Solar flares are perhaps the most remarkable transient events within the solar system. A century of observation has done little to elucidate their true nature. Their secrets are hidden even from the sophisticated satellite experiments which have kept up an intensive surveillance for the last decade. These experiments have, however, produced an indigestible mass of data. From these we must try to synthesis an overall picture of the flare and identify the physical processes responsible. In this thesis two aspects of the flare problem are considered. The first concerns hard X-ray emission during the impulsive phase of the flare. The electron trap model of the hard X-ray source is analysed in detail and the predicted directivity and polarisation of its emission found to be compatible with hard X-ray observational data. Secondly, a self-consistent model of the soft X-ray flare is developed. Mass motion, which has previously been ignored in such models, is shown to be of vital importance. In Chapter I, the observational evidence concerning all types of flare emission is summarised. The coherency of a picture of the flare in which energetic electrons play a central part is pointed out and the significance of hard X-ray emission as an indicator of the properties of these electrons noted. Current hard X-ray source models are described in Chapter II and their predictions for the flare X-radiation outlined. Other topics of importance to the hard X-ray problem - bremsstrahlung radiation, the albedo effect and modulation of the X-ray flux - are also discussed here. Finally, the predictions of the source models are compared with observation and important areas of experimental and theoretical research suggested. The electron trap hard X-ray source model is analysed in Chapter III. This model, whose properties have only been guessed at until now, postulates that high energy electrons are trapped in a coronal magnetic arch where they emit bremsstrahlung radiation while decaying collisionally on the time scale of the hard X-ray burst decay. Directivity and polarisation of the emission are predicted for a variety of trapped electron distributions over energy and pitch angle. Predicted properties of the hard X-ray emission are presented in Chapters IV and V. Chapter IV is concerned with the total X-ray flux from the trap while Chapter V deals with some aspects of the spatially resolved emission, in particular the predicted "behind-the- limb" X-ray flux. In both chapters, results are compared with the observational data available and observations which could help to discriminate between this and other source models suggested. In Chapter VI a model of the soft X-ray flare is developed. The model consists of a high density coronal filament into the centre of which energy is injected during the impulsive phase of the flare. First, the potential importance of mass motion in this situation is demonstrated by dimensional analysis. Then a numerical treatment of the fluid dynamic equations is developed. Computational results describing the evolution of the filament, under a variety of conditions, are presented in Chapter VII. Conclusions drawn from the dimensional analysis are vindicated and deeper insight into the energy transport processes operating in the filament obtained. The soft X-ray differential emission measure is examined and it is suggested that the form is compatible with that inferred from observation.

This thesis probes the role of environment in galaxy evolution, focussing particularly on understanding the links between the truncation of star formation, the transformation of galaxy structure, and environment. This study utilises deep HST imaging, photometric and spectroscopic data for galaxies within ten high-z cluster fields, which form part of the ESO Distant Cluster Survey (EDisCS). I first compare the mass--size relations of cluster and field galaxies, to address the dependence of galaxy size on environment observed from z~2.5, and which seemingly disappear at lower redshifts. I find no significant difference in the size distributions of cluster and field galaxies of a given morphology, or with similar rest-frame B-V colours. I rule out average size differences larger than 10--20 % in both cases. Thus, I conclude that if the size difference at higher-z reported in the literature is real, the growth of field galaxies seem to have caught up with that of cluster galaxies by z~1. Any putative mechanism responsible for galaxy growth has to account for the existence of environmental differences at high redshift and their absence (or weakening) at lower redshifts. I then move on to analyse the effects of the global cluster/field environment on the internal structure of galaxies and their colours. I introduce quantitative non-parametric measures like the residual flux fraction (RFF) and the asymmetry in galaxy residuals (A_res) which measure deviations from symmetric light distributions using HST images, to explore the internal structure of galaxies. I also obtain complementary information on the probable causes of structural disturbances, both internal and external in nature, by performing visual classifications of cluster and field galaxies. Combining these two approaches of measuring galaxy structure, it is found that the RFF is a good proxy for `roughness' in the surface brightness distribution, while A_ res is more sensitive to the causes of the structural disruption. Incorporating visual morphology and environment, it was found that the external causes of disturbances were most often associated with star formation in spiral galaxies. When adding information on the star formation activity of galaxies, I discover two complementary subpopulations of galaxies abundant in clusters: visually undisturbed passive spirals, and undisturbed star-forming lenticulars. In addition to being visually symmetric, these passive cluster spirals are also consistently smoother than their star-forming counterparts. These observations, therefore, strongly advocate gentle physical processes acting on the gas content to modify the star formation properties of galaxies accreted into clusters, without large-scale disturbances in their stellar structure. Considering the variations of quantitative galaxy structure with the star formation history of galaxies, I find that the young, star-forming galaxies are consistently rougher and more asymmetric than the galaxies with older passive stellar populations. Further, the galaxies with different average stellar ages seem to have similar distributions of RFF and A_res over cluster/field environments, thereby emphasising that the star formation history of galaxies is strongly linked to their intrinsic structure alone. Finally, complementing the global cluster/field environment, I explore the projected phase--space as a tool to investigate possible variations in galaxy structure and their stellar ages over the internal cluster environment. The analysis with the projected phase--space shows a decrease in the fraction of galaxies with younger stellar populations in the cluster core when separated by morphology, especially for spirals. This trend, however, is less pronounced in the observed distributions of RFF and A_res across the projected phase--space and the field. All these observations, when put together, signify that the star formation in galaxies is shut down as they get accreted into clusters, while the internal structure of galaxies remains more or less unaffected. The actual morphological change in galaxies, therefore, will follow later after the star formation has already been truncated. Furthermore, the physical mechanisms driving these transformations would, therefore, be gas extinguishing ICM processes like ram-pressure stripping and starvation, which leave the galaxy structure undisturbed.

This Thesis is written in three parts. The first part describes the analytic calculation of the unequal-time correlator of cosmic strings and superstrings. The first efficient constraint analysis of all string and superstring network parameters is performed. By studying the effect of cosmic strings on the cosmic microwave background (CMB) radiation it is discovered that cosmic strings must make up a vanishingly small proportion of the energy density of the universe. The constraints on string network parameters are all skewed toward reducing the magnitude of energy density arising from strings. Also in this Part, a better comprehension of the unconnected segment model (USM) was gained. In particular, a greater understanding of the string scaling parameter $L_f$ was garnered, as well as finding the reason why the USM tends to provide greater power than simulations of Nambu-Goto cosmic strings. The second part contains a detailed description of statistical cosmology and how differences between parameter constraints from different data sets can lead to misleading quantification of discordance. The majority of this part describes different methods of quantifying differences between probability distributions and how these can be interpreted. In particular, using the most up-to-date data possible, differences between parameter constraints using the CMB and probes of large scale structure (LSS) in the universe can be measured. With current data the discordance can be interpreted as a low level of disagreement, but the application of prior ranges on well known parameters can force the tension to be greater. Using data from earlier work, this issue is considered in greater detail, with extensions to the accepted LCDM model added to test if the discordance can be alleviated. These extensions include the addition of active or sterile neutrinos and even ad-hoc changes to the primordial power spectrum. Although there are slight hints that these may help, when considering only the new data it might be unwise to believe that the discordance between parameter distributions from different data sets exists to a degree where the modifications are necessary. Finally, application of deep learning to astrophysical observations is discussed. Using neural networks to learn about specific problems is de rigueur and their use in astronomy and cosmology is a promising field of study. In particular, applying raw data to neural networks can often outperform, or add enhanced features, to what is possible with current, non-empirical feature detection. The classification of supernovae from their light curves can be achieved using a specific machine learning architecture called a recurrent neural network (RNN). Using the raw data from supernova light curves, the RNN is able to learn about features in sequences which can be used to classify types of supernova. Although a large training set is needed to perform as well as current techniques, one major advantage the RNN method has is the possibility of early detection. Rather than needing the entire light curve to perform statistical fits to categorise the supernova type, relatively little information from the early observation data is needed to classify using the RNN. Installing RNN on machinery for observation would save a vast amount of time by early classification since only supernovae of interest can be concentrated on.

The analysis of nuclear flares is unfortunately difficult. Contamination of supernova surveys by active galactic nuclei (AGN) variability, and the difficulty in detecting transients in the high surface brightness nuclei of galaxies, has led to many surveys avoiding nuclear transients entirely. Even in cases where transients are detected, their identification and classification remains complex, with many possible progenitor pathways, overlapping models and wide ranges of observed properties to explain. Here I consider a sample of these events, placing them in the wider context of transient astronomy. The detection of a class of relativistic tidal disruption flare, thought to be the capture and disruption of Sun-like stars that also powers a moderately relativistic jet, has prompted the search for more of these events. Within this work I analyse the properties of one such candidate, Swift J1112.2-8238, conforming its extragalactic origin and showing it came from a galaxy at a redshift of z = 0.89. Its high energy and optical properties are consistent with the previous candidates and its position, close to the centre of a likely star-forming host, continues to support the tidal disruption flare origin of these events. The rates of these events suggest that only a small fraction of tidal disruption flares launch similar jets. Prompted by these findings, I proposed and obtained medium resolution spectroscopy and radio observations of the source, and analysed high-resolution HST imaging to further constrain the position of the transient within its host. The HST imaging shows that the host has a complex morphology, perhaps due to an interaction with another galaxy, with the transient loosely consistent with the centre of compact bulge-like component. I confirm the host's redshift and determine its nature as a star-forming galaxy. Radio emission detected coming from the host, that is too luminous to be associated with star-formation, shows evidence of variability, suggesting that it is associated with the transient flare and thus is perhaps confirmation of the jetted nature of the event. In almost all respects, Swift J1112.2-8238 remains an excellent candidate relativistic tidal disruption flare. Finally, I analyse HST imaging of a number of flares with unusual properties. I greatly improve the astrometric tie of ASASSN14ae and ASASSN14li to the nuclear regions of their hosts and show that their properties are still most consistent with a tidal disruption flare origin. In the case of CSS100217 and ASASSN15lh however, the “accepted" classification of their origins as superluminous supernovae appears to be at odds with their host galaxies, with ASASSN15lh in particular coming from a massive host with minimal star-formation. I show that CSS100217 has undergone a significant drop in apparent quiescent-level emission following the flare, indicating the possibility that the flare may have directly impacted, or been caused by, a change in the accretion of the known AGN. I consider the possibility that both flares could be associated with unusual tidal disruption flares or AGN variability, though the current observations make it difficult to make strong claims about either flare's true origins.

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.

The first direct detection of dark matter is anticipated in coming years by one of a range of experimental strategies. Because the identity of dark matter remains unknown, the strategy that will be successful in this one cannot say. However beneath this fundamental particle physics uncertainty lies another uncertainty with regard to the structure of the dark matter halo of the Milky Way that must be confronted when interpreting data from terrestrial experiments. However these astrophysical uncertainties might only be resolved with the very same experiments; in fact, directly detecting dark matter represents the only way to probe the ultralocal structure of the halo. This thesis explores the impact of astrophysical uncertainties on the particle physics goals of dark matter detection but also the extent to which we might in the future be able to resolve those uncertainties. The discussion is framed around the detection of three types of particle, two of which are dark matter candidates: weakly interacting massive particles (WIMPs), neutrinos and axions. In the case of WIMPs I consider how upcoming directionally sensitive experiments can be used to probe the full 3-dimensional velocity distribution to learn about dark matter substructure. A range of model dependent and independent statistical approaches are tested under various astrophysical benchmarks. I also explore prospects for WIMP direct detection when faced with the ultimate neutrino background, as expected in the next generation of experiment. In this eventuality the uncertainties in the neutrino flux are essential in predicting the WIMP models inaccessible due to the background. However the same is true of astrophysical uncertainties. Once astrophysical uncertainties are accounted for the neutrino floor limit is raised in cross section by up to an order of magnitude and the accuracy of any potential WIMP particle measurement is greatly increased. Addressing these concerns, I demonstrate how one should go about subtracting the neutrino background. This involves a return to directional detection. I find that even non-ideal circumstances, the neutrino and WIMP signals can be distinguished and the neutrino floor overcome. Finally in the context of axions, I discuss the prospects for microwave cavity haloscope experiments to perform "axion astronomy". Haloscopes measure the direct conversion of axions into photons and hence can make potentially much finer measurements of the dark matter halo compared with WIMPs. I develop a technique to extract astrophysical parameters, such as the halo velocity dispersion and laboratory velocity, as well as learn about properties of substructure from tidal streams and axion miniclusters. I show that a level of precision can be achieved in relatively short duration haloscope experiments that can match or improve upon that of astronomical observations.

There are a host of open questions in the study of massive clusters relating to cluster formation and evolution. Understanding these processes can be useful in studying their host galaxies and how they have evolved. The formation of globular clusters is also a well debated topic, which is yet undecided and requires many more observations to constrain the theories. Here I present the work carried out during my PhD, with the goal of furthering our understanding of cluster formation and evolution using observations of massive clusters of various ages. Firstly, HST WFC3 data of the well studied face-on spiral galaxy M 83, combined with an existing cluster catalogue, was used to investigate the timescale by which young massive clusters become free of gas. This has implications for globular cluster formation theories, in addition to the survival of clusters at young ages. The presence of Wolf-Rayet stars was also investigated within the clusters and the unreliability of Hα photometry in young cluster age and mass fitting was explored. Secondly, the cluster population of NGC 1566 was used to investigate the cluster mass function and disruption in the galaxy. Whether the mass function has a truncation in the form of a Schechter function and whether disruption is environmentally and mass dependent are two questions that still persist in this area. For NGC 1566 I find that the mass function does show a truncation and using the observed luminosity function in conjunction with models, that an underlying Schechter mass function fits the observations well. Additionally the galaxy shows evidence for environmentally dependent disruption as the average timescale for the disruption of a 10⁴ msun cluster varies with galactocentric radius. A difference in age in radial bins is also indicated in a colour change found with U-B between consecutive bins, that shows more young clusters towards the centre of the galaxy and fewer at the edge. Finally, low resolution FORS2 spectra of two intermediate age massive clusters in the SMC (Lindsay 1 and Kron 3, 6-8 Gyr old) were used to look for the signatures of multiple populations (MPs), as observed in ancient GCs. The main driver behind this project was to investigate the possibility that YMCs can be considered young GCs and used to constrain their formation, and also to explore the role cluster age has in determining the presence of MPs. A subpopulation of N-enriched stars was found in each cluster, consistent with the presence of MPs. This indicates that MPs are not limited to ancient GCs and their formation mechanism must be operating until a redshift of at least 0.65, much later than the peak of GC formation at ≈ 3. It hints at a common formation mechanism between massive clusters of varying ages, including GCs, and suggests that YMCs can be used to constrain GC formation. The publications from these projects have contributed mainly to constraining GC formation theories and provides evidence for commonality in the formation mechanism used to produce GCs and YMCs alike.

A hundred years after Einstein predicted the existence of gravitational waves, the first direct detection was made from gravitational waves emitted by a binary black hole system. Other potential sources for an advanced gravitational-wave detector network include core-collapse supernovae. Due to complicated simulations of the physics involved in core-collapse supernovae, the exact waveform of a core-collapse supernova signal is unknown. A detection of a core-collapse supernova signal is challenging, as noise of non-astrophysical origin contaminates the science data taken by the advanced detectors. Noise transients in the detectors limit the false alarm rate of astrophysical detections, and could potentially mimic a core-collapse supernova signal. They can reduce the duty cycle of the detectors, which is particularly harmful for core-collapse supernovae detections due to their low event rate. Prompt characterization of instrumental and environmental noise transients will be critical for improving the sensitivity of the advanced detectors during observing runs. During the science runs of the initial gravitational-wave detectors, noise transients were manually classified by visually examining the time-frequency scan of each event. Here, we present a Bayesian model selection algorithm designed for the automatic classification of noise transients in advanced gravitational-wave detectors. The algorithm is tested on simulated data sets and real non-Gaussian, non-stationary Advanced LIGO noise, and we demonstrate the ability to automatically classify transients by frequency, SNR and waveform morphology. A classification of noise transients as data is taken can lead to an improvement in data quality during an observing run and determine their origin. In this thesis, we show how Bayesian model selection can be used to determine if a core-collapse supernova candidate gravitational-wave signal is a noise transient, a core-collapse supernova signal or other astrophysical transient. If the signal is a core-collapse supernova detection, we show how the core-collapse supernova explosion mechanism can be determined using a combination of principal component analysis and Bayesian model selection. We use the latest three-dimensional simulations of gravitational-wave signals from core-collapse supernovae exploding via neutrino-driven convection and rapidly-rotating core-collapse. We show that with an advanced detector network, we can determine if the core-collapse supernova explosion mechanism is neutrino-driven convection for sources in our Galaxy, and rapidly-rotating core collapse for sources out to the Large Magellanic Cloud.

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.