Accurate stellar parameters are important not just to understand the stars themselves, but also for understanding the planets that orbit them. Despite the availability of high quality spectra, there are still many uncertainties in stellar spectroscopy. In this thesis, the finer details of spectroscopic analyses are discussed and critically evaluated, with a focus on improving the stellar parameters. Using high resolution, high signal-to-noise HARPS spectra, accurate parameters were determined for 22 WASP stars. It is shown that there is a limit to the accuracy of stellar parameters that can be achieved, despite using high S/N spectra. It is also found that the selection of spectral lines used and the accuracy of atomic data is crucial, and different line lists can result in different values of parameters. Different spectral analysis methods often give vastly different results even for the same spectrum of the same star. Here it is shown that many of these discrepancies can be explained by the choice of lines used and by the various assumptions made. This will enable a more reliable homogeneous study of solar-like stars in the future. The Rossiter-McLaughlin effect observed for transiting exoplanets often requires prior knowledge of the projected rotational velocity (v sin i). This is usually provided via spectroscopy, however this method has uncertainties as spectral lines are also broadened by photospheric velocity fields known as “macroturbulence” (vmac). Using rotational splitting frequencies for 28 Kepler stars that were provided via asteroseismology, accurate v sin i values have been determined. By inferring the vmac for 28 Kepler stars, it was possible to obtain a new calibration between vmac, effective temperature and surface gravity. Therefore macroturbulence, and thus v sin i, can now be determined with confidence for stars that do not have asteroseismic data available. New spectroscopic vsini values were then determined for the WASP planet host stars.

Gravitational-waves provide a unique probe of the stellar remnants---black holes and neutron stars---left behind at the end of massive stars lives. On the 14th September 2015 the Advanced Laser Interferometer Gravitational-wave Observatory (LIGO) made the first direct detection of gravitational waves from a merging stellar mass binary black hole, GW150914, during its first observation run (O1). In this thesis we present Compact Object Mergers: Population Astrophysics and Statistics (COMPAS). COMPAS is a platform incorporating astrophysical modelling tools and statistical analysis tools to extract information from the population of merging binary black holes we observe. We demonstrate that the masses, spins and observed rate of binary black holes can offer new insights into their formation.

This thesis describes the design, construction and characterisation of a spectrograph operating in the optical range 4000-8000 Å and its application in the observation and classification of transient astronomical sources.

The formation and growth of galaxies through star formation is an unsolved problem in astrophysics. Emission from galaxies in the far-infrared is known to be strongly correlated with obscured star formation and makes up a significant fraction of the cosmic infrared background. The Herschel Space Observatory was designed to cover the wavelengths where this background peaks and gave an unprecedented view of the Universe at this part of the electromagnetic spectrum. This thesis uses observations conducted for the Herschel Multi-tiered Extragalactic Survey (HerMES) by the Herschel Space Observatory's SPIRE instrument to catalogue sources of far infrared emission at 250μm, 350μm and 500μm which will allow models of galaxy evolution to be constrained. Source detection and extraction is performed with both blind and prior-driven algorithms on two newly constructed maps and, for the first time, on the largest of the HerMES' maps, HeLMS. Modifications are made to the algorithms to compensate for cirrus contamination. The accuracy and completeness of both algorithms are assessed and compared. In addition, emission from a galaxy across the three SPIRE bands is known to be strongly correlated. The extent of the covariance between the SPIRE images is explored using Principal Component Analysis, finding that up to 95% of the variance in the SPIRE maps can be explained by one linear combination of the three SPIRE bands. This new projection of data could allow previously unfeasible multi-dimensional probability of deflection analyses to be performed through a reduction of the data dimensionality. Looking forward, the Herschel Extragalactic Legacy Project (HELP) will be using Herschel observations from both HerMES and the H-ATLAS collaboration to create homogenous, multi-wavelength data products. In the final chapter, a comparison between datasets to test the homogeneity was undertaken, leading to a preliminary recalculation of the HerMES angular correlation function, a result that has been in contention between collaborations. Catalogues created as part of this thesis are available at hedam.lam.fr/HerMES/.

One of the most fundamental probes of the physics that underpins galaxy evolution is the star formation rate (SFR) as a function of cosmic time. In addition, the statistical prop- erties of galaxy populations are another important key to understand how the universe has been evolving. It is known that the far-infrared emission from galaxies is strongly correlated with obscured star formation and forms a significant part of cosmic infrared background. We thus investigate the variation of the SFR of galaxies over time by com- paring predictions of the L-Galaxies semi-analytic model with observations of the far infrared (FIR) luminosity and number counts. In the first part of this thesis, we follow the star formation histories (SFHs) of galaxies and use these to construct stellar spectra in post-processing. We then contrast model SFHs from the Millennium Simulation with observed ones from the VESPA algorithm as applied to the SDSS-7 catalogue when this has been characterized by mass and colour of galaxies. In order to investigate the SAM model prediction, I extend L-galaxies to predict far infrared fluxes and construct mock catalogues which are fed into SMAP in order to provide simulated maps. LFs have also been estimated for model galaxies at different redshifts. The results are compared with observations from Herschel. To conclude, our model under- estimates the number density of galaxies at bright sources (e.g fluxes above 0.02 Jy) also does not produce high luminosity objects especially at higher redshifts (e.g z > 1) . We show that by fitting the simulated IR luminosity function to observed LIR, our model is able to produce more bright sources at high redshifts and match reasonably well to the observed number counts.

This thesis is conceptually divided into two parts. The first and main part concerns the generation of gravitational radiation that is emitted from merging black-hole binary systems using Numerical Relativity methods. The second part presents the methodology of the search for gravitational-wave bursts that are emitted in core-collapse Supernovae. My approach to Numerical Relativity is to utilise the late-time gravitational radiation of merging binary black holes to extract key astrophysical parameters from such systems via standard parameter estimation techniques. I begin with an introductory chapter that outlines the standard theories of stationary and perturbed black holes. In addition, up-to-date techniques in performing binary black hole simulations, the current and near-future status of the global network of gravitational-wave detectors, as well as the most promising gravitational-wave sources. I conclude the chapter with elements on parameter estimation techniques, such as Bayesian analysis and the Fisher information matrix. In Chapter 2, I discuss detection issues and parameter estimation results from the late-time radiation of colliding non-spinning black holes in quasi-circular orbits, and propose a practical test of General Relativity, as well as of the nature of the merged compact object. Chapter 3 involves similar parameter extraction calculations, but involves a more realistic approach, whereby the effect of the various angular parameters on parameter estimation is considered, placing an emphasis on the actual science benefit from measuring the gravitational radiation from perturbed intermediate and supermassive black holes. In Chapter 4 we present the results of an extensive Numerical Relativity study of merging spinning black hole binaries and argue that the mass ratio and individual spins of a non-precessing progenitor binary can be measured solely by observing the late-time radiation. Chapter 5 presents the methodology in carrying out searches for gravitationalwave bursts (GWB) from core-collapse Supernovae with a dedicated for GWB searches pipeline (X-Pipeline) and presents the sensitivity performance of XPipeline in detecting GWBs associated with certain Supernova candidates during the two most recent LIGO-Virgo-GEO Science Runs.

In this thesis, I present molecular line & continuum data of a sample of small, southern, isolated cores. I present a multi-wavelength view of the cores, by utilising optical images, 2MASS extinction maps, CO integrated intensity maps and 1.2mm continuum images of each of the cores. Spitzer data are used to identify young stellar objects local to each core, which may influence the evolution of the core. Column densities and masses are calculated for each core. The column densities calculated from the CO and 1.2mm continuum tracers are shown to be in excellent agreement with each other, and with the peak extinction, as seen on the 2MASS extinction maps. A comparison of column density derived from 1.2mm continuum and C18O observations suggest that a fraction of the gas has frozen out onto the dust grains in the densest parts of the core. The masses derived from 13CO, C18O and 1.2mm continuum observations are compared with the virial mass calculated from the observed linewidths. The cores are found to be within 3� of virial equilibrium in all cases, which suggests that all cores may be gravitationally bound. I find that the observed linewidths of the isolated cores are consistent with models of star formation by turbulent dissipation. The C18O linewidth is observed to be narrower than the 13CO linewidth, which is narrower than the 12CO linewidth in all cases. This suggests that as the density of the tracer increases, the linewidth decreases. Therefore, turbulent support against collapse is removed from the inside out, resulting in stars forming in the densest parts of the cores. I also present a proposed evolutionary diagram, based on the observed 12CO and ratio of 12CO/C18O linewidths. I hypothesise that a young core will have large 12CO and C18O linewidths. In an older core, the turbulence will have had time to dissipate in the core centre, and so the C18O linewidth will be narrower. For the oldest cores, the dissipation of turbulence will have occurred in the outer parts of the core and so the 12CO/C18O ratio will be small, indicating a more evolved core.

The way in which stars form from the interstellar medium is poorly understood. In this thesis we investiage the process star formation in molecular clouds via core fragmentation using Smoothed Particle Hydrodynamics (SPH). The initial conditions of the simulations are informed as closely as possible by observations of Ophiuchus. We run large ensebles of individual core simulations and compare the collective results with observations of stars and brown dwarfs. We use observations of Ophiuchus by Motte et al. (1998) and Andre et al. (2007) to calibrate a lognormal distribution from which we draw correlated masses, sizes and velocity dispersions. We assume that the cores are intrinsically triaxial. The distribution of core shapes is then inferred by fitting a single parameter family of ellipsoidal shapes to the observed core aspect ratios. Each core is given the density profile of a critical Bonnor-Ebert sphere and a turbulent velocity field which is modified to include ordered rotation and radial excursions. We evolve one hundred of the model prestellar cores using the Seren SPH code (Hubber et al., 2011). The simulations are repeated with continuous accretion heating, no accretion heating and episodic accretion heating (Stamatellos et al., 2012). We find that simulations with episodic accretion heating can reproduce the general features of the Chabrier (2005) initial mass function. This includs the ratio of stars to brown dwarfs and the turn-over at 0:2M�. We demonstrate that the mass of a star is not related to the mass of the prestellar core in which it formed. Low mass cores with Mcore � 0:1M� tend to collapse into single objects whereas higher mass cores with Mcore & 1M� can fragment into tens of objects. We finally show that the multiplicy statistics of the protostars formed in these simulations are well matched by observations. Multiplicity frequencies are higher than those of field stars and we note the presence of long-lived quadruple, quintuple and sextuple systems.

The thesis goes on to extend the work of Bretherton (2003) and Moore et al. (2007) on the W3 Giant Molecular Cloud, by performing NH3 follow up of a sample of the cores discovered in the 850um SCUBA map and observing the whole cloud in 13CO(J=1-0) and C18O(J=1-0). - The NH3(1,1) and NH3(2,2) observations of the SCUBA cores used the fact that NH3 only traces the densest regions of the cloud, and hence can be used to find the temperature and kinematics of the cores themselves. This was used to test if the individual cores were virially bound, and from this find if cores in the more densely star-forming region of the cloud (High-Density Layer, HDL) were more likely to be bound than those in the Low-Density Layer (LDL). There are a mixture of virially bound and unbound cores in both the HDL and the LDL but no statistical difference in ratio of these between the two regions. This has an important bearing on models of environmentally-dependent star-formation, which divide into two categories; those, such as Collect and Collapse (Dale et al., 2007), which state that external pressures create dense structure, and those such as Radiatively- Driven Instability (RD I) (Bertoldi, 1989), which state that those external pressures cause dense structure to collapse. The evidence from this thesis favours models in which dense structure is created according to the Collect and Collapse scenario.

The aim of this thesis is to investigate evidence of heating and radiative feedback in local Gould Belt star-forming regions. I discuss what impact, if any, radiative feedback is having on the star formation. I primarily use Submillimeter Common-User Bolometer Array 2 (SCUBA-2) observations from the James Clerk Maxwell Telescope (JCMT) Gould Belt legacy Survey (GBS) of nearby star-forming regions. I analyse this data in conjunction with catalogues of candidate young stellar objects (YSOcs) from mid-infrared surveys with Spitzer IRAC and MIPS surveys. I use the ratio of SCUBA-2 fluxes to calculate dust temperature, given a constant value of dust opacity spectral index, following the method of Reid & Wilson (2005). I employ a two-component beam (2CB) cross convolution to map the temperature of the Serpens MWC 297 region, achieving a resolution of 19.9′′. I employ a convolution kernel to map the temperature of the majority of the JCMT GBS, including the Aquila W40 complex, achieving a resolution of 14.8′′. I use the fellwalker clump finding algorithm to produce a global catalogue of 619 SCUBA-2 850 μm clumps across 26 distinct sub-regions of the JCMT GBS, calculating real temperatures where available. I was the PI of a proposal to observe 12CO 3-2 line emission, with the aim of decontaminating the SCUBA-2 850 μm band. I find 12CO 3-2 line contamination has a significant impact, increasing the dust temperatures calculated per pixel, on average, by 3 K where contamination is less than 10%, and by 16 K where contamination is greater than 10% (in the Aquila W40 complex). I find evidence for 12 outflows in this region, associated with active star formation. I also use archival VLA data to decontaminate both SCUBA-2 bands of free-free emission associated with massive star formation. Where compact free-free sources are sufficiently bright and optically thick, for example the B1.5Ve star MWC 297, their contribution can lead to prominent bright sources at the submillimeter wavelengths detected by SCUBA-2 and lower temperatures around Herbig stars. I present published studies of the Serpens MWC 297 region and the Aquila W40 complex. In both cases I find evidence that the presence of young OB stars is raising the temperatures of nearby clumps. Examining clumps across the JCMT GBS, I find that those clumps isolated from OB stars have a mean temperature of 15±2 K, a value that is consistent with gas temperatures (Friesen et al., 2009) and Bonnor-Ebert sphere models (Kirk et al., 2006). I find no evidence of heating from embedded low-to-medium mass YSOs. Clumps that lie within 3 pc of OB stars have a mean temperature of 21±4 K and O type stars heat clumps over the greatest range. By remodelling the heated clumps with a temperature of 15 K, I calculate that up to 10% of clumps in the JCMT GBS are no longer Jeans unstable, indicating that radiative feedback from OB stars is potentially suppressing fragmentation and allowing for the formation of more massive stars.