of the dI galaxies and the absence of strong dynamical processes a homogeneous chemical composition is normally assumed. The main argument is that the gas in the interstellar medium of the galaxies are probably not too much affected by internal dynamics and could therefore be well mixed. This hypothesis has been investigated. Optical spectra for various star-forming regions (HII regions) in five dI's have been obtained and analysed. For all the HII regions, the chemical abundances of the whole region have been determined. Spectra of four of these HII regions are of very high quality which allow an additional study of possible variations of the chemical abundances within the regions. It could be concluded that the existence of variations in the chemical abundances, especially at large scales, may depend on thephysical properties of the galaxy and its environment. Another important aspect to study is the behaviour of the dI galaxies in the metallicity-luminosity plane. A deep study of previous relationships for this kind of objects were performed as well as the possible sources of dispersion from this relationship. With the results on the chemical abundances of eight galaxies and the latest distance determinations a new, weak, relationship between the metallicity and the luminosity was derived. The influence of the environment and other physical properties of the galaxies have also been studied.

This thesis is focused on the origin and evolution of blue compact galaxies (BCGs). BCGs are low mass galaxies characterised by blue colours, low chemical abundances and high star formation rates. The gas consumtion time scale is considerable shorter than the age of the universe and thus the high star formation rate must be transient. Such a non sustainable star formation rate is commonly referred to as a "starburst". It has been proposed that BCGs are genuinely young galaxies or alternatively that theyoccasionally undergo starbursts, separated by long quiesent periods. It is of importance for the understanding of the evolution of galaxies, and in particular dwarf galaxies, to understand under what conditions a galaxy can become a starbursting BCG. The properties of in particular intrinsically luminous BCGs have been examined photometrically and kinematically. The results show that the studied galaxies have a population of old stars underlying the luminous starbursts. Thus they are not young galaxies. It is shown that infrared photometry is a powerful method to detect underlying old populations in BCGs. Kinematical studies of the velocity field suggests that the galaxies have been perturbed by mergers, and it is proposed that mergers betweengas rich dwarf galaxies is the triggering mechanism for starbursts in luminous BCGs. Low surface brightness galaxies are promising ingredients in such mergers. Observations with the Hubble Space Telescope of the BCG ESO 338- IG04 reveal the presence of a very rich population of globular clusters (GCs) of various ages. Apparently GC formation is still going on in this starbursting galaxy. The age dating of GCs offers an efficient way of studying the starburst history of BCGs and galaxies in general. The relation between BCGs and other types of low mass galaxies is discussed.

The local space density of the faintest stars, including M dwarfs, white dwarfs and brown dwarfs has been studied by two different methods. By using dark nebulae as opaque outer screens, the luminosity functions of M dwarfs and white dwarfs were studied as volume-limited and dynamically unbiassed foreground samples. The surveyed volume corresponds to 464 pc3 in the solar neighbourhood and contains 21 M dwarfs and 7 white dwarfs. The derived M-dwarf luminosity function is consistent with previous findings, showing no substantial upturnbeyond Mv = 16. The white dwarf space density is consistent with a 20% fraction of the dynamical matter in the solar neighbourhood residing in white dwarfs. This is in line with the most recent independent results obtained with different methods, making the white dwarfs the best explanation to the Milky Way dark matter at present. The brown dwarfs were studied in the Pleiades open cluster. The distance and age of the Pleiades make the rapidly fading brown dwarfs still rather bright and easy to detect. 850 arcmin2 were covered in a deep RIJK survey. Nine new possible cluster members were discovered, four of which are below the brown dwarf limit. The faintest of these has an estimated mass of 0.040 M. and is thereby the lowest mass brown dwarf identified in the Pleiades cluster at present. The derived Pleiades substellar luminosity function is consistent with a mass function index between 0 and 1, making the brown dwarfs unlikely to contribute more than a few percent to the cluster mass, which is also consistent with recent dynamical results.

The thermal infrared and microwave emission from asteroids have many applications. Fundamental physical properties can be investigated, which have implications for the geophysical evolution of the asteroids, and the early history of the solar system. A new thermophysical model of asteroids is presented. A number of physical processes previously neglected in the Standard Thermal Model are now considered. The new model predicts the thermal emission of asteroids, from mid-infrared to microwave wavelengths. The irregular shapes of asteroids are modelled in detail, and the spin state of the asteroids is taken into account, whereby it is possible to calculate model thermal lightcurves. The heat conduction into the surface material in general lowers the mid-infrared flux from main-belt asteroids. The model is able to explain the observed "beaming" of the emission into the solar direction, under the assumption that it is caused by the small-scale surface roughness. Subsurface scattering processes are considered when calculating the directional- and wavelength-dependent emissivity. The model is applied to a large database of observations in the wavelength range of 7-2 000 µm. The purpose is to derive the thermophysical properties of ten asteroids, selected as calibration targets for the instruments on board the Infrared Space Observatory. The results indicate very rough and porous surfaces, with low levels of heat conduction. The emissivity variessignificantly with wavelength.