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In 2019, the Event Horizon Telescope Collaboration (EHTC) released the first image of the shadow of a black hole based on observation. This is the closest astronomers have come to imaging a celestial object from which no light can escape. This dissertation offers an account of its formation and early reception based on historical analysis of black hole images published since 1973 in addition to fieldwork and interviews performed at institutions where members of the EHTC work in Harvard and Radboud University. A study of the research techniques and iconography built up over several decades demonstrates that earlier research played a significant role in both the production of [the image], reflected in visual and argumentative strategies deployed by EHTC scientists, and its reception amongst scientists and a broader public. First focusing on points of views in black hole imaging, I show how the framing of the observer in General Relativity took the role of a 'photographer' of black holes, and then changed as it was believed that observation from Earth could be possible if using multiple telescopes across the globe. Examining the portrayal of spatial depth in these representations, I trace several ways in which perspective techniques were employed to make spaces close to black holes intuitive to researchers and other audiences, identifying when scientists used approaches which did not rely on forms of perspective familiar from art history. I then focus on the roles spatial relations played in the social and epistemic aspects of the production of images within the EHTC. Finally, I analyse the reception of [the image] with a focus on how the existing iconography of black holes influenced the ways various audiences perceived and reframed the first image from observation.

In this thesis, we propose a formula for the quantum entropy function of the BMPV black hole using localisation methods. We use the framework of off-shell N = 2, D = 5 supergravity to study the quantum entropy function of a black hole proposed by Sen. We analyse the localization equations relevant to the quantum entropy of the BMPV black holes. The precise problem is to classify all solutions to the off-shell supersymmetry equations in N = 2 supergravity coupled to nv + 1 vector multiplets around the near-horizon black hole. We rewrite these equations in terms of the bosonic spinor bilinears that exist in the geometry for an arbitrary background. We then focus on the vector multiplet fluctuations around the near-horizon attractor region of the supersymmetric black hole, and classify all smooth solutions to the lo-calisation equations in this background for different choices of analytic continuation. For the choice of analytic continuation consistent with the 4d/5d lift, we find that the most general localisation solution for the five-dimensional black hole problem is an (nv + 1)-dimensional manifold, which is precisely the lift of the localisation manifold for supersymmetric black holes in four-dimensional asymptotically flat space. We then evaluate the action on the black hole, including the vector multiplet fluctuations. We lift the localisation solutions from 4d for the Weyl multiplet and evaluate the second derivative action. We then propose a formula for the full action, including higher derivatives, over the full localisation manifold. By comparing with the low energy logarithmic corrections found by Sen, we propose an ansatz for the one loop determinant. Lastly, we evaluate the quantum entropy function at second derivative level.

In the centres of the Milky Way and M83, the global environmental properties thought to control star formation are indistinguishable within observational uncertainties. Despite this, present-day star formation rates in the centres of each galaxy differ by an order of magnitude. In this thesis, I explore the gas kinematics of both regions to understand the origin of this difference. In Chapter 2, I present an overview and data release of the spectral line component of the SMA Large Program, CMZoom. The CMZoom survey targeted 10 dense gas and shock tracers in all gas within the Central Molecular Zone (CMZ; inner few hundred pc of the Galaxy) above a column density of N(H2) >= 10^23 cm^-2, between an observing frequency of 216 -232 GHz, including three CO isotopologues, multiple transitions of H2CO, SiO, OCS and SO. I extracted spectra from all compact 230 GHz continuum sources (cores) in the previously published CMZoom catalogue and fit line profiles to the spectra. After detailed quality controls, I used the fit results from the H2CO 3(0,3)-2(0,2) transition -- which represents 88.8% of the total mass of CMZoom cores -- to determine the core kinematic properties. I find that only four cores are self-gravitating, but that the remainder are consistent with being in hydrostatic equilibrium and confined by the high external pressure in the Galactic Centre. Using star formation tracer associations from an forthcoming publication, I estimate a present-day star formation rate of 0.009 M yr^-1 for all cores in the Galactic Centre with an H2CO detection. I find that the line ratios of CMZoom cores in the Galactic Centre are indistinguishable from CMZoom cores which lie outside the Galactic Centre. This suggests that the chemical differences observed between the CMZ and disk clouds does not propagate down to core scales. I find only two convincing proto-stellar outflows throughout the survey, ruling out the possibility of a previously undetected population of forming high-mass stars. Despite having sufficient sensitivity and resolution, in the large CMZoom survey area I find no high-velocity compact clouds (HVCCs) which have been claimed as evidence for intermediate mass black holes interacting with molecular gas clouds. In Chapter 3, I use ALMA observations of HCN (1-0) and HCO+ (1-0) to trace the dense gas at the size scale of individual molecular clouds (0.54"', 12pc) in the inner ~ 500 pc of M83, and compare this to gas clouds at similar resolution and galactocentric radius in the Milky Way. I find that both the overall gas distribution and the properties of individual clouds are very similar in the two galaxies, and that a common mechanism may be responsible for instigating star formation in both circumnuclear rings. Given the remarkable similarity in gas properties, the most likely explanation for the order of magnitude difference in SFR is time variability, with the Central Molecular Zone (CMZ) currently being at a more quiescent phase of its star formation cycle. I show M83's SFR must have been an order of magnitude higher 5-7 Myr ago. M83's `starburst' phase was highly localised, both spatially and temporally, greatly increasing the feedback efficiency and ability to drive galactic-scale outflows. This highly dynamic nature of star formation and feedback cycles in galaxy centres means (i) modeling and interpreting observations must avoid averaging over large spatial areas or timescales, and (ii) understanding the multi-scale processes controlling these cycles requires comparing snapshots of a statistical sample of galaxies in different evolutionary stages.

Black holes: cosmic bodies of extreme gravity that nothing, not even light, can escape. Einstein first told the world about the existence of these enigmatic objects in his groundbreaking theory of General Relativity. It was not until 2015 that black holes were probed directly through the first observation of gravitational waves. The treasure trove of knowledge gleaned from this observation revolutionised our understanding of the cosmos. Since this Nobel prize winning observation, the LIGO Scientific, Virgo and KAGRA collaborations have observed fifty gravitational waves. The aim of this thesis has been to take advantage of the growing population of gravitational wave sources to answer the following fundamental question: do binary black holes undergo spin-induced orbital precession? This has significant implications on our understanding of how binary black holes form in nature. To answer this question, I first introduced a brand new formalisation for modelling a gravitational wave that originated from a precessing system. I then introduced the 'precession signal-to-noise ratio' which naturally followed from this unique description. This novel tool quantified, for the first time, the significance of precession in an observed gravitational wave. I then used the 'precession signal-to-noise ratio' to demonstrate a) that there is no evidence for precession in any of the binary black hole candidates from the first gravitational wave catalog and b) how this lack of precession allows us to constrain the properties of black holes. I then presented the properties of potentially the first neutron star-black hole binary observed -- a system which is most likely to have measurable precession as a result of the asymmetric component masses. I then calculated the 'precession signal-to-noise ratio' for all gravitational wave candidates observed in the first half of the third gravitational wave observing run, and demonstrated that three observed gravitational waves could have originated from precessing systems. I then used the gravitational wave data from the second gravitational wave catalog to determine the most likely spin distribution of black holes. By doing so, I was able to determine whether the population of binary black holes are likely to undergo spin-induced orbital precession. Finally, I presented a new and innovative software package to analyse, display and combine posterior samples. This package has become one of the major workhorses of the LIGO Scientific, Virgo and KAGRA collaborations and is widely distributed through the gravitational wave data analysis computing environment.

Star formation (SF) has been continuous since the Universe was 200 million years old. It occurs in the interstellar medium (ISM) - the gas and dust between stars within galaxies. The majority of SF occurs inside giant molecular clouds (GMCs) - the most massive agglomerations of dense gas within the ISM - typically the stars form in clusters. Initially the SF is governed solely by a GMC's morphology, but, as stars form, the energy and momentum they inject into their surroundings - stellar feedback - affects ongoing star formation within the GMC. The effects of this feedback not only help to break up the cloud, but affect the wider ISM, and hence influence both neighbouring GMC evolution and future GMC formation. This thesis explores how two forms of stellar feedback - photoionisation and supernova (SN) - affect Milky Way-like spiral arm regions through the use of nu- merical hydrodynamic simulations. The numerical initial conditions are created by extracting a 500 pc2 region from simulations of whole galaxies. This means the simulations begin with a 'realistic' arrangement of neighbouring GMCs. The ISM is affected by the warm (104 K) HII regions that form and expand around massive photoionising stars and the hot (106 K) SNe ejecta that are emitted from the same stars at the end of their lifetimes. In these simulations photoionisation breaks GMCs and the denser clumps in their substructure up into a larger number of objects while, at the same time, increasing the total mass of dense ISM. This results in more rapid, and partially displaced, SF when compared with simulations without stellar feedback. The main cause of these effects is the compression of dense, but non-star forming, gas from multiple sides by HII regions. SNe have little effect on SF on spiral arm scales. However, SNe are able to heat large regions of the ISM to high temperatures, but only if the gas has already been exposed to photoionising feedback.

Observational studies of star formation reveal spatial distributions of Young Stellar Objects (YSOs) that are `snapshots' of an ongoing star formation process. Using methods from spatial statistics it is possible to test the likelihood that a given distribution process could produce the observed patterns of YSOs. I determine the sensitivity of the spatial statistical tests Diggle's G function (G), the `free-space' function (F), Ripley's K and O-ring for application to astrophysical data. To do this I applied each test to simulated data containing 2D Gaussian clusters projected on a random distribution of background stars. By varying the number of stars within the Gaussian cluster and the number of background stars I determined the ability of the tests to reject complete spatial randomness (CSR) with changing signal-to-noise. Ripley's K and O-ring were shown to be much more sensitive to Gaussian clusters than G and F. I then apply the O-ring test to determine if column density alone is sufficient to explain the locations of Class 0/I YSOs within Serpens South, Serpens Core, Ophiuchus, NGC1333 and IC348. Star formation is known to occur more readily where more raw materials are available, a relationship that is often expressed in the form of a 'Kennicutt--Schmidt' relation where the surface density of Young Stellar Objects (YSOs) is proportional to column density to some power, μ. Using the O-ring test as a summary statistic, confidence envelopes were produced for different values of μ from probability models made using the Herschel column density maps. The YSOs were tested against four distribution models: the best-estimate of μ for the region, μ = 0 (i.e. random) above a column density threshold and zero probability elsewhere, μ = 1, and the power-law that best represents the five regions as a collective, μ = 2.05 ± 0.20. Serpens South and NGC1333 rejected the μ = 2.05 model on small scales of ~ 0.15 pc which implies that small-scale interactions may be influencing their distribution. On scales above 0.15 pc, the positions of YSOs in all five regions can be well described using column density alone.

This thesis concentrates on efforts to identify and characterize compact mergers involving neutron stars using wide-field survey instruments. With the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) and the Asteroid Terrestrial-impact Last Alert System (ATLAS), searches for compact mergers, including kilonovae, have been performed. The Pan-STARRS search identified two rapidly evolving transients with similar decline rates to the first kilonova, AT2017gfo, in PS15cey and PS17cke. While model fits of black hole-neutron star mergers can match the peak luminosity of PS15cey, a kilonova interpretation is less likely than a rapidly evolving supernova. PS17cke may be a plausible kilonova candidate, however, based on supporting modelling. The ATLAS search has returned no plausible kilonova candidates, but has identified a unique optical transient in AT2018kzr whose peak luminosity and rapid decline are difficult to describe using a traditional Nickel-56 driven explosion models. This necessitates the inclusion of an additional powering source, such as the spin-down of a magnetic neutron star remnant, to provide a plausible model fit to the bolometric lightcurve. Spectral modelling suggests the object is characterised by intermediate mass elements (including oxygen, silicone and magnesium) which disfavours a neutron star-neutron star merger scenario, but is consistent with predictions of white dwarfneutron star mergers. The lack of a kilonova detection by ATLAS has prompted the development of an efficiency simulation to understand the challenges of observing rapidly evolving and intrinsically faint transients with all-sky survey telescopes. Such a simulation has been used to constrain the volumetric rate of AT2017gfo-like transients in the local universe, with the results being consistent with other optically-derived rates estimates and rate of binary neutron star mergers measured gravitational wave observatories. Given the available information, it is predicted ATLAS will need to continue observing for at least a decade in order to observe a kilonova.

In this thesis we have studied static black holes in direct product spacetimes. In this setting, there exists a universal sector describing black holes which are uniformly extended along the internal space. This sector naturally exhibits a hierarchy of length scales and dynamical instabilities of the Gregory-La amme type appear when one of those lengths is much larger than the other. At the onset of the instability there exists a 0-mode that hints on the existence of a new branch of black holes that are non-uniformly extended along the internal space. Typically, this new family of black holes cannot be found analytically and one has to rely on numerical methods. In particular, we have numerically constructed these `non-uniform black holes' in Kaluza-Klein theory and Anti-de Sitter spaces times a sphere. Moving along the space of solutions, eventually the branches of non-uniform black holes merge with another branch of black holes which have di erent horizon topology. These are also constructed in the Kaluza-Klein case. In this thesis we have focused on a detailed study of the extreme black holes very near the critical (or merger) point. It had been predicted that the physical properties of black holes near the critical solution are controlled by a local Ricci- at cone that governs, locally, the singularity at the merger. We verify this prediction by extracting the critical exponents of various physical quantities in the Kaluza-Klein setting in D = 10. In this particular case, properties of black holes can be computed by solving a dual super Yang-Mills theory on the lattice. In another study, we consider critical non-uniform black holes in AdSp Sq for (p; q) = (5; 5) and (p; q) = (4; 7), which are the relevant cases for the gauge/gravity duality, and compute, for the rst time, the critical exponents. Remarkably, in these two cases our study suggests a non-Ricci- at cone, which is consistent with the presence of nontrivial uxes in the setting. Our results are new and non-trivial predictions of the gauge/gravity duality without supersymmetry.

Type Ia supernovae (SNe Ia) are still poorly understood, despite being widely-studied across many fields of astrophysics. Explosion simulations have shown that sub-Chandrasekhar mass white dwarfs are promising candidates for the progenitors of SNe Ia, and can reproduce a number of the observed features of SNe Ia. The 'double detonation' explosion mechanism is a widely discussed scenario by which an explosion in a sub-Chandrasekhar mass white dwarf may be ignited. In this scenario, helium accreted onto the surface of a carbon-oxygen white dwarf detonates, igniting a secondary carbon detonation in the white dwarf core. However, apparent discrepancies are found between models and observations due to strong absorption caused by the products of the helium shell detonation. In particular, red colours and strong Ti II absorption features are predicted that are not observed in normal SNe Ia. In this work, we present new simulations of the double detonation scenario with the aim of determining whether improved simulation methods can reduce these discrepancies. We have carried out radiative transfer simulations using the 3D code ARTIS to make predictions of the synthetic light curves and spectra for a series of double detonation explosion models. We initially tested whether improvements to the level of sophistication in the hydrodynamic explosion simulations can improve agreement with observations, making use of the standard version of ARTIS (ARTIS-classic), which approximates non-LTE. However, we found that these results were similar to previous work, and discrepancies still remain between models and observations. We tested the full non-LTE implementation of ARTIS (ARTIS-NLTE) at phases around maximum light for the first time. We found that ARTIS-NLTE produces quantitative differences in the predicted synthetic observables compared to ARTIS-classic. These differences were particularly significant for double detonation models. The heavy elements in the ejecta outer layers, produced in the helium shell detonation, are strongly affected by the improved treatment of ionisation in ARTIS-NLTE. The full non-LTE simulations predict significantly less absorption by the products of the helium shell detonation, such that colours become less red, and show better agreement with those of normal SNe Ia. However, we still find strong Ti II spectral features, not observed in normal brightness SNe Ia. While the full non-LTE treatment has reduced the discrepancies between models and observations, further work is required to determine whether the helium detonation products (particularly Ti) can be reduced. We also make predictions of helium spectral features for the double detonation scenario, and compare to observations. These results indicate that the direct detection of helium spectral features is a potential signature of the double detonation.

In this thesis I will present observational studies of transient systems that produce mildly to extremely relativistic outflows through a coupling to an accretion flow. I will focus on the analysis of data taken from three source classes: black hole X-ray binaries (BHXRBs; particularly the system MAXI J1820+070), gamma-ray bursts (GRBs; particularly the system GRB 171010A), and tidal disruption events (TDEs; particularly the systems ASASSN-14li and Swift J1644+57). I will present an extensive radio monitoring campaign on MAXI J1820+070 utilising five different interferometers, along with extensive X-ray observations, during the system's 2018 outburst. Together these data allowed me to probe the coupling between accretion and jet production throughout an entire outburst cycle, as well as during multiple hard accretion state only re-brightenings, tracked over a two year time frame. As MAXI J1820+070 transitioned from the hard to soft accretion state, contemporaneous time-series indicators of the launch of bipolar relativistic ejections were observed at both radio and X-ray frequencies (manifesting as a radio flare and an evolving quasi-period oscillation, respectively). I then confirmed the presence of these ejecta utilising multiple interferometers, and was able to track the evolution of both the approaching and receding ejecta for over ~150 d. Through utilising interferometers sensitive to very different angular scales, I was able to infer the internal energy of the ejection, and found it to be much larger than the value implied from the state transition radio flare. This is strong evidence for ongoing particle acceleration as ejections interact with the surrounding interstellar medium. In addition to the study of MAXI J1820+070, I will also present a broader population study of state transition radio flares from black hole X-ray binaries, and demonstrate that commonly employed models (which attribute flares to an optical depth evolution from an expanding region) are not appropriate for the majority of flares in the sample studied. I describe the ability of extended periods of particle acceleration to explain the flare profiles. GRB 171010A was a luminous and nearby long GRB detected at early times by the Arcminute Microkelvin Imager Large Array interferometer, as well as with the Swift X-ray telescope. Long GRBs produce highly relativistic outflows that are best studied through their interaction with the circumburst material. This interaction produced a broadband synchrotron afterglow. I present a study of the radio through X-ray afterglow of GRB 171010A in the context of the fireball model (which details the interaction of the jet and interstellar medium). By fitting the time evolving spectra, the values and evolution of the characteristic synchrotron frequencies can be inferred. GRB 171010A is one of the most energetic GRBs detected below z~0.5, allowing for our theoretical understanding of afterglows to be investigated. While I find general agreement with the canonical models (particularly the spectral indices either side of the minimum energy frequency) a number of deviations are seen. I discuss possible solutions to these deviations, which likely include the addition of a second spectral component resulting from a reverse shock. Finally, I will present late time radio and X-ray observations of the thermal TDE ASASSN-14li, and late time radio only observations of the relativistic TDE Swift J1644+57. Tidal disruption events occur when a star passes too close to a supermassive black hole and is torn apart by tidal forces. Approximately half of the stellar material is accreted and the rest is unbound. ASASSN-14li is a radio bright thermal TDE, and the origin of this radio emission is disputed. I will show that the late time radio properties are now consistent with background AGN activity, but that while the TDE was the dominant radio component, the radio emission was correlated with the X-ray emission. This provides evidence that thermal TDEs produce jets. It is not disputed that the relativistic TDE Swift J1644+57 produced a jet, and said jet's radio emission has now been monitored for ~10 yrs. I will present the most recent monitoring of Swift J1644+57 in the context of previously proposed jet models for the source. I will additionally discuss the up-to-date population of radio loud TDEs. My conclusions contain a comparison of the outflows produced by these sources, and how they are analysed in different frameworks.

Supermassive black hole (SMBH) masses correlate tightly with various properties of their host galaxies, implying close co-evolution. However, the mass measurements underpinning this conclusion have large uncertainties and are limited by selection biases. With the advent of modern millimetre/sub-millimetre interferometers, molecular gas dynamics are now suitable tracers of the SMBH-dominated regions of galaxy potentials. In this thesis, I present new mass measurements for the SMBHs in the nuclei of the local elliptical galaxies NGC 524 and NGC 7052. These measurements exploit exceptionally high angular resolution observations of carbon monoxide (CO) J=2−1 emission using the Atacama Large Millimeter/sub-millimeter Array (ALMA). I refine earlier dynamical modelling methods, allowing for non-axisymmetric gas distributions and accounting for previously underestimated formal uncertainties when fitting models with very large numbers of constraints. The measurement in NGC 524 is consistent with an earlier stellar dynamical measurement, while that in NGC 7052 shows that an earlier gas dynamical measurement underestimated the SMBH mass (possibly by neglecting pressure support). The dynamical models of these galaxies also reveal a small inflow/warp in NGC 524, and that the central hole in NGC 7052 is consistent with the region within which tidal forces disrupt molecular clouds. Using spatially-integrated CO spectra from both single-dish telescopes and interferometers I further show a correlation exists between large-scale CO emission line widths and SMBH masses. This new correlation is no less tight than most, allowing it to be used to estimate SMBH masses in the absence of the (often harder to obtain) data required to exploit tighter correlations. Finally, I discuss the dynamical features revealed in new ALMA observations of 11 other SMBH mass measurement candidate galaxies, and estimate the SMBH masses in the three most promising. This thesis demonstrates the potential of molecular gas dynamics to advance our understanding of SMBH-host coevolution, and indicates the next steps in understanding the molecular contribution to AGN fuelling and feedback.

The classical picture of an accretion disc is of a geometrically thin, nearly axisymmetric, fluid flow undergoing supersonic circular motion and slowly accreting due to angular momentum transport by the disc turbulence. There are, however, strong theoretical and observational grounds for considering non-circular motion in accretion discs. In this thesis I study the dynamics of such non-circular accretion discs using a mix of analytical and semi-analytical methods. One such example of a non-circular accretion disc is an eccentric disc, where the dominant fluid motion consists of slowly evolving, nested, confocal, Keplerian ellipses. I focus on the dynamics of these eccentric discs, along with the dynamical non-axisymmetric vertical structure that they set up. In the first part of this thesis I consider eccentric waves in ideal fluid discs. I present a secular Hamiltonian theory describing the evolution of the disc orbits due to pressure gradients and show that it can be used to calculate the eccentric standing wave patterns in the disc (the eccentric modes). I derive a ``short wavelength'' theory for nearly circular orbits that nevertheless can have substantially non-axisymmetric surface density/pressure due to nonlinear eccentricity gradients and disc twist. I use this to show that the pressure profile and precessional forces (such as general relativistic apsidal precession) can focus eccentric waves, causing them to become highly nonlinear in the inner disc. In the second part of this thesis I consider the action of non-ideal terms on the eccentric disc, such as excitation and damping by viscosity. I derive the ordinary differential equations describing simple horizontally invariant ``laminar flows'' in a local model of an eccentric disc, which can be used to study the disc's dynamical vertical structure. I also move beyond the purely hydrodynamic models of the preceding chapters and consider the action of magnetic fields in a local model. Finally I apply the theory presented in this thesis to the highly eccentric discs expected from the tidal disruption of a star by a supermassive black hole. It is currently an open question how, and indeed if, the discs in tidal disruption events circularise. As a step towards understanding the evolution of the disc orbits, I calculate the dynamical vertical structure of highly eccentric discs, emphasising the role of radiation pressure and thermal stability, and showing that magnetic fields may be important where the disc is highly compressed near the periapsis.

This thesis investigates the question of whether supernovae (SNe) are important contributors of dust to the interstellar medium (ISM). Using far-infrared (FIR) observations of 190 supernova remnants (SNRs) covered by the Herschel Infrared Galactic Plane Survey (HiGal), we augment the current sample of dusty SNRs with an additional 38 from which dust emission is newly detected. This gives a 21 per cent detection rate of dust within SNRs in the Galactic Plane, although this is expected to be a lower limit given confusion with the ISM in this region. The majority of detected SNRs are young (less than 5 kyr), as expected since these objects tend to be more compact and have higher surface brightness, making them more easily detected above the level of the ISM. Nevertheless, we expand the age range of SNRs containing dust to roughly 100 kyr. Of our detections, 13 are from core collapse (CC) and 2 from Type Ia SNe (24 are of unknown type). Consistent with previous observations and theoretical models, we find that dust detected within the Type Ia SNRs is not expected to be of ejecta origin, but is more likely to have been swept up from the ISM. It is likely that many of our other detections are also dominated by ISM, although there are several objects within which we expect that the FIR emission originates from freshly formed ejecta material, including G350.1-0.3. We make new discoveries of ejecta dust within pulsar wind nebulae (PWNe) associated with three SNRs: G11.2-0.3, G21.5-0.9, and Kes 75. Using point process mapping (PPMAP) to analyse the FIR emission indicates the presence of dust within each object with temperatures between 20 and 40 K, and a PWN dust mass of 0.34 +/- 0.14 solar mass, 0.29 +/- 0.08 solar mass, 0.51 +/- 0.13 solar mass for G11, G21, and Kes 75 respectively. This suggests that SNe are important contributors of dust to the ISM, and PWNe in particular may play an important role in this. Although some of this dust may yet to be destroyed, there is evidence that the reverse shock has already reached the centre of G11, leaving behind a considerable mass of surviving dust. We use PPMAP further in an attempt to examine the dust properties, by studying the emissivity index, beta. The signal-to-noise in the longer wavebands is too low to constrain b for G11 or Kes 75; however, we find some evidence of a variation in the dust properties of ejecta dust within G21 (beta = 1.4 +/- 0.5), compared with that of the surrounding ISM (beta = 1.8 +/- 0.1). We study the dust within 11 of the other detected SNRs which are in regions with limited ISM variation, making background subtraction and thus dust analysis possible. By producing temperature maps, using their 24 - 70 micron flux ratios, we reveal the presence of warm dust (25 - 40 K) within all of these objects. For the objects with associated distance estimates, this analysis gives dust masses of between 0.2 and 340 solar mass, some of which are extremely large compared to the typical combined mass of freshly formed and swept up material. It is most likely that, despite careful background subtraction, the largest of these masses are overestimated due to contaminating ISM, which reduces the average dust temperature. As the dust mass is highly sensitive to even small changes in the estimated temperature, ISM confusion is the largest barrier to overcome when analysing the dust content of SNRs within the Galactic Plane. We detect a region of dust coincident with a strange radio source at the centre of G351.2-0.1. Previous studies have suggested that this could a PWN, or simply an unrelated HII region; we therefore attempt to determine its nature using FIR observations. Its spectral energy distribution (SED) suggests that the region consists of two dust components: (3.9 +/- 0.4) x 10^(-6) solar mass of warm dust with a temperature of 242 +/- 7 K, and 0.18 solar mass of cool dust with a temperature of 45.8 K. The temperature of the dust is higher than typical for a HII region, although not unheard of, and when considering the IR colours the emission from the core is consistent with either a HII region or SNR. We therefore cannot rule out either option based only on the IR emission, and the G351.2 core could be either an unrelated HII region, or a PWN with a considerable dust mass. A large region of dust was detected within a very unusual object known as the Tornado, G357.7-0.1. This is an object with a very uncertain past, which, it has been suggested, could have been formed by a SN or an X-ray binary. We detect FIR emission from multiple components of the Tornado: a filamentary structure within the tail region, peaks within the head, and an unrelated HII region dubbed the 'eye'. Using PPMAP we reveal complex dust structures within the Tornado head and tail with temperatures between 15 and 61 K, and a total dust mass for the head of 16.7 solar mass. Within the head there is clear variation in the IR emission: an IR peak is detected to the north-west where the head is thought to be expanding into dense molecular material, and there is IR emission along radio filaments found towards the east. Comparing the IR and IR - radio colours highlights a variation in the emission from the two regions, suggesting that the dust properties or heat processes are different. When considering the nature of the Tornado, the head can be explained by a SNR aged between 2 and 8 kyr, with a large mass of dust swept up from the dense surrounding material. However, the tail is bizarre and more difficult to explain with a SN origin. We find it unlikely that it was formed by a progenitor prior to exploding. The structures in the tail could have been formed by jets from an X-ray binary, and we propose that the Tornado is similar to the W50 - SS 433 system, in which the jets of an X-ray binary plough into the SNR shell. This thesis has found a considerable dust mass contained within numerous Galactic SNRs, with several examples of newly detected ejecta dust. In particular, PWNe seem to be significant in the search for dusty SNRs. These findings suggest that SNe are important contributors of dust to the ISM.

Stellar flares are explosive phenomena caused by reconnection events in the magnetic fields of stars. They emit across a wide range of wavelengths but notably in the optical, UV and in X-rays. They are some of the highest energy events seen from other stars and they regularly dwarf those seen from the Sun. They are regularly seen on low mass stars and provide a way of studying the magnetic activity of these systems. Their role in exoplanet habitability (in particular for low mass stars) has become increasingly relevant in recent years, necessitating studies of their energies and occurrence rates. In this thesis I present observations and analysis of stellar flares detected with the Next Generation Transit Survey (NGTS). These flares come from stars ranging from G to L spectral type. I also present the detection and analysis of the transiting brown dwarf NGTS-7Ab, discovered in part because of the multiple flares detected from the M dwarf host star. These flare detections include the first ground-based CCD detections of superflares from a G-type star, NGTS J030834.9-211322. I used the high cadence NGTS observations to apply a Solar inspired empirical flare model, one which does not require arbitrary break points between the flare rise and decay. I also present the detection of a giant flare from a pre-main sequence M star. This flare was one of the most energetic detected from an M star and displayed quasi-periodic pulsations in the flare peak. I apply solar techniques to analyse these oscillations and identify their cause. I present the first detection of a white-light flare from an L2.5 dwarf. This is the coolest star to ever show a white-light flare to date and shows strong tranxiii sient magnetic activity can persist to the brown dwarf boundary. I also show how full frame images in wide-field exoplanet surveys, provided they are a high enough cadence, can be used to detect flares from the faintest and coolest stars. I present the detection of NGTS-7Ab, an ultra-short period brown dwarf around a tidally locked and active M star. I analyse the entire system, showing through a kinematic and photometric analysis that it is likely a hierarchical triple system formed of two active M stars (NGTS-7A and NGTS-7B) and a transiting brown dwarf (NGTS-7Ab). This is the shortest period transiting brown dwarf discovered to date and it has spun up and tidally locked its host star, placing the system in a state of spin-orbit synchronisation. Finally, I discuss the detection of white-light flares from pre-main sequence stars associated with the Orion complex. These stars have an average age of 4 Myr. I measure the average flare occurrence rate for M0-M3 stars, finding a non-linear relationship between flare occurrence rate and cluster age. My work shows how the ground-based NGTS observations can rival those available from space for high energy white-light flare events. Throughout this work I also discuss the possible effects flares may have on nearby exoplanets, along with how the detected flare events relate to other signs of magnetic activity such as starspots.

Variable X-ray and radio emission is frequently observed from flares on the Sun as well as low mass, young stellar objects known as T-Tauri stars. These T-Tauri flares are several times more luminous than their solar analogues; a result of their more active magnetospheres. The temporal relationship between the thermal X-ray and nonthermal radio emission seen in flares is described by the Neupert effect, where the thermal X-rays are produced following the plasma heating by the non-thermal particles. The Gudel-Benz relation also correlates the peak of the thermal X-ray and ¨ non-thermal radio emission from flares. This relation has long been applied to solar flares as well as extended to flares on other main sequence stars. However, combining observations of T-Tauri flares from multiple sources suggests that the radio emission produced in the magnetospheres of T-Tauri stars is far greater than expected. Modelling of T-Tauri flares is carried out, using fast gyrosynchrotron codes to calculate the gyrosynchrotron emission produced from flux tubes that connect the star with its accretion disk. Initially, the effect of varying peak flaring parameters on the radio and X-ray emission is investigated. The resulting peak X-ray and radio luminosities of logLX [erg s -1 ] = 30.5, logLR [erg s-1 Hz -1 ] = 16.3 agree well with observations and also fall below the Gudel-Benz relation. A secondary model, that incorporates time dependent ¨ effects of the radio emission using 3D MHD data, also produces large radio luminosities that agree with observations. This model also provides the first multi-frequency (1-1000 GHz) intensity and circular polarisation predictions for a flaring T-Tauri star over time. In both models, the peak flux occurs around 30 GHz. The peak flux at each increasing time step is also found to decrease and move to lower frequencies. These models provide important constraints on the physical properties of the flaring star-disk environment, as well as predictions for the observable fluxes from different viewing angles.

Wolf-Rayet (WR) stars are the helium core burning descendants of massive O stars. Their strong and dense stellar winds remove outer hydrogen layers from the atmosphere, revealing core burning products, which appear as broad emission lines. As the final evolutionary phase of high mass stars, they can reveal much about massive star origins, evolution and fates. In this thesis, we study Galactic and extragalactic WR stars using results from Gaia Data Release 2, which contains parallaxes and proper motions for >1 billion Galactic stars. We find distances for 383 Galactic WR stars, of which 187 have reliable distances and absolute magnitudes at optical or IR wavelengths. Absolute magnitudes were used to calculate subtype calibrations, which can be applied to estimate distances to other WR stars. Additionally, we find 31 potential runaways by searching for stars far from the Galactic plane. We also explore WR star origins, using membership of Galactic clusters, associations and star forming regions. At least 59% of WR stars are probably isolated. Simulations suggest they likely result from sparse (~10 Msun pc^3) associations, which dissolve to field densities (~1-10 Msun pc^3) during the WR star lifetime. Many massive stars may therefore form outside cluster environments. We also determine age estimates of clusters containing WR stars. New distances allow us to determine emission line luminosity calibrations at 4600-4700Ang and 5600-5900Ang . These can be applied to determine the numbers and subtypes of WR stars within unresolved regions of external galaxies. Finally, we surveyed the WR population of the spiral galaxy NGC6946 with Gemini GMOS. We find 92 photometric candidates and confirm 61 WC stars spectroscopically using templates. These results are consistent with populations derived from emission line calibrations. We use spectroscopic nebular data to estimate the central metallicity (12+log(O/H)=8.76+/-0.2) and extinction, AV =1.88+/-0.09 towards the galaxy, consistent with other studies.

Early Universe cosmological phase transitions, in particular first-order transitions which proceed through the nucleation of bubbles, provide an environment for interesting physics with potentially detectable consequences. This thesis focuses broadly on bubble nucleation rates and the determination of a successful transition. After providing the groundwork for understanding these transitions and their key characteristics, this work can be considered in two main parts: improving viability of the extra-dimensional holographic phase transition, and quantifying the effect of black holes acting as nucleation sites for thermal phase transitions. Extra dimensional models recently gained traction as attractive options for solving the hierarchy problem, one of physics' most prominent issues. Finite temperature studies of the Randall-Sundrum model, one of the most popular models, revealed the existence of the holographic phase transition, a first-order transition described by the radion field. Typically, however, this transition struggles to complete while maintaining a consistent theory. Applying knowledge from more conventional settings, this thesis firstly describes work to alleviate this problem through the introduction of brane localised curvature, altering the kinetic term of the radion field. Ultimately, it is shown that nucleation rates sufficient for a successful phase transition can be achieved over a large region of parameter space. Nevertheless, difficulties remain when describing the role of black holes in holographic phase transitions. Inspired by these issues, the second key work of this thesis studies the possibility of black holes acting as nucleation sites in thermal phase transitions. Within the thin-wall limit, it is found that lower mass black holes could drastically improve nucleation rates. Consequently, future studies of cosmological phase transitions should carefully consider the effect of black holes; particularly in regards to phenomenological outcomes such as gravitational waves.

Pulsars are fascinating objects that enable a variety of high-precision experiments. A key aspect of pulsar science is the continued search for more of them, with an emphasis on unusual specimens that challenge and expand our understanding of the laws of physics. This thesis starts with a search of archival survey data acquired with the Parkes radio telescope (Chapter 2), using a highly optimised GPU-based search code and an accurate candidate classification algorithm, resulting in the discovery of 23 pulsars missed in previous searches. We empirically noticed that longer period sources were less easily detected by the standard method based on Fourier transforming the data, leading us to experiment with another technique: the fast folding algorithm (FFA). In early tests of the code, we found an underlying periodicity to a sequence of faint pulses recorded at Parkes; these turned out to be from a previously unknown pulsar, PSR J2251-3711, whose spin period of 12.1 seconds was the longest known at the time of discovery. An analysis of the unusual properties of this source is presented in Chapter 3. The idea that the FFA could fill the sensitivity gap of the standard method in the long-period regime is not new. In Chapter 4 we demonstrate analytically an even stronger result: the FFA is the most sensitive method to all periodic signals. Furthermore, we show that the sensitivity of some past surveys, an important ingredient to models of the Galactic pulsar population, has been overestimated; the population might thus be larger than previously thought. We also release our FFA search pipeline implementation, which is shown to be fast enough to process the data generated by modern surveys. Lastly, we contributed to building MeerTRAP, a backend for the MeerKAT radio telescope that continuously searches for radio transients and pulsars, with a wide field of view and sensitivity unmatched in the Southern hemisphere. The project is ambitious with many technical challenges; our contribution consisted of developing the high-level control system software for the MeerTRAP processing cluster, which is the subject of Chapter 5. The fast radio transient search pipeline is now functional, and once finished, a search for periodic signals will be added, eventually providing the FFA with an ideal hunting ground.

In this thesis, I study the evolution of low-mass (around 2 M⊙) solar-metallicity stars including the effect of rotation and magnetic fields. These stars produce a significant amount of elements heavier than iron via the so-called s process and thus have a large impact on galactic chemical evolution. In the last decade, researchers have been able to obtain rotational properties from asteroseismic observations of stars. These observations cannot be reproduced by current stellar evolution models. It is now generally accepted that a process of transport of angular momentum is missing from the current implementations of rotation in stellar evolution models. The aim of the thesis is to explore the impact of rotation on the evolution and nucleosynthesis of low-mass stars, and to use the asteroseismic and s-process nucleosynthesis observations as constraints. To do so, I calculated rotating and non rotating models, with and without the Tayler-Spruit dynamo. To constrain the missing process of angular momentum, I included an additional, artificial viscosity to models. The main findings are the following. I determined the amount of additional viscosity needed for the cores within my stellar evolution models to rotate within the asteroseismically constrained rotation rates of core helium burning stars and white dwarfs. The value I had to use for such viscosity is ?add =106-107 cm2 s-1, several orders of magnitude higher than the value found to match observations for lower mass stars. I then calculated for the first time the s-process nucleosynthesis of stellar evolution models that match these constraints on rotation rates. I concluded that the effect of rotation on the s-process production of low mass AGB stars is negligible, which is in agreement with s process observations. I also placed constraints on the mixing of chemical elements by the missing process of angular momentum, and I have listed future work involving magnetic dynamos.

In an attempt to reveal whether any coevolution between a central SMBH and its host galaxy exists, the literature has sought to identify whether the growth rates of the two are connected (witnessed as Active Galactic Nuclei, AGN, and star-formation respectively). However, depending on the sample selection method, there appears to be a contradiction in the results, with those studies selecting a galaxy sample reporting a positive correlation and those studies selecting an AGN sample reporting a at relationship. In order to include non-detections in the analysis, the majority of these studies resort to a binning-and-averaging approach and thus investigate how the average star-formation changes across AGN power bins, or vica versa. However, binning and averaging both have limitations. In this thesis, we conduct a detailed statistical analysis of the relationship between the SMBH accretion rate and the star formation rate (SFR) of the host galaxy. We firstly investigate how the full distribution of SMBH accretion rates changes between galaxies with excess star formation (i.e., starburst galaxies) and those with lower levels of star formation. Secondly, we investigate how the full distribution of star-forming properties changes between high and low power AGNs, before moving on to present a binning-free methodology to investigate how the same distribution changes continuously with AGN power. Therefore, conducting analyses that moves beyond the binning-and-averaging approach. In general, we �nd a statistical connection between SMBH accretion rate and SFR likely exists such that more rapidly accreting SMBHs tend to reside in galaxies with higher levels of star formation. We propose that this scenario is consistent with a proposed coevolution of SMBH and galaxy growth in that they are both cogoverned by cold molecular gas availability in the host galaxy.

This thesis consists of two parts. In the rst part we examine pole-skipping, a phenomenon observed in thermal Green's functions in quantum eld theories with gravity duals. We begin by analysing the near horizon behaviour of bosonic elds in asymptotically Anti-de Sitter spacetimes before presenting the detailed analysis of fermionic elds in such backgrounds. We nd that at negative imaginary Matsubara frequencies and special values of the wavenumber, there are multiple solutions to the bulk equations of motion that are ingoing at the horizon and thus the boundary Green's function is not uniquely de ned. At these points in Fourier space a line of poles and a line of zeros of the correlator intersect and we derive the generic form of the Green's function near such locations. We then consider explicit examples where the correlator is known explicitly and also discuss the special case of a fermion with half-integer mass in the BTZ background. In the second part we study the microscopic degrees of freedom of a particular black hole through the lens of the fuzzball proposal. In particular we construct a new class of smooth horizonless microstate geometries of the supersymmetric D1-D5-P black hole in type IIB supergravity. We rst work in the AdS3 S3 decoupling limit and use the fermionic symmetries of the theory to generate new momentum carrying perturbations in the bulk that have an explicit CFT dual description. We then use the supergravity equations to calculate the backreaction of these perturbations and nd the full nonlinear solutions both in the asymptotically AdS and asymptotically at case. These new geometries have a simpler structure than the previously known superstrata solutions. We conclude with a discussion and an outlook for possible generalizations of the results.

Core-collapse supernovae (CCSNe) are the explosive endpoints of massive (>8 M_Sun) stars that manifest as a bright optical display that lasts for a period of weeks to several months, leaving behind a stellar-mass black hole or a neutron star corpse. Currently supernova science is in a phase of unprecedented activity and productivity, with a new generation of wide-field survey telescopes that scan the entire night sky every few days. As a result, we have begun to discover huge diversity in the observational and physical explosion properties of supernovae that do not neatly fit into the standard classification scheme derived nearly a century ago. Some supernovae produce relativistic jets and are accompanied by long-duration gamma-ray bursts (LGRBs), and superluminous supernovae (SLSNe) have radiated luminosities that exceed CCSNe by an order of magnitude, suggesting an energetic and poorly understood explosion mechanism. In this thesis, I take advantage of new unbiased supernova samples to provide insights into the explosive endpoints of massive stars through their host galaxy environments. I use public imaging surveys to gather multi-wavelength photometry for a spectroscopically-complete sample of 150 CCSN host galaxies from the All-Sky Automated Survey for Supernovae (ASAS-SN) and fit the resulting spectral energy distributions (SEDs) to derive stellar masses and integrated star formation rates. I find that 33(+ 4)(- 4) per cent of CCSNe take place in dwarf galaxies (stellar mass, M < 10^9 M_Sun) and 2(+ 2)(- 1) per cent in dwarf starburst galaxies (specific star formation rate, sSFR > 10^(-8) yr^(-1)). CCSNe provide a census of all massive star formation. Thus, this new CCSN catalogue is an ideal `control' sample to compare with exotic supernovae (SLSNe and LGRBs) at low redshift. I reanalyse SLSN and LGRB hosts from the literature (out to z< 0.3) in a homogeneous way to compare against the CCSN host sample. I conclude that the relative SLSN-I to CCSN supernova rate is increased in low-mass galaxies and at high specific star formation rates. These parameters are strongly covariant, and it was not possible to securely identify which factor (low mass or high sSFR) was more strongly associated with an enhanced SLSN or LGRB rate. In addition, I present my work to help to build future supernova catalogues with the Zwicky Transient Facility (ZTF), including the Bright Transient and Superluminous Supernova surveys. I have integrated the Liverpool Telescope with ZTF which allows follow-up spectroscopy and photometry to be requested and reduced via an automated reduction and image-subtraction pipeline. These new transient catalogues will be larger, and they will provide better control of systematics, enabling detailed future studies.

Strongly lensed supernovae (gLSNe) are a remarkable phenomenon capable of cutting edge cosmological and astrophysical science. The time delays between the multiple lensed images are a direct and independent probe of the Hubble constant (H₀). As a standardisable candle, Type Ia gLSNe promise to measure H₀ with tightly constrained systematics. This is vital to resolve the ongoing tensions between indirect early-Universe and direct late-Universe measurements of H₀, potentially signalling physics beyond the ∧ cold dark matter (∧CDM) model of cosmology. A 1% measurement of H₀ acquired through gLSNe Ia provides a stringent test of ∧CDM. Additionally, gLSNe discovered before the reappearance of the supernova (SN) explosion in the later lensed images provides a unique window into observing the earliest moments of SNe. Such early observations are critical in constraining the progenitor populations of SNe. This thesis investigates the practicality of using gLSNe to make the above observations. I begin with an introduction to the Hubble constant, supernovae and strong lensing to provide the reader with the context for my research. I then detail the research from Foxley-Marrable et al. (2018), where we discuss the usefulness of gLSNe Ia as a cosmological probe, given the obstacle of stellar microlensing. We show that by considering a sample ∼ 140 gLSNe Ia with asymmetric image configurations, we can measure H0 with systematics constrained at the 0.5% level. I then present research from Foxley-Marrable et al. (2020), where we discuss whether gLSNe can be used to observe the earliest moments of SNe. We predict that Legacy Survey of Space and Time (LSST) will find ∼110 candidate systems per year, with 11.7+29.8 -9.3 days between discovery and the SN reappearance. We argue that whilst this will be a challenging undertaking, with significant investment from the astronomical community, deep observations of gLSNe are capable constraining the progenitor populations of SNe. This thesis demonstratively proves the future power of gLSNe when applied to both astrophysics and cosmology.

Many hot Jupiters have been discovered from the ground and Kepler discovered many exoplanets around fainter stars, but searches push for more smaller planets around bright stars. Transit photometry also relies on knowledge of host star properties, less certain for later spectral types. This thesis aimed to investigate detecting planets smaller than Jupiter around bright, later spectral types and to help better understand M dwarf properties. NGTS aims to discover Neptune-sized exoplanets around K and M dwarfs. In Chapter 3, I investigated the brightest stars, saturated in the standard pipeline. While relatively few, these targets are highly valuable. I investigated whether saturated star fluxes could be recovered by using custom apertures, finding tailored rectangular apertures captured them well. I show the difference in noise between bright unsaturated stars and saturated stars is reduced using rectangular apertures rather than the standard pipeline, and is the same for a number of cases. However, to perform saturated star photometry, NGTS needs to change its operational gain setting, to allow charge to be conserved. Earth-sized habitable zone exoplanets are easier to detect around later spectral types as smaller, cooler stars are more favourable for transit photometry. However, their exoplanets are difficult to find in wide-field transit surveys due to their inherent comparative faintness. For this reason, I conducted a targeted survey of mid-late M dwarfs, presented in Chapter 4. Six relatively bright M dwarfs with spectral types M5−M8 were intensely observed, aiming for 50% phase coverage into their habitable zones. Some late M dwarfs also exhibit high flaring frequencies, so for the highest flaring rate target, I investigated the habitability for any habitable zone planets, finding them likely uninhabitable. Stellar properties are a large source of potential uncertainty in determining exoplanet properties, especially for mid-to-late M dwarfs, where stellar evolutionary models show inconsistency with observed data. Testing these models requires independently determined parameters, which is possible using EBLMs from transit surveys. In Chapter 5, I fit two EBLMs from SuperWASP. I find both secondary stars sit close to the bottom of the main sequence, where there are few other measured stars. However, properties of the higher mass (primary) star can still have large uncertainties, affecting uncertainties for the secondary star, and values vary depending on how primary star mass and radius are determined. Overall, this thesis shows some ongoing areas of interest within transit surveys. I determined how saturated star fluxes in NGTS could be used to search for valuable transiting exoplanets around bright stars. I conducted a survey of relatively bright mid-to-late M dwarfs specifically searching for Earth-sized exoplanets within their habitable zone. While no exoplanets were found, transit injections showed that we were sensitive to these planet sizes around at least some of our targets. I also fitted two low mass eclipsing binaries with the aim of better informing low mass stellar evolutionary models. I find that both secondary stars have potential but increased precision on their primary star properties is still needed.

In this thesis I study compact binary systems containing white dwarfs, looking at both detached systems and those that are accreting. These systems are representative of a diverse set of white dwarf binaries in the Galaxy, from which thermonuclear supernovae and other varieties of exotic stellar object emerge. Following a description of the observational techniques and data sources used, I begin with a study of a sample of white dwarfs that show evidence for excess infrared emission in their spectral energy distributions. The targets formed a subset of a larger sample after removal of systems resolved from the Hubble Space Telescope (HST). The period distribution ranges from 0.141 to 9.16 d and peaks near 0.6 d. The original sample therefore contains two sets of binaries, wide orbits (≈ 100 − 1000 au) and close orbits (. 1 − 10 au), with no systems found in the ≈ 10 − 100 au range. This observational evidence confirms the bimodal distribution predicted by population models and is also similar to results obtained in previous studies. I find no binary periods in the months to years range, supporting the post common envelope evolution scenario. The work on detached systems revealed the remarkable utility of modern wide-field photometric surveys. These are revealing rare and unusual types of white dwarf binary systems previously missed through faintness and inconspicuous photometric behaviour. I look at examples of these in the following three chapters. Chapter 4 presents a study of V1460 Her, a star only identified as an accreting binary following a rare, low amplitude outburst. V1460 Her is revealed as a remarkable system, which hosts a donor star far from thermal equilibrium, the remnant of a phase of rapid mass loss. In addition, its white dwarf shows a strong coherent signal on a period of 39 seconds in HST ultraviolet data. It shares these two aspects with the hitherto unique system AE Aqr which also has a rapidly spinning white dwarf, but, unlike V1460 Her, is propelling accreting matter out of the system altogether. Chapters 5 and 6 present new observations on magnetic CVs. V808 Aur is a polar is undergoing changes in its orbital period as is shown through ten years of photometric observations. I also derive masses for both components in the system and determine the spectral type of the donor. CRTS J215256.1+191658 is a newly discovered 3.9 hr period polar. I present high speed photometry showing the eclipse in both a high and low state and Doppler maps of the emission features revealing the flow of the accretion stream through the system.

Herbig Ae/Be stars are pre-main sequence sources, canonically defined as having masses 2 Msun < M < 10 Msun, which are key to understanding the differences in formation mechanisms between low- and high-mass stars. The study of the general properties of these objects is hampered by the lack of a well-defined, homogeneous sample, and because few and mostly serendipitously discovered sources are known. As a consequence, many open problems involving high-mass star formation suffer from biases and lack of completeness. I study the general properties of the 252 known and proposed Herbig Ae/Be stars with parallaxes from Gaia DR2 at the start of this doctoral thesis. High-mass stars were found to have a much smaller infrared excess and optical variability compared to lower-mass stars, with the break at around 7 Msun. Different or differently acting (dust-) disc-dispersal mechanisms are proposed for this break. The variability indicator developed in this dissertation shows that approximately 25% of all Herbig Ae/Be stars are strongly variable. Evidence is provided to support the hypothesis that this variability is in most cases due to asymmetric dusty disc structures seen edge-on. Using that well-characterised sample of Herbig Ae/Be stars as a training sample for a bespoke machine learning algorithm, a homogeneous and well-defined catalogue of 8470 new pre-main sequence candidates was obtained. In parallel, a catalogue of 693 new classical Be candidates was produced. At least 1361 sources are potentially new Herbig Ae/Be stars according to their position in the Hertzsprung-Russell diagram. This increases the number of known objects of the class by an order of magnitude. In addition, I discuss the results of independent spectroscopic observations conducted for a selection of 145 new Herbig Ae/Be and 14 new classical Be stars. These independent observations further confirm the quality and the accuracy of the classification. I conclude with an analysis of the general properties of the new catalogues that validates the results and conclusions obtained for the set of previously known Herbig Ae/Be stars.

Despite their name, there is clear theoretical evidence that black holes emit radiation. The nature and spectrum of this radiation depends on many features of the black hole and the emitted particles. With the profound implications that black hole evaporation has on our understanding of the marriage of quantum mechanics and general relativity, it is essential to study the details of this radiation in new contexts. In this thesis we examine certain important modifications to the spectrum of radiation from black holes that can occur. Firstly, we study the case that the particles emitted by the black hole are spatially extended, rather than pointlike, and find that the rate of emission is suppressed by a factor which depends sensitively on the size of the particle. We then study the emission of electrically charged particles from black holes from a perspective that elucidates the physical picture of emission; namely, in terms of tunnelling of particles both through the horizon and the external electric field. We finally consider the cosmological consequences of modifying the mass-temperature relation of black holes by supposing that they can radiate into higher dimensions. In particular, we argue that current constraints on the density of black holes in the universe are evaded in certain extra-dimensional theories, in which black holes are substantially colder than their four-dimensional counterparts.

We calculate the dynamical tides raised by a close planetary companion on non-rotating stars of 1 M⊙ and 1:4 M⊙. Using the Henyey method, we solve the fully non-adiabatic equations throughout the star, both for the case of frozen convection and for the case where an approximation for the perturbation to the convective flux is used. The horizontal Lagrangian displacement is found to be 10 to 100 times larger than the equilibrium tide value in a thin region near the surface of the star. This is because non-adiabatic effects dominate in a region that extends from below the outer edge of the convection zone up to the stellar surface, and the equilibrium tide approximation is inconsistent with non-adiabaticity. We derive analytical estimates which give a good approximation to the numerical values of the magnitude of the ratio of the horizontal and radial displacements at the surface. We calculate the conversion of these oscillations into observable spectroscopic and photometric signals. Observables are calculated for some real planetary systems to give specific predictions. Time-dependent line broadening and the radial velocity signal during transit are both investigated as methods to provide further insight into the nature of the stellar oscillations. The photometric signal is predicted to be roughly proportional to the inverse square of the orbital period, P-2, as in the equilibrium tide approximation. However, the radial velocity signal is predicted to be proportional to P-1, and is therefore much larger at long orbital periods than the signal corresponding to the equilibrium tide approximation, which is proportional to P-3. The prospects for detecting these oscillations and the implications for the detection and characterisation of planets are discussed.

Over 200 molecules have been detected in the interstellar medium (ISM) but the formation pathways for most of these molecules remains elusive. It is now believed that a major route of molecular synthesis lies within the processing of interstellar ices formed on microscopic dust grains created in the cycle of star birth and death. However, despite decades of experimental research on molecular synthesis in ice films, the lack of a systematic approach to ISM analogue experiments means that there is still no comprehensive understanding of the influence of discrete experimental parameters on the synthesis of any molecular system under ISM conditions. To demonstrate the advantages of performing a systematic study, this thesis presents a comprehensive analysis of thermal and electron processing of CO₂:NH₃ binary mixtures as a function of mixing ratio. A combination of mid-IR and VUV spectroscopy is used to identify chemical products and explore the morphology within the ice mixture demonstrating the advantages of the complementarity of the two spectroscopic methods. Mid-IR spectroscopy of the thermal processing of CO₂:NH₃ mixtures revealed that the ice structure is ratio dependent and that products formed by electron processing of such ices is dependent on the initial ratio of the ice. VUV spectroscopy revealed a thermal reaction which is not observed from the mid-IR spectroscopic studies and allowed new products to be identified. This thesis therefore establishes the need to adopt a more systematic approach to ISM analogue experiments. Such comprehensive and coherent experimental data will improve the interpretation of the next generation of astrochemical observations whilst simultaneously allowing more accurate models of molecular synthesis in the ISM to be developed which in turn allows for a better understanding of processes involved in star and planet formation.

The atmospheres of Chemically Peculiar A stars provide a fascinating area of research on many levels: their slow rotation, strong magnetic fields and lack of turbulent convection give rise to stratification of different ionic species of the same element through a process of radiative levitation. In particular, this phenomenon has already been seen in the abundance disequilibria of Iron, and rare earth elements such as Neodymium (Nd) and Praseodymium (Pr). The research work described in this thesis involves the development of a semi-automated set of routines coded in the programming language IDL to measure equivalent widths of selected Fe lines, and the application of these routines to perform an abundance survey of Fe i and Fe ii species on the high-resolution spectra of approximately 350 Ap stars collected between 2007 and 2010 on the FEROS Echelle spectrograph housed at the 2.2–m telescope at European Southern Observatory at La Silla, Chile. The results show strong correlations with effective temperature as a consequence of atmospheric strat- ification. The approach is then extended to the ionic pairs of rare earth species Pr ii / Priii and Ndii / Ndiii to investigate whether these abundance disequilibria in the rapidly oscillating Ap stars are a consequence of the relatively cool temperatures of those stars, or rather a signature of pulsation. We believe that the onset of this disequilibrium phenomenon observed in the cooler Ap stars is more likely to be a temperature effect, rather than a signature of pulsation.

In this thesis I present the results of my PhD research into the physics of accretion (infall) and ejection (outflow) of matter in compact objects. Accretion disc winds are formed of hot and ionised material launched by magnetic forces, radiation pressure or thermally from the discs of accretors. They have been discovered in most types of accreting systems including supermassive black holes, classical X-ray binaries and Ultraluminous X-ray sources. Outflows form a ubiquitous and important part of the accretion flow, carrying away a considerable fraction of the originally infalling mass. With velocities as high as 30 per cent of the speed of light, their kinetic energy budget can also be significant and have a strong impact on the accretor surroundings. Ultra-fast outflows from supermassive black holes could contribute to or even drive active galactic nucleus feedback in galaxies. Accretion disc winds can be observed through Doppler-shifted spectral lines in the X-ray part of the electromagnetic spectrum, the wind physical properties can thus be studied with X-ray spectroscopy. In this work, I present the detection of disc winds and I study their physics in a number of accreting systems. I particularly make use of the high-spectral resolution Reflection Grating Spectrometer onboard the XMM-Newton observatory. For efficient data analysis I develop and use systematic automated routines for search of wind signatures in X-ray spectra. The first two chapters of this thesis introduce the theory of accretion and describe different accreting systems, as well as the X-ray observatories and data analysis methods used in this study. The third chapter contains a systematic search for disc winds in a sample of Ultraluminous X-ray sources, powered by super-Eddington accretion onto stellar-mass black holes and neutron stars. In the fourth chapter I achieve the first discovery of an ultra-fast wind in a neutron star Ultraluminous X-ray source. In the following two chapters I present the detection of ultra-fast outflows from two accreting supermassive black holes PG 1448+273 and 1H 0707-495. The outflow energetics show that these winds are more than capable to drive feedback in the accretor host galaxies. Furthermore, both show evidence for a multi-phase wind structure, PG 1448+273 also exhibiting variability in time. In the seventh chapter, I present the discovery of a disc wind in the unique X-ray binary Hercules X-1 known for a precessing, warped accretion disc. I leverage the warped disc precession to sample the vertical distribution of the disc wind, constraining its launching angle as well as the total mass outflow rate, two crucial quantities which are difficult to measure in other accreting systems. The final chapter contains the conclusions of the thesis as well as potential future research avenues in this field and the promising upcoming X-ray observatories.

Good knowledge about cool star magnetic activity, topology and cycles is crucial to find a second solar system and to better understand ours. The Zeeman-Doppler-Imaging (ZDI) surveys, which unveil the stellar magnetic field topology, have now run for long enough to detect solar-like activity cycles. This is a good point to review what ZDI detects robustly and how to interpret the resulting ZDI maps. As ZDI only detects the large-scale magnetic field an important question to answer is: What can we learn from the large-scale field topology about the small-scale field for solar-like stars? I connect 3D non-potential flux transport simulations based on the Sun with the observational ZDI technique. First, I decomposed the magnetic field topology of the simulations into different length-scales. I discovered that the large-scale field reflects global properties of the small-scale field emergence for slowly-rotating solar-like stars. Second, I used synthetic line profiles modelled from the simulations as input for ZDI. I showed that ZDI can recover the hints of the small-scale flux emergence in the observable large-scale field for slow rotators but recovers approximately one order of magnitude lower magnetic energy. The maximum entropy regularisation used in ZDI prevents the correct reconstruction of the magnetic energy distribution but ZDI can recover the fractions of the different field components reasonably well. To examine if ZDI can recover solar-like cycles, I applied ZDI to non-potential flux transport simulations modelling the solar magnetic field over 15 years. I discovered that the axisymmetric poloidal fraction and the axi- and non-axisymmetric energy are the best parameters to track solar-like activity cycles while the averaged large-scale field or the total energy show no or misleading trends.

Among the many mysteries of our Universe, one still unanswered question is how globular clusters form. Globular clusters are very dense agglomerates of hundreds of thousands of stars and they host some of the oldest known stars in our Universe. Since they are luminous, old and found in all massive galaxies, they are a fundamental piece of the puzzle to understand galaxy formation and evolution processes. Traditionally, globular clusters were thought to be simple stellar systems, in which all stars were born at the same time and have the same chemical composition. %Therefore, globular clusters have been considered the perfect laboratory to study how stars evolve. However, in the last few decades, it has been shown that stars within a given globular cluster display inhomogeneities in their chemistry. Every massive old globular cluster located in the Milky Way, for which high precision and deep observations were obtained, was found to host several different stellar populations, i.e. multiple populations. Each stellar population is characterized by specific chemical patterns observed in the atmospheres of individual stars. Only certain elements are found to vary, and they do not do so randomly, but rather the variations are observed to correlate between the elements. The stellar population that has enhanced nitrogen (N) content, also has enhanced sodium and helium abundances but has a depletion in carbon and oxygen, to cite a few examples. At the same time, the iron content is found to be constant among the different populations. Such chemical patterns are often called anomalies. More interestingly, it seems like such chemical anomalies are unique to globular cluster systems, i.e. dense stellar systems, since they are basically not found in other stars located in the field. Knowing how such multiple populations form and how they impact the evolution of globular clusters is crucial to understand the formation of stars and clusters themselves and, more broadly, the formation and evolution of galaxies. Many theoretical scenarios have been proposed to explain the origin of the chemical anomalies in globular clusters. Most models treat the origin of this phenomenon as multiple events of star formation. In such models, a first generation of stars forms from the collapse of a giant molecular cloud which is homogeneous in its chemical composition. The winds of the massive stars from this first generation sink in the centre of the cluster to collapse and provide material for a second generation of stars, which then forms with a different chemical composition. While theoretically straightforward, such scenarios (which involve many types of massive stars) fail in reproducing many of the observed properties of multiple populations in globular clusters. Hence, the formation mechanism for the origin of multiple populations remains an open question. Most studies of multiple populations focused only on ancient globular clusters, aged up to ~13 Gyr. However, many dense and massive younger star clusters are observed in nearby galaxies. Is the multiple populations phenomenon limited to the ancient globular clusters, i.e. could this be a cosmological effect? The goal of this thesis has been expanding the search for multiple populations to star clusters that are significantly younger than the old globular clusters, i.e. up to 10 times younger. Indeed, a compelling line of investigation is to look for multiple populations depending on certain global properties of the clusters, such as age, mass, metallicity. The first major result presented in this work is that multiple populations are found also in the young clusters, down to ~2 Gyr old objects, showing that the phenomenon of multiple populations is not only restricted to the early Universe. Another interesting result I report is that the extent of the multiple populations (in chemical abundance spread) is a strong function of age, with older clusters having larger chemical variations. Additionally, I show that there is no difference in age between the populations in a young star cluster. Such results represent fundamental constraints for the origin of multiple populations and might point towards a new and fresh direction into the onset of this complex phenomenon. An important and related question is whether the young massive star clusters are the same type of stellar systems as the ancient globular clusters, just observed at a different stage of their lifetimes. If confirmed, this could provide important constraints on star cluster formation studies. Therefore, in this thesis I explored clusters at younger ages in order to address the fundamental question whether the star (and cluster) formation conditions were different in the early Universe. The results presented here represent an important hint that ancient and young clusters share the same origin and are only separated in age. I show that star clusters do not require special conditions in which to form, so that they can be used as tracers for the formation and evolution of galaxies.

This thesis comprises the dynamical study of star clusters in the Milky Way and Magellanic Clouds from early to ancient times. Much is unknown about the formation of high-mass stars and clusters — our understanding is deeply hindered by the obscuration of stars by thick columns of dust and gas. One can infer the motions of stars in star-forming regions, however, via radio observations of ionised gas. By way of example, I examine a young, bipolar H II region in the Galactic disc which lies at the centre of a massive (∼ 10³ M⊙) infrared-dark cloud filament. Intriguingly, the region known as G316.81– 0.06, displays a large velocity gradient (47.81 ± 3.21 km s−1 pc−1) along the same direction as the filament — a phenomenon scarcely observed at this stage of evolution. Based on a qualitative comparison between G316.81–0.06 and simulations of young star-forming regions, the velocity gradient can be explained by rotation, inferred to be a direct result of the initial angular momentum of the natal molecular cloud. If true, this kinematic signature should be common in other young (bipolar) H II regions and may help to discern the scenario by which star clusters form and evolve. Star clusters at ancient times (i.e. globulars) appear in an entirely different form. Rid of their natal gaseous cocoons, globulars visibly contain 10⁵ − 10⁶ stars, held together by their mutual gravity. One particular conundrum appeared in recent decades: observed mass-to-light ratios (M/L) of metal-rich globular clusters (GCs) disagree with theoretical predictions. This discrepancy is of fundamental importance since stellar population models provide the stellar masses that underpin most of extragalactic astronomy, near and far. Using integral-field unit data from the WAGGS project, I have extracted radial velocities for 1,622 stars located in the centres of 59 Milky Way GCs — twelve of which have no previous kinematic information — in order to calculate dynamical masses and M/L_V ratios via N-body modelling. Most importantly, the sample includes NGC 6528 and NGC 6553, which extend the metallicity range of GCs with measured M/L up to [Fe/H] ∼ −0.1 dex. The results confirm that metal-rich clusters have M/L_V more than two times lower than what is predicted by simple stellar population models, and thus the discrepant M/L– [Fe/H] relation remains a serious concern. I have explored the potential origin of the divergence, and it appears that dynamical effects are the most likely explanation. With great technological advances in recent years, the internal kinematics of more distant star clusters can also be probed, such as massive star clusters in the Magellanic Clouds. These clusters are as young as ∼ 1 Myr and are thought to be the progenitors of ancient globulars. Thus, this provides a unique opportunity for the study of globular formation at a relatively unexplored snapshot in time. I have carried out a preliminary study of eleven (young, intermediate-age and old) massive clusters in the Clouds as an extension of the M/L–[Fe/H] study of GCs. With this, I can then test stellar population models and improve constraints on theories of dynamical evolution at early times. Newly discovered Gaia star clusters present another avenue for novel research. Home to a new area of parameter space, these clusters appear to be old and compact, yet they are faint (V-band magnitude < −2.5 mag). This is an exciting opportunity to advance our knowledge of (heavily dissolved) star clusters which seem to be approaching the end of their lifetime.

Cepheid variable stars have long been used as distance indicators due to their strong period-luminosity relation. However, the period of a Cepheid is affected by its metallicity so any difference in metal-content between calibration Cepheids and Cepheids being used for distance measure would lead to a systematic error in the distance calculated. Metallicity measurements are traditionally achieved by spectroscopic analysis but this can lead to considerable uncertainties. Therefore, the first part of this thesis looks at using double-mode, or beat, Cepheids to measure metallicity. Specifically, to measure the metallicity gradient across the Triangulum galaxy, M33. Beat Cepheids can be used to trace metallicity because their period ratio is sensitive to metal abundance which is well described by pulsation models. To find variable stars, image subtraction techniques are applied to observations of two separate datasets of M33. PSF fitting photometry is carried out on the data to produce calibrated light curves. The is also done to cross-calibrate the two datasets but an amplitude discrepancy arises in many of the Cepheid light curves. This amplitude problem is not caused by mismatching of the stars between the datasets, via either pixel matching or WCS coordinate transform. Nor is there any correlation with the Cepheid position in M33. Further investigation, beyond the scope of this thesis, into this issue is required. The amplitudes are corrected using a scaling factor so that an Analysis of Variance routine can be applied on the light curves to find Cepheid periods. Cepheids are only kept if the ratio between their two strongest periods lies within an appropriate range. 3 beat Cepheids are found, on top of another 5 already known beat Cepheids, in M33. The metallicities of the beat Cepheids are determined by comparing the period ratios with beat Cepheids of known metallicity in the Milky Way and Magellanic Clouds. The galactocentric distances of the stars are determined by deprojecting their celestial coordinates with M33's inclination and position angles along with the distance. Therefore, the metallicity of M33 as a function of radius can be immediately obtained, yielding the metallicity gradient across the galaxy. Using this method the metallicity gradient of M33 is found to be steeper than measurements made by recent spectroscopic analysis of HII regions. This is more in line with what is expected from recent work deriving the Cepheid Period-Luminosity relation for M33. There exists a period-age relation for Cepheid variable stars. The second part of the thesis aims to derive this empirically using observations of 6 stellar clusters in the Large Magellanic Cloud. The age for each cluster is taken from literature and were determined by Isochrone fitting. The same image subtraction techniques as used on the M33 data are used to find Cepheids and their periods in the LMC clusters. Cepheids are only considered to be cluster members if they fall within the half-light radius of the cluster and have proper motions matching their host cluster. The mean periods of the Cepheids in each cluster along with the cluster ages is then used to derive the period-age relation. The period-age relation derived here shows a steeper gradient than those produced by models or previous empirical derivations. There is a large spread in periods of the Cepheids in any given cluster that increases inversely with cluster age. This effect cannot be replicated if the cluster's population is recreated with a single stellar model. However, the period spread can be described using stellar models of multiple initial rotation rates.

In this Thesis, I present a search for eclipsing companions to white dwarf stars, with the aim of studying and more accurately constraining the properties of evolved planetary systems. I first undertake an independent, ground-based survey of metal-rich white dwarfs with similar physical properties to WD1145+017, the first discovered white dwarf hosting a disrupting planetesimal. While no transit-like features are detected in the survey, photometric variability is discovered in two objects. I investigate the nature of such variability in both objects. At the time of writing this work, I am awaiting spectroscopic data for one of the targets in order to search for evidence of an unseen companion. The second object reveals no radial velocity variations, thus the most likely source of variability is determined to be accretion events from the surrounding environment. Moreover, I take part in analysing the white dwarf sample in Campaign 14 of the Kepler K2 mission, sampling 368 white dwarfs brighter than 20 mag. In the sample, I ?nd a brown dwarf totally eclipsing a white dwarf with a period of ~133 minutes. Only two other eclipsing white dwarf + brown dwarf systems were known previous to this discovery. Finally, I present the discovery of two post common-envelope binary systems in the Next Generation Transit Survey. The ?rst system consists of an M4 star orbiting a cool (~7500 K) white dwarf with a period of ~13.85 hours. Only one other system similar to this had been previously discovered. The second binary is a younger, hotter system, in which an ~18,000 K white dwarf is eclipsed by a late-K or early-M main sequence companion.

This thesis contains a study of the circumstellar discs around intermediate mass stars. Through observational data and protoplanetary disc modelling, the physical structure and composition of protoplanetary discs are investigated with regards to their capacity for planet formation. In-depth analysis of millimetre-wavelength interferometric observations are carried out on the circumstellar environment of two Herbig stars. Firstly, the distribution of gas and dust in the gas-rich, potentially planet-hosting disc of HD100546 is characterised. Using ALMA observations of 1.3mm continuum and CO isotopologues tracing the disc midplane, estimates of disc mass are calculated, constraints on the size of dust grains inferred and evidence for midplane counterparts to scattered light features are identified. Secondly, an analysis of the more evolved circumstellar disc around Herbig star HD141569 is made in order to investigate the mass content of the disc and inform the debate as to its evolutionary stage. New ALMA observations presented in this thesis and new midplane structures in the gas and dust that support an intermediary stage of evolution between the protoplanetary disc and debris disc regimes. Finally, modelling of the pre-main sequence evolution of stars across the stellar mass range at which exoplanet detections peak is combined with Monte Carlo radiative transfer and modelling of the evolution of midplane gas and dust in order to study the impact of stellar evolution on the midplanes of protoplanetary discs. Variations in midplane temperature profles result in different locations of key snowlines in the disc, which in turn produces variations in the molecular composition of the local disc. The results quantify how snowline locations depend on stellar luminosity evolution. This modelling procedure is applied to the system of HR8799 in order to put constraints on the time and location within the disc at which wide-orbit planets could have formed based on their atmospheric C/O ratio. The results support an early formation time, within around 1 Myr, for the carbon-rich exoplanet HR8799b.

Pulsars are named for the periodic pulse of electromagnetic energy we detect from point sources in the sky and are considered exceptionally good, but not perfect, cosmic clocks. We use sensitive radio telescopes to measure pulse time-of-arrivals (ToAs) and construct pulsar timing models containing rotational, astrometric and stellar binary system parameters. Pulsar timing compares measured ToAs with the predictions from the model and analysing the differences can provide insights into a wide range of astrophysical phenomena. The key instrument in pulsar observations is the Pulsar Timing System (PTS) and in this thesis we describe our development of two new pulsar timing systems. The first is a robust, low cost, next generation system using consumer grade GPUs as the compute engine and we have instruments installed in Ghana and at Jodrell Bank Observatory in the UK. We consider the apparent relationship between pulsar characteristic age and spectral index and we seek to make pulsar flux density measurements at 5GHz and 6.7GHz (in C-band) using the AVN radio telescope at Kuntunse in Ghana to explore this relationship further. We also consider searching for pulsars in the Galactic centre region where the potential benefits of using pulsars to probe the region and SgrA* would be very significant. Our second pulsar timing system combines the observations from the six medium sized, geographically dispersed, e-MERLIN telescopes into a single incoherent beam providing increased sensitivity to pulsar signals. We are making pulsar timing observations of eight pulsars that exhibit transient behaviour in the form of glitches (a sudden increase in the rate of rotation) or mode-switching (changes to the pulse profile). The e-MERLIN facility now offers a 'Pulsar LOFT-e Mode' as an option for external researchers based on our work on this PTS.

Pulsar timing, which tracks the rotational phase of pulsars over many years, has established pulsars as precise celestial clocks and enabled the tests of theories of gravity and the search for nano-Hz gravitational waves. For these applications, a higher timing precision can put a tighter bound on alternative theories of gravity or even detect the gravitational waves. The timing precision is expected to increase when a new generation of telescopes is rolling out, while an inspection on the method itself may further improve the precision. This thesis aims to improve the precision of pulsar timing arrays by inspecting the existing timing models, the correlations due to red noise, and the subtle variations of the pulsars per se. Firstly, the precision of the timing models in the Solar system is discussed, using the widely used software package 'tempo2' as an example. 'tempo2' was designed to achieve a precision of 1~ns over 20 years, but some components, especially wet delay and ionospheric delay, could induce errors above this level. The current atmospheric delay does not include the wet delay and could lead to an elevation-dependent error up to about 7~ns. Besides that, the existing atmospheric delay is also prone to errors around the horizon. Due to its strong dependence on the elevation and meteorological conditions, one may consider real-time observations of atmospheric delay via other means, like the global positioning systems. Also, there is no modelling of ionospheric delay although errors up to 100~ns could be induced, while the time delay of Solar wind is oversimplified and can lead to huge inaccuracy when the line-of-sight is close to the Sun. Secondly, the origin and detectability of the spin frequency second derivative are discussed. A comprehensive survey on the kinematic and dynamical origins was conducted for the pulsars in the International Pulsar Timing Array. It found that the contribution related to the radial velocity is dominant in most cases with a magnitude of 10⁻³⁰-10⁻²⁹~s⁻³. The measurement error of the spin frequency second derivative heavily depends on the noise in the timing residuals as well as the time span ($T$) of the data. In the case of white noise, the error decreases as T⁻⁷/², while if correlated noise exists, the error is proportional to T⁻³.⁵⁺⁰.⁵α for power-law red noise with a power index of α\le4. In addition to the power-law model, the possibility of using a squared exponential kernel to model the red noise was also discussed. Finally, I investigated potential shape variations of the pulse profile of PSR~B1937+21, using 637 observations spanning nearly 6 years. In this study, a Gaussian process was used to infer the shape variations between consecutive observations. Systematic variations with an amplitude of 1 per cent of the main peak were found near the shoulder of the main pulse and 0.5 per cent for those near the peak of the interpulse. The reason for such variations is not clear, but both polarisation impurities in the receiver and intrinsic shape changes are possible.

The present thesis reveals a novel connection of Diophantine approximation arising from small divisors to general relativity, more precisely, the Strong Cosmic Censorship conjecture. The main results provide theorems which resolve a linear scalar analog of the Strong Cosmic Censorship conjecture in general relativity for Λ < 0. The proofs are intimately tied to small divisors and the resolution crucially depends on suitable Diophantine conditions. A further ingredient is the novel scattering theory on black hole interiors established at first. The thesis consists of three parts. In the first part we develop a scattering theory for the linear wave equation on the interior of Reissner–Nordström black holes. The main result shows the existence, uniqueness and asymptotic completeness of finite energy scattering states on the interior of Reissner–Nordström. The past and future scattering states are represented as suitable traces of the solution ψ on the bifurcate event and Cauchy horizons. Finally, we prove that, in contrast to the above, on the Reissner–Nordström–(Anti-)de Sitter interior, there is no analogous finite T energy scattering theory for either the linear wave equation or the Klein–Gordon equation with conformal mass. This part is joint work with Yakov Shlapentokh-Rothman (Princeton University). The second and third parts are motivated by the Strong Cosmic Censorship Conjecture for asymptotically AdS spacetimes. We consider smooth linear perturbations governed by the conformal wave equation on Reissner–Nordström–AdS and Kerr–AdS black holes, respectively. We prescribe initial data on a spacelike hypersurface of a Reissner–Nordström–AdS and Kerr–AdS black hole and impose Dirichlet (reflecting) boundary conditions at infinity. It was known previously by work of Holzegel–Smulevici that such waves only decay at a sharp logarithmic rate (in contrast to the polynomial rate in the asymptotically flat regime) in the black hole exterior. In view of this slow decay, the question of uniform boundedness or blow-up at the Cauchy horizon in the black hole interior (and thus the validity of the linear scalar analog of the C0-formulation of the Strong Cosmic Censorship conjecture) has remained up to now open. In the second part of the thesis, we answer the question of uniform boundedness in the affirmative for Reissner–Nordström–AdS: We show that |ψ| ≤ C in the black hole interior. In this setting, this corresponds to the statement that the linear scalar analog of the C0-formulation of Strong Cosmic Censorship is false. The proof follows a new approach, combining physical space estimates with Fourier based estimates exploited in the scattering theory developed in the first part. In the third part of the thesis, we show that |ψ| → ∞ at the Cauchy horizon of Kerr–AdS if the dimensionless black hole parameters mass and angular momentum satisfy certain Diophantine properties. This is in stark contrast to the second part as well as previous works on Strong Cosmic Censorship for Λ ≥ 0. In particular, as a result of the Diophantine conditions, we show that these resonant black hole parameters form a Baire-generic but Lebesgue-exceptional subset of parameters below the Hawking–Reall bound. On the other hand, we conjecture that, as is the case for Reissner–Nordström–AdS, linear waves remain bounded at the Cauchy horizon |ψ| ≤ C for a set of black hole parameters which is Baire-exceptional but Lebesgue-generic. This means that the answer to the above question concerning uniform boundedness or blow-up on the Kerr–AdS interior is either negative or affirmative depending on the parameters considered. Thus, in this setting, the validity of the linear scalar analog of the C0 -formulation of Strong Cosmic Censorship depends in an unexpected way on the notion of genericity imposed.

Stellar rotation, convection, and magnetism are intricately linked in low-mass stars like the Sun. In their outer convective envelopes, the interplay of rotation and convection form a magnetic dynamo capable of sustaining both large and small scale magnetic fields. The strength of these magnetic fields are observed to grow with increasing rotation rate. The coronae of low-mass stars are heated by these magnetic fields (the exact mechanism of which remains under debate), such that the thermal pressure drives a quasi-steady outflow of plasma, referred to as a stellar wind. Due to the interaction of the large-scale magnetic field with the outflowing plasma, stellar winds are able to efficiently remove angular momentum from these stars. Therefore, the evolution of rotation for low-mass stars (on the the main sequence) is governed by their stellar winds, and by interrelation, the evolution of their magnetic activity and stellar wind output. In this thesis I attempt to better constrain the angular momentum-loss rates of the Sun and other Sun-like stars through the use of magnetohydrodynamic simulations combined with a broad range of observations. Though I do not find a concrete value for the solar case, I reduce the uncertainty in its value to within a factor of a few by locating key factors/quantities which limit our predictions, and further highlight the importance of understanding the solar angular momentum-loss rate in an astrophysical context. For the other Sun-like stars, I find the simulation results largely under-predict the angular momentum-loss rates implied by current rotation-evolution models. The reason(s) for this are uncertain, but likely involve uncertainties in both the observed magnetic field strengths and mass-loss rates of these stars, along with the under-prediction of how much of the surface magnetic field is ``opened'' by the stellar wind.

Hot subdwarf B (sdB) stars are evolved core He-burning stars. The sdBs are formed by binary interactions on the red giant branch (RGB) which cause the stars to lose most of their H envelopes. Over half of all observed hot subdwarf B stars are found in binaries, many of which are found in close configurations with orbital periods of $10\,$d or less. These short period systems are formed by common envelope evolution. In order to estimate the companion masses in these predominantly single-lined systems, tidal locking has frequently been assumed for sdB binaries with periods less than half a day. Observed non-synchronicity of a number of close sdB binaries challenges that assumption and hence provides an ideal testbed for tidal theory. The stars have convective cores and radiative envelopes. Tidal dissipation in such systems is not particularly well understood. We solve the second-order differential equations for detailed 1D stellar models of sdB stars to obtain the tidal dissipation strength and hence to estimate the tidal synchronization time-scale owing to Zahn's dynamical tide and the equilibrium tide. The results indicate synchronization time-scales longer than the sdB lifetime in all observed cases using standard input physics. Asteroseismological analysis of NY Vir suggests that at least the outer 55 per cent of the star (in radius) rotates as a solid body and is tidally synchronized to the orbit. Detailed calculation of tidal dissipation rates in NY Vir fails to account for this synchronization. Recent observations of He core burning stars suggest that the extent of the convective core may be substantially larger than that predicted with theoretical models. We conduct a parametric investigation of sdB models generated with the Cambridge STARS code to artificially extend the radial extent of the convective core. These models with extended cores still fail to account for the synchronization. Tidal synchronization may be achievable with a non-MLT treatment of convection. Several sdB stars have been both predicted and observed to pulsate with multiple frequencies. Asteroseismological analysis of the observed pulsations shows that they do not quite fit with the theoretical models, especially in the close binary systems. We present a method for computing tidal distortion and associated frequency shifts. Validation is by application to polytropes and comparison with previous work. For typical sdB stars, a tidal distortion of less than 1% is obtained for orbital periods greater than 0.1\,d. Application to numerical helium core-burning stars identifies the period and mass-ratio domain where tidal frequency shifts become significant and quantifies those shifts in terms of binary properties and pulsation modes. Tidal shifts disrupt the symmetric form of rotationally split multiplets by introducing an asymmetric offset to modes. Tides do not affect the total spread of a rotationally split mode unless the stars are rotating sufficiently slowly that the rotational splitting is smaller than the tidal splitting.

Diffeomorphisms in general relativity can act non-trivially on the boundary of spacetimes. Of particular importance are the 'BMS group' of transformations, originally found by Bondi, Metzner, van der Burg and Sachs. They describe the symmetries of asymptotically flat spacetimes. We first consider symmetries such as these that act non-trivially at null infinity. We look in detail at the BMS group and then describe the conformal symmetries of asymptotically flat spacetime. These represent an extension of the BMS group that we call the 'conformal BMS group'. Second, we explore the action of these large diffeomorphisms in the context of black holes. The emergence of Virasoro algebras as the asymptotic symmetry algebras of black hole spacetimes suggests a fundamental link to conformal field theory. For the case of the generic Kerr black hole, we hypothesize that the black hole is itself a thermal conformal field theory which transforms under a Virasoro action. A set of infinitesimal 'Virasoro_L ⊗ Virasoro_R' diffeomorphisms are presented which act non-trivially on the horizon. Using the covariant phase space formalism, we can construct the corresponding surface charges on the black hole horizon and find the central terms in their algebras. Ambiguities in the construction of the charges allow for the addition of extra terms. Wald and Zoupas have provided a general framework for these counterterms, although the precise form is left undetermined. In computing the horizon charges, certain obstructions to the integrability and associativity of the charge algebra arise, calling for some counterterm to be used. A consistent counterterm is found that removes these obstructions and gives rise to central charges c_L=c_R=12J. On the assumption that there exists a quantum Hilbert space on which these charges generate the symmetries, one can use the Cardy formula to compute the entropy of the conformal field theory. This Cardy entropy turns out to be exactly equal to the Bekenstein-Hawking entropy, providing a potential microscopic interpretation for this macroscopic area-entropy law. The results are generalised to the Kerr-Newman black hole with the addition of charge.

Observations were made of the massive stellar cluster Westerlund 1 (Wd1) using the Australia Telescope Compact Array (ATCA), with a total on source integration time of ~ 16 hours, collected over the autumn and winter of 2015, with follow up observations carried out in the summer of 2016. This thesis presents a full radio census of the massive stellar cluster. This thesis aims to investigate in full the dataset of radio observations, looking at the radio emission detected from the most massive stars in the cluster, as well as the majority of the post-main sequence massive stars present, including several Wolf-Rayet stars, yellow hypergiants, red supergiants, and a luminous blue variable. A full description outlining the data reduction process and the analysis carried out showcases current methods for handling radio interferometric data from ATCA when considering a crowded field full of extended and complex sources. The scientific results gained provide a follow-up to previous ATCA observations taken in the late 1990s and early 2000s. The more recent observations that are the focus of this thesis demonstrate an increase in both the sensitivity and resolution possible with the use of ATCA. New radio detections are made for ten members of the known stellar population of Wd1. The radio observations could be compared to complementary observations at millimetre wavelengths to determine spectral indices and index limits, which allowed for the nature of the radio emission to be constrained. Conclusions could be made from the spectral indices and index limits about the potential binarity of the stellar sources. For all stars where the radio emission could be validly approximated to a thermally emitting stellar wind, updated constraints could be found for the mass-loss rates of these sources. Comparisons between radio and millimetre wavebands allowed for a consideration of clumping factors.

Emission from accreting black holes of all sizes is often highly variable, and strongly dependent on the energy band of observation. Particularly at accretion rates close to a few percent of the Eddington rate, multiple components become important in constructing the energy spectrum, and the signals can vary greatly on short timescales. For both X-ray binaries and Active Galactic Nuclei (AGN), the accretion geometry at low luminosities has become controversial, as spectral contributions from the thermal disc, relativistic reflection and Comptonization continua are often highly degenerate, with instrumental effects throwing doubt on the strongest pieces of evidence. However, with the latest generation of instruments it has become possible to analyse both the time-variability of distinct energy bands, and the correlations between these bands, combining information from the orthogonal energy and time domains. For the X-ray binaries, the Rossi X-ray Timing Explorer (RXTE) and XMM-Newton have facilitated observations at high energy resolution down to the millisecond timescale - close to the dynamical period in these sources. Meanwhile for Active Galactic Nuclei, a series of high-cadence, long timescale campaigns using the Hubble Space Telescope, the Neil Gehrels Swift Observatory and a multitude of ground-based telescopes has produced data on the broadband continuum shape of a growing sample of accreting sources with unprecedented timing quality. In this thesis I will present modeling of these spectral-timing data, with a view to constraining the physical mechanisms and size scales of the accretion regimes in both the X-ray binaries and AGN. By jointly modeling the energetic and timing aspects of the data, the models in this thesis will aim to provide a more self-consistent picture of the accretion structure at low luminosities, and rule out those pictures which cannot simultaneously explain the spectral and temporal information.

The aim of this research project has been to search for disrupted or disintegrating exoplanets; this search has been carried out using archival data from the Super-WASP (Wide Angle Search for Planets) transit photometry detection programme. The main aim of the archival search was to look for objects similar to Kepler-1520b (Rappaport et al., 2012) and Boyajian’s star (Boyajian et al., 2016), which were both discovered using the transit method of detection and both show very unusual signals when compared to a typical transiting exoplanet. The transit signal of Kepler-1520b appears to change in depth between transits, where transits have a depth between 0% and 1.3% of the flux. Boyajian’s star shows highly chaotic behaviour in the lightcurve with no distinct transit period and no consistent transit depth. I develop bespoke routines to search the SuperWASP archive, based on the characteristics of Kepler-1520b and Boyajian’s star, to find transits that vary in depth. From these searches, objects with the largest transit depth variability were highlighted for possible follow up. The South African Large Telescope (SALT) was used to observe the spectra of four of the most interesting objects highlighted from the archive searches. These spectral observations provide stellar characteristics of the system and the subsequent WASP lightcurve analysis led to the discovery of two possible tidally disrupted transiting objects, which have been named Tidally Disrupted Transitors (TDTs) in this thesis. These objects appear to show large transit signals of up to 68% of the flux occurring frequently and in a distinct pattern for between 1-2 months. The orbital configuration of these objects also seems to suggest that they may have originated from outside the current host star’s system.

This thesis will address the problem of measuring stellar radii, which is ubiquitous across many fields of modern astrophysics. A technique is introduced which integrates the area beneath the stellar spectral energy distribution (SED) of a star to measure its luminosity, and the shape of the SED to measure its temperature - from which follows its radius. This method addresses many of the problems facing of existing methods, which are reviewed, as it provides accurate measurements of stellar radius using only multiband photometry and precision parallaxes. It is well known that the radii and temperatures of M-dwarf prescribed by models are in disagreement with observations, both on the pre-main-sequence (pre-MS) and the main-sequence (MS). This methodology is applied to pre-MS M-dwarfs in the Pleiades and Praesepe clusters to perform a direct comparison to the radii predicted by stellar interiors. Assessment of the physicality and accuracy of the stellar atmosphere models is also performed by comparing synthetic spectra generated from them to flux--calibrated spectroscopic observations. The parameters for the synthetic spectra are provided by the SED fitting, allowing verification of the methodology itself to be performed. The advent of Gaia DR2 means that reliable distances are now available for field M-dwarfs, permitting the extension of this investigation to MS stars. Through this investigation, the nature of radius inflation in MS M-dwarfs is studied as a function of mass. This crucially allows insight into the physics behind the observed radius inflation, allowing current theories underpinning radius inflation to be critically assessed. The conclusion of this investigation is that magnetic models are currently unable to explain radius inflation in M-dwarfs. Given the successful application of the SED fitting methodology in measuring the stellar radii of miscellaneous field stars, this work is built upon to address the problem of determining the stellar parameters of exoplanet host radii. In doing so, it is demonstrated that the SED fitting technique extends well to the mass range of stars currently being scrutinised to discover and characterise exoplanets. Given its wide applicability for exoplanet host characterisation, the potential systematic errors that may prove problematic are reviewed and methods for their mitigation are suggested.

One of the main issues in astrophysics is to understand how galaxies form and evolve. Deep photometric studies help the investigation of the evolution of resolved stellar contents of nearby systems. Hence the properties of these regions represent an archaeological record of the processes that shape a galaxy over cosmic time. So one can interpret from the star formation history the evolution of the star formation rate throughout the galaxy and the evolution of the mass and metallicity distributions. The system that has been studied in this project is the nearby galaxy M33, located in the Local Group. The photometric data was taken in the Canada-France-Hawaii Telescope with the MegaPrime/MegaCam wide-field mosaic imager and it is available for the filters g’, r’ and i’. The data analysis is presented in this work with the purpose of recovering its star formation history. Over one million point sources were identified in each filter. The program chosen for this process is DAOPHOT (Stetson, 1987). PSF-fitting photometry was performed using a few hundreds of point sources, selected from non-crowed areas, to fit the point-spread functions. This process, however, was repeated a couple of times in order to get a well adjusted point-spread function with the least residuals possible. The instrumental magnitude was then determined. A selection cut enabled spurious sources to be discarded based on the photometric errors (σ), residuals scatter (Χ2) and image quality (sharpness). Aperture and offset corrections were applied in the magnitudes before the transformation to the standard photometric system. A completeness test to examine the effects of crowding in the images was conducted in each photometric filter. The bias in the observed magnitudes and in the stellar counts due to high stellar density affects the final star formation history, resulting in the miss-assumption of the stellar age, metallicity and initial mass function. The artificial stars test (Williams et al., 2009) is a standard technique used to that end and consists of inserting synthetic stars in the images, with the routine ADDSTAR (Stetson, 1987), and performing again the photometric reductions in those synthetic images to compare the known inserted brightness with the recovered ones. The completeness is given by the ratio of the number of retrieved artificial stars over the number of added ones. Stars of all evolutionary stages lose mass and the mass recycled in the interstellar medium will be part of the next generation of stars and planets. The study of mass loss is quite well understood for metal-rich stars populating the asymptotic giant branch, though there is still a lot to be understood about the metal-poor stars losing mass during the red giant phase. The understanding of the mass loss process that happens in red giant stars of globular clusters might help us to better estimate the post-main sequence stellar evolutionary stages and the intra-cluster gas enrichment. Since the 70’s it has been known that the Galactic globular cluster ω Centauri shows an extremely complex stellar chemistry, with a wide variation in metallicity, [Fe/H] ≈ -2 to [Fe/H] ≈ -0.6, and light elements (like He, C, N...). Indeed, the properties of ω Cen favours the hypothesis that this is a remnant of a dwarf galaxy orbiting the Milky Way and tidal interactions partially disrupted it. With observations from the Infra-Red Array Camera aboard of the Spitzer telescope, investigations on red giant stars in ω Centauri are carried out to identify infrared colour excess originating from the emission of a circumstellar envelope surrounding the stars (e.g. Frogel & Elias, 1988; Origlia et al., 1996). This study is based on a proper combination of ground-based and original Spitzer photometric data as well as results from previous spectroscopic surveys. Prior to the selection of the dust excess stars, the magnitudes from the SDSS photometric system are converted to the TCS system based on the colour relations of Carpenter (2001) and Alonso et al. (1998) as the colour-temperature equations used to calculate the effective temperature are in different photometric filter systems. Bolometric corrections and the effective temperature are needed for comparisons between observations and theory and both parameters were derived according to Alonso et al. (1999). After selection, 34 giant stars presented colour excess in (K - 8) with metallicities ranging from -1.9 < [Fe=H] < -0.7; metallicities that were interpolated from PARSEC isochrones (Bressan et al., 2012). Field stars were rejected based on the proper motions from GAIA, which reduced to 18 the number of mass-losing candidates. The large amount of field stars excluded from the sample is due to the difference in spatial coverage from GAIA and Spitzer. The stellar synthetic spectral distribution of those stars is modelled and used to calculate its mass loss rate, using a modified version (Origlia et al., 2007) of the radiative transfer code DUSTY (Ivezic et al., 1999; Elitzur & Ivezic, 2001). The mass loss rates derived from our sample are in the range of 10−8 to 10−7 M yr−1, which is slightly off the values proposed by Origlia et al. (2002) and Boyer et al. (2008). The mass loss rates seem to increase with increasing luminosities and its dependency with metallicity is minimal. Only a fraction of red giant stars are losing mass indicating an episodic mass loss.