The chemical fingerprints of old, metal-poor stars can be used to unravel the events of the newborn Universe and help us understand the properties of the first stars and star clusters. The study of nearby stars to infer properties in the distant past is often referred to as Galactic archaeology. However, the chemical composition of stars cannot be observed directly, but must be inferred by means of spectroscopic modelling. Traditionally, this modelling utilises one-dimensional (1D) stellar atmospheres in hydrostatic and local thermodynamic equilibrium (LTE). Today, we know that departures from LTE (known as NLTE), and differences between 1D model atmospheres and their hydrodynamical three-dimensional (3D) counterparts, become increasingly severe at lower metallicity. The development of NLTE modelling of spectral line formation in 3D atmospheres is still in its infancy, but constitutes a remarkable step forward that has been made possible by parallelised codes and supercomputers. The central theme of this thesis is the application of NLTE analyses to metal-poor stars, to help usher the field of Galactic archaeology forward with important consequences for the nature of the first stellar generations. I present a theoretical NLTE study of aluminium, where I validate the analysis using a set of bright standard stars and provide calculated NLTE effects for a large parameter space. I perform 3D NLTE calculations for the solar spectrum to better constrain the zero-point of the cosmic abundance scale, and find excellent agreement with the meteoritic aluminium abundance. I also present NLTE analyses of metal-poor stars in the globular clusters NGC 6397 and M4. While globular cluster stars were long expected to form from a chemically homogeneous medium, star-to-star abundance variations of light elements indicate multiple epochs of star formation. Massive first-generation stars polluted the interstellar medium from which later generations formed, and I use the observed abundance variations to deduce the properties of the polluting stars. Among the heavier elements, I uncover evolutionary abundance variations that match predictions of stellar evolution models with atomic diffusion. The results indicate that the chemical abundance ratios of unevolved metal-poor stars are affected by gravitational settling, with a bias of the order 25-50 %, increasing towards lower metallicity. This atmospheric depletion mechanism is a probable explanation to why the stellar abundances of lithium fall short of the predictions from standard Big Bang nucleosynthesis. Finally, I apply a 3D NLTE abundance analysis to the red giant SMSS 0313-6708, which is the most iron-deficient star known. The chemical abundance pattern of this star indicates that it formed from gas affected only by Big Bang nucleosynthesis and a single faint supernova. Comparison of the inferred abundance pattern to theoretical predictions leads to constraints on the explosion mechanism and the mass of the metal-free progenitor star.

This thesis investigates the potential for detection and identification of primordial stars, galaxies, and supernovae at high redshift. Simulations indicate that the first Population III stars should appear in minihalos of mass M = 105-106 Msol at z ≈ 10-30. To assess the detectability of these objects, theoretical models of these stars and their surrounding HII regions are used. We assess the plausibility of detection with the upcoming James Webb Space Telescope (JWST), using the gravitational lensing provided by the galaxy cluster MACSJ0717.5+3745. The conclusion is that the detection of these objects is highly improbable but not impossible. To investigate the prospects of detecting and identifying the first galaxies, the spectral synthesis code Yggdrasil is introduced. According to this code, JWST may be able to detect Population III galaxies with stellar masses as low as 105 Msol at z ≈ 10 in unlensed fields. We find that, over limited redshift intervals, it could be possible to use Hubble Space Telescope (HST) and/or JWST broadband color criteria to single out Population III galaxy candidates. The prospects of detecting gravitationally lensed Population III galaxies with JWST and HST is investigated. A lower limit to detect ≈1 Population III galaxy of ε ≈ 10-2 (HST/CLASH) and ε ≈ 10-3 (JWST using MACS J0717.5+3745 as lens) is derived, where ε is the baryon fraction converted to Population III stars in a host halo. By fitting HST/CLASH data to Yggdrasil and comparison grids, two Population III galaxy candidates are discovered. These two candidates are the first Population III galaxy candidates discovered at z &gt; 6.5. A highly-magnified and doubly lensed extremely high-redshift (z ≈ 7.8) object is also identified. Finally the prospects of detecting core-collapse (CC) supernovae (SN) from the first galaxies at z ≈ 5-12 are investigated. The prediction is that no primordial SN is detectable, but 2-3 CC SN should be discovered by the HST/CLASH. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 5: Submitted. Paper 6: Submitted. Paper 7: Manuscript.</p>

La formation des premières étoiles, quelques centaines de millions d'années après le Big Bang, marque la fin de l’âge sombre. Actuellement, nous n’avons aucune observation de la formation de ces étoiles, appelée popIII, mais d’après des simulations numériques de différents groupes, il semblerait que ces étoiles primordiales étaient très massives: plusieurs centaines de masses solaires. Ces premières étoiles, ont produits aussi des sursauts gamma (GRBs). Ainsi, l’étude des GRBs produits à partir des popIII, pourraient permette d’étudier directement le stade final des étoiles primordiales. Les télescopes d'aujourd'hui ne peuvent pas regarder assez loin dans le passé cosmique pour observer la formation des premières étoiles, mais la nouvelle génération de télescopes permettra de tester des idées théoriques sur la formation des premières étoiles.Les GRBs sont liés à la mort d’étoiles massives et qu'ils sont connectés avec des supernovae. En ce sens, les GRBs sont l'une des classes de processus explosifs en physique stellaire et devraient suivre les mêmes lois physiques que l'explosion des supernovae. Ce travail tente d'aborder le problème des GRBs comme un problème d'explosion stellaire et utilise les données d’observation sur les spectres et les courbes de lumières notamment.Dans le cadre de cette thèse, des outils spécifiques ont été développés pour étudier les explosions stellaires: un code numérique pour résoudre les réactions nucléaires a été incorporé dans le code hydrodynamique existant. Ces outils ont été utilisés dans les simulations de supernovae afin d’étudier les connections avec les sursauts gamma: analyse spectrale et étude statistique en fonction du redshift. / The formation of the first stars hundreds of millions of years after the Big-Bang marks the end of the Dark Ages. Currently, we have no direct observations on how the primordial stars formed, but according to modern theory of stellar evolution these stars should be very massive (about 100 Msun) Population III stars have a potential to produce probably most energetic flashes in the Universe - gamma-ray bursts. GRBs may provide one of the most promising methods to probe directly final stage of life of primordial stars. Today's telescopes cannot look far enough into the cosmic past to observe the formation of the first stars, but the new generation of telescopes will test theoretical ideas about the formation of the first stars.Thanks to many years of observations we have good GRB's data -statistics of occurrence, spectrum, lightcurves. But there are still a lot of questions in the theory of GRBs. We know that GRBs are related to the death of stars and that they are connected with supernovae. So gamma-ray bursts are one of the classes of explosive processes in stellar physics that should have a lot of common with supernovae explosions. In that case GRBs should follow the same physical laws of explosion as supernovae. This work tries to approach the problem of GRBs as a problem of stellar explosion.Necessary instruments of studying stellar explosion were developed as a part of doctoral research: code for solving systems of nuclear reaction equations was incorporated into hydrodynamical code. These tools were applied for supernovae simulations in order to find possible connection with GRBs. Basing on analysis of supernovae simulations spectral analysis of GRBs was performed.