In this thesis I present the research carried out on X-ray binaries in the Local Universe. These X-ray binaries are found in a wide variety of environments and display an array of interesting characteristics. I present strong evidence for the discovery of a new black hole X-ray binary in a globular cluster of a nearby elliptical galaxy. At the time of discovery it was only the second such system known. I also present the first X-ray spectroscopic analysis of a source found in the central region of the nearby starburst galaxy, M82. This source is most likely a high mass X-ray binary system and displays peculiar X-ray characteristics relative to its behaviour at other wavelengths. I study the temporal and spectroscopic behaviour of the first black hole binary found in a globular cluster. The spectra of this source show evidence for a strong, soft X-ray emission line. I also study the global properties of X-ray binaries. I analyse the X-ray binary population of NGC4472, a nearby elliptical galaxy. I compare these sources to those of other galaxies and also analyse the characteristics of subpopulations of X-ray binaries within the galaxy

Neutron stars are some of the most magnetic objects that have ever been observed, and so they provide physicists with unique environments where fundamental laws of physics can be tested. Neutron stars are typically thought to have magnetic fields between 108 and 1014 G. The effects of the quantum electrodynamics are important above the quantum critical field (BQED) of 4.4×1013 G. In this thesis, I provide evidence that there may be many more neutron stars with B > BQED than previously thought, and that all neutron stars in binary systems that are close to spin equilibrium follow the same relationship between spin period (P) and magnetic field. In Chapter 2, I determine the long-term average X-ray luminosity, spin period, and rate of change of spin period for 42 Be X-ray binaries (BeXB) in the Small Magellanic Cloud (SMC). I use this information, combined with orbital data, to show that the neutron stars in all of these systems are disc-accreting, and that 85% are close to spin equilibrium. All systems with P & 100 s are predicted to have B > BQED. This applies to 2/3 systems. These predicted magnetic fields are higher than those of neutron stars in Galactic BeXB that have had their magnetic fields directly measured via cyclotron resonance scattering features (CRSF). I conclude that this is because the CRSF sources are not close to spin equilibrium. In Chapter 3, I look at pulse-profiles for the neutron stars discussed in Chapter 2 and find that they contain an array of features that vary both across and within individual systems. I suggest that BeXB containing neutron stars with relatively longer spin periods transition from a pencil to a fan beam at lower luminosities. In Chapter 4, I apply the methods used in Chapters 2 and 3 to LXP187, a BeXB in the Large Magellanic Cloud (LMC) that is not close to spin equilibrium. Results for LXP187 help confirm the conclusions of Chapter 2 - that � 2/3 BeXB contain neutron stars with B > BQED.

Observations have shown that galaxies have undergone intense transformations over the past 11 Gyr, increasing both their size and stellar mass in the process. Uncovering and understanding the mechanisms behind such changes remains one of the aims of modern astronomy. This Thesis presents an investigation into two mechanisms - star-formation and galaxy mergers - which may be responsible for these observed changes. This is achieved through the analyses of several publicly a available semi-analytic models of galaxy formation and evolution, combined with a large sample of approximately 350,000 galaxies at 0.005< z <3.5. Firstly, a comprehensive study is detailed comparing two methods which aim to connect galaxies across cosmic time, to ascertain the best method of tracing the true evolution of a galaxy population's most fundamental properties across large redshift ranges. This is done using a suite of semi-analytic models and selecting galaxies at either a constant stellar mass, or a constant cumulative number density ranked by stellar mass. It is found that the latter selection is better at tracing the true evolution in stellar mass and star-formation rate of a galaxy population, both forwards and backwards in time, compared to the former method. The method allows these properties to be recovered within a factor of 2-3 across a redshift range of 0< z <3, with the systematic o set proportional to the redshift range probed. This contrasts with a constant stellar mass selection - used throughout the literature - which often overestimates these physical properties by up to a factor of ~20, depending on the mass range probed. Secondly, this Thesis introduces a method allowing for the measurement of the close-pair fraction for galaxies selected by stellar mass from a flux-limited survey. Previous measurements of the merger fraction suffered from small volumes or uncertain statistical corrections for projected close-pairs of galaxies. The method presented herein, adapted from that presented in Lopez-Sanjuan et al. (2015), uses the full redshift probability distribution to measure the pair fraction of galaxies at >1010M, and at a constant cumulative number density of 10-4 Mpc-3, representing the best constraints on the pair fraction at z < 3.5 to date. Major and minor merger pair fractions approximately a factor of ~ 2 smaller than previous works are found and subsequently converted to merger rates. The major merger rate is found to be similar for galaxies at >1011Mand>1010M, while the minor merger rate is larger for the most massive galaxies by a factor of ~ 2. Finally, the relative role of galaxy mergers and star-formation in the build up of stellar mass is explored. Using star-formation rate estimates, a statistical estimation of the star-formation rate density and the merger accretion rate density of stellar mass-selected samples are compared and contrasted. From this analysis, it is found that star-formation remained the dominant source of stellar mass growth in massive galaxies until z ~ 0.5, with major merger becoming comparable in more recent times and minor mergers a factor of ~ 10 smaller even today. Furthermore, simple virial arguments are used to show that major and minor mergers are likely not the dominant mechanism in the size evolution of massive galaxies at z < 3.5, increasing their sizes by a factor of ~ 1.6 at most. In summary, the results presented in this Thesis explore the stellar mass, star-formation and size evolution of massive galaxies over the past 11 Gyr, and shed new light on the mechanisms responsible. By taking advantage of the latest wide-area, deep surveys, the largest sample of galaxies is used to constrain the merger histories of massive galaxies and infer their role in the evolution of massive galaxies in a consistent manner.

This thesis presents sub-arcsecond resolution observations of massive Young Stellar Objects (YSOs) in two satellite galaxies of the Milky Way, the Magellanic Clouds. With metallicities of »0.5 and »0.2 Z¯ for the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC), respectively the Magellanic Clouds present a unique opportunity to study star formation in environments which differ significantly from those of our own Galaxy. 19 targets in the SMC and 3 targets in the Hii region LHA 120-N113 in the LMC were observed with the near-infrared integral field spectrograph SINFONI at the VLT. Archival SINFONI data towards 7 targets in the Tarantula nebula in the LMC were also obtained. These observations reveal a wide variety of spatially extended emission line morphologies, indicative of outflows and compact Hii regions. Additionally, the most direct indications to date for the presence of discs in massive YSOs in the Magellanic Clouds are presented. Previously obtained optical spectra towards massive YSOs in the SMC have been analysed, as well as newly obtained spectra using the Robert Stobie Spectrograph (RSS) at the Southern African Large Telescope (SALT). Fabry-Perot interferometric observations obtained with RSS at SALT towards two star forming Hii regions in the Magellanic Clouds, are also presented, along with a new Fabry-Perot data reduction pipeline. Through analysis of the optical spectra in the context of the SINFONI data the massive YSOs in the SMC appear to reside in a porous ISM, allowing a large mean-free-path for energetic photons. Through a comparison of massive YSOs in the Magellanic Clouds and a previously obtained Galactic sample, evidence of significantly enhanced accretion rates towards the YSOs in the Magellanic Clouds is detected. Whilst the underlying mechanism of this enhancement is uncertain, there appears to be a convincing correlation between metallicity and accretion rates in massive YSOs.

In July of 2012 the WASP-South instrument was modified to allow it to collect data on brighter stars. This change was motivated by the dearth of planets known to orbit bright hosts in the southern hemisphere and the depth of study possible for HD209458 b and HD189733 b. These two planets orbit very bright stars in the northern hemisphere and have lead to a wealth of discoveries thanks to the relative ease with which they can be studied. My initial work with the Wide Angle Search for Planets (WASP) project was to contribute to updating the existing automated data reduction pipeline. I investigated the effects of, and helped to implement, various changes made to the pipeline in order to optimise its performance for brighter stars. During these investigations I also contributed fixes for several pipeline bugs. As a result of these changes the pipeline provides a level of precision with the new data as the previous version did with data from the unmodified instrument. The modified instrument was initially intended to collect data for 3 years. I performed an investigation in order to see if there was value to be gained in extending this period. I injected simplified transit signals into data gathered with the modified instrument that showed no known or candidate signals. These transit signals had a variety of different periods and depths, ranging from those we would expect to detect to those we would expect to be below our detection limits. My results showed that we can expect to recover between 42% and 72% of hot-Jupiters with transit depths between 0.5% and 2% and periods shorter than 10 days. This is comparable to a similar study of WASP-South before the modifications. I have also contributed to the follow-up of several newly discovered and previously known planets. I have presented the tools used by WASP to discover and characterise exoplanetary systems and have applied them to the investigation of these systems. With new data, I update and refine parameters for 7 previously known WASP planets. I resolve discrepancies in previous studies of WASP-31, show that WASP-42 may be an important probe of planet migration theory and show that seemingly “typical” systems may warrant re-observing. I present three newly discovered systems: WASP-120 b, a system with a star showing variable activity and a possibly eccentric planet orbit, WASP-122 b, which offers a good opportunity for atmospheric study, and WASP-123 b, which orbits an old star, ∼ 7 Gyr.

The spectra of Ap and Bp stars show evidence of non-homogenous distributions of chemical elements both vertically and horizontally, along with the presence of largescale ordered magnetic fields. The atomic diffusion theory in stellar atmospheres explains the presence of the non-homogenous element distributions as a result of the magnetic field’s effect on the radiative pressure in the photosphere. Recent modelling of the abundance distributions in Ap and Bp stars has questioned the results determined theoretically. In addition, there has been a debate over the uniqueness and reliability of the results determined using Zeeman/Magnetic Doppler Imaging (Z/MDI). To provide the tools necessary to determine further observational constraints for diffusion theory and to check the uniqueness and reliability of current MDI measurements, this thesis presents the development of two codes: Sparti Simple for the analysis of spectra formed in non-magnetic stellar atmospheres; and Sparti for the analysis of Stokes IQUV profiles formed in magnetic stellar atmospheres. Before the application of these two codes to observational data, testing was carried out to confirm the functionality and ability to cope with the challenges introduced as a result of the observation of Stokes IQUV profiles. The analysis of non-magnetic stellar spectra is a first step in the analysis of magnetic stellar spectra and also provides important observational constraints to diffusion theory. The member stars of the cluster NGC6250 were analysed using Sparti Simple as part of a larger collaborative effort to analyse the member stars of a variety of open stellar clusters. A cluster membership analysis of the stars in NGC6250 was performed and the fundamental parameters and photospheric chemical abundances were determined for each of the 19 member stars. Finally, the magnetic roAp star HD24712 and Ap star HD137909 were analysed using Sparti to determine its ability to recover unique and reliable results.

Novae are binary systems containing a white dwarf (WD) and a less-evolved companion star, either a main-sequence, sub-giant or red giant star. The WD accretes matter from the companion through Roche lobe overflow or via a stellar wind. As material is accreted, the pressure and temperature at the base of the accreted envelope increase until a thermonuclear runaway occurs. This causes a sudden increase in brightness (the outburst), which ranks among the most luminous stellar astrophysical phenomena. Following the outburst, some novae form detectable dust in the ejecta. Observationally, there is a correlation between the dust-formation timescale and the time it takes the nova to fade optically by two magnitudes, which was emphasised in a study of infrared emission from novae in the Andromeda Galaxy (M31). In the first part of this thesis, a simple theoretical model is presented, which considers the higher-energy photons produced by the nova being absorbed by neutral hydrogen in the ejecta, before they can reach the potential dust-formation sites. This new model successfully replicates the observed trend between these two parameters and agrees well with the observational data. The majority of novae are thought to consist of a WD and a main-sequence star, although some systems harbour a sub-giant (SG-novae) or red giant (RG-novae) companion instead. In the Milky Way galaxy, relatively few RG-novae have been confirmed, although in many systems, the evolutionary state of the secondary is simply not known. There is evidence that the progenitors of some Type Ia supernovae (SNe Ia) may be RG-nova systems (e.g. SN PTF11kx), therefore it is important to understand the population of such systems. In this thesis, archival Hubble Space Telescope (HST) data are used to search for RG-novae in M31. Many more novae are discovered in M31 each year (~30) than in the Milky Way (~10). Distance determination is a major complication when studying Galactic novae. However, at the distance of M31 all the novae may be considered to be at the same distance, making M31 an excellent environment for studying nova populations. We conducted a survey of 38 spectroscopically confirmed M31 novae in quiescence. We determined that 11 of these systems had a coincident progenitor candidate whose probability of being a chance alignment with a resolved source in the HST data was ≤5%. As the main sequence and the majority of the sub-giant branch are not resolvable in the HST data, this implies that a significant proportion of these systems contain red giant secondaries. The light curves of several M31 novae are also presented here, some of which use HST data to extend the light curves far deeper than is typically possible for extragalactic systems. A statistical study was then carried out to test the results of the survey and derive an estimate of the proportion of M31 novae associated with a resolved source in the HST data. This includes, for example, models of the spatial distribution, speed class and peak magnitude of the M31 nova population, as well as considering biases introduced by the HST coverage of M31. The initial results suggest about 0.38 of M31 novae are associated with a source in the HST data, a class of objects expected to be dominated by RG-novae. This is a much greater proportion than that observed so far in our Galaxy, and will be important when considering such systems as potential SN Ia candidates. The spatial distribution of novae that have resolved progenitor candidates is consistent with these systems being associated with the M31 disk, rather than the bulge. The method used to locate the progenitors of M31 novae was also used to study three additional systems. The M31 nova, M31N 2008-12a, which appears to be a recurrent nova (RN) with a very short inter-outburst period, produced an outburst in November 2013. This outburst was studied and a candidate progenitor system was found in HST data when it was apparently in quiescence, supporting its classification as a RN with a high accretion rate. The method was also used to explore upper limits on the brightness of the progenitor of SN 2014J, a SN Ia in M82, although no progenitor was found, a RG-nova (or in-fact any type of system) could not be ruled out due to the limitations of the data. For the M31 transient TCP J00403295+4034387, which showed an unusual spectrum, archival HST data were used to show the object was probably a blend of two objects with a very small apparent separation. Finally, the thesis is summarised, and future work on both dust formation and the progenitor search are discussed.

A detailed understanding of the process of star formation is crucial for modern astrophysics. Stars form from the gravitational collapse of molecular gas clouds; it is the process by which cold molecular gas is transformed into the stars and planets that make up the many billions of galaxies in the observable Universe. However, there are a number of open questions that have yet to be answered and a comprehensive theory that explains and predicts how, where and why stars and their clusters form proves elusive. One such open question is how does the environment, on both local scales and galactic scales, influence star formation? The enormous radiative and mechanical outputs of high-mass stars (M > 8M_sol ) are known to have a strong impact on their surroundings and are able to erode their natal molecular clouds via their stellar winds, ionizing radiation and supernovae. It has been proposed that the shock fronts at the edges of expanding HII regions might trigger subsequent generations of star formation (e.g. Elmegreen & Lada, 1977; Bertoldi, 1989), and there are observational studies to support this (e.g. Thompson et al., 2012). It has also been proposed that large-scale effects such as the spiral structure of galaxies like the Milky Way might trigger the formation of stars in otherwise quiescent gas (e.g. Dobbs et al., 2008), though observations within the Galaxy appear to suggest that spiral arms are playing only a minor role, if any, in the triggering of star formation (e.g. Moore et al., 2012; Eden et al., 2015). To answer this question, and others concerning star formation, large samples of imminently and currently star-forming regions are required, and surveys of the plane of the Milky Way in various tracers are providing the data to acquire these. Molecular clouds are the initial conditions for star formation, and a complete theory of star formation must necessarily involve a detailed understanding of molecular clouds. In this thesis a survey of molecular gas in the Inner Galaxy known as CHIMPS is presented; these data provide measurements of denser and more optically thin molecular gas at a higher angular resolution than preceding surveys and over a significant area of the first quadrant of the Galactic plane. The combination of CHIMPS data with data from other surveys, such as Hi-GAL, allows the star-forming content of clumps of dense molecular gas to be studied. The clumps of molecular emission identified within CHIMPS appear to be highly turbulent in nature, and are over-pressurized with respect to the encompassing neutral gas. This would appear to suggest that they are transient features in a highly dynamic interstellar medium. The efficiency of star formation within the CHIMPS clumps is not found to vary significantly on kiloparsec scales between the spiral arms and their inter-arm regions, with the exception of the Scutum-Centaurus arm, within which the current level of star formation per unit gas mass appears to be somewhat suppressed. On a clump-to-clump basis, the distribution of star formation efficiency is log-normal, indicating that the efficiency is determined by many random processes, with no single dominant agent. The conclusion is that it is turbulence that controls the star formation efficiency, which is powered on a wide range of scales from the feedback of high-mass stars to the shear induced by the rotation of the entire Galaxy.

I present an investigation into Type Ia Supernovae (SNe Ia). The aim of this investigation is to explain the physics and diversity of SNe Ia, motivated by the fact that, although SNe Ia are known to come from a thermonuclear explosion of a C+O Chandrasekhar mass (Ch-mass) White Dwarf (WD), their exact explosion scenario is one of debate, and their full diversity is not fully understood. As SNe Ia are used as cosmological distance probes, understanding their explosions and progenitors systems in more detail could have important consequences. To examine the diversity of SNe Ia, I first present a large sample analysis of their B and V - band light curves, separated by host galaxy type. A new method for calculating host galaxy extinction is implemented and the width luminosity relation (WLR) is examined. After correction for host galaxy extinction, ‘normal’ SNe Ia (∆m15(B) < 1.6 mag) fill a larger parameter space in the WLR than previously suggested. Even excluding fast declining SNe, ‘normal’ (MB <−18mag) SNeIa from star forming(S- F) and passive galaxies are distinct. This may indicate that various progenitor channels are prevalent in different galaxy types. Furthermore, it was also confirmed that sub- luminous SNe Ia tend to favour passive galaxies, which implies that this subset of SNe Ia come from an older progenitor system. There was a lack of transition SNe Ia in the dataset used in this project. These are SNe Ia with a luminosity between normal and sub-luminous SNe Ia. Understanding transitional SNe Ia is important in determining whether sub-luminous SNe Ia are a totally different population. With the aim of understanding how normal SNe explode, I first turn my attention to SN 2014J. SN 2014J was the closest type Ia in the last 410 years, and it was a once in a life time opportunity to study. Therefore, a detailed spectroscopic and photometric analysis and abundance stratification modelling of SN 2014J is presented. SN 2014J is a spectroscopically normal type Ia SN with a B band decline rate of 0.95 mag, before correction for extinction. It was located in the dusty starburst galaxy M82, and does not follow the average Galactic extinction law of Rv = 3.1. With the knowledge about the diversity of SNe Ia and the ability to carry out de- tailed modelling, SN 1986G was next chosen to be modelled. SN 1986G sits in an interesting area of parameter space in the WLR. It is located in the ‘gap’ between normal and sub-luminous SNe Ia. It has been theorised that sub-luminous SNe Ia come from a different progenitor system than standard SNe Ia. Therefore, understanding SN properties in this ‘gap’ is important for determining at which point SNe Ia properties begin to diverge from the normal scenario. A full abundance tomography modelling of SN 1986G was carried out. It was found that this SN is a low energy Chandrasekhar mass explosion. It had 70% of the energy of a standard W7 model. These findings raise the possibility that only SNe Ia with very large decline rates deviate from a Chandrasekhar mass.

Filamentary structures are observed to be common over a wide range of spatial scales and are strongly linked to star formation. In this thesis I present the results of a range of numerical simulations which investigate the stability, collapse and fragmentation of filaments. The global longitudinal collapse timescale for filaments is found to be considerably longer than for equally dense spheres, allowing sufficient time for local collapse to occur, and to solely occur via the distinctive end-dominated mode. A new freefall timescale equation for filaments is presented, as well as a semi-analytic model of longitudinal collapse. The fragmentation of accreting filaments is found to be more complicated than that of equilibrium filaments, and is dominated by the behaviour of longitudinal gravo-acoustic oscillations. This results in the fastest growing perturbation mode being independent of filament width. The non-equilibrium model presented here allows observers to estimate the age of a fragmenting filament and the mass accretion rate. Simulations of filaments accreting from a inhomogenous, turbulent medium show that turbulence has a large impact on the fragmentation of a filament. When the turbulence is sub-sonic, a filament fragments in a two-tiered hierarchical manner. As the energy in the turbulent field increases, the filament fragments into elongated fibre-like sub-structures. The formation of these fibre-like structures is intimately linked to the vorticity of the velocity field in the filament and the accretion onto the filament. In addition, I present synthetic C18O observations and show that the fibrelike sub-structures appear as velocity-coherent structures, well separated in velocity space, similar to the fibres observed by Hacar & Tafalla (2011).