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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.

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.

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.

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.

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.

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.

Wave scattering phenomena are ubiquitous in almost all Sciences, from Biology to Physics. Interestingly, it has been shown many times that different physical systems are the stage to the same processes. The discoveries of such analogies have resulted in a better understanding of Physics and are indications of the universality of Nature. One stunning example is the observation that waves propagating on a flowing fluid effectively experience the presence of a curved space-time. In this thesis we will use this analogy to investigate, both theoretically and experimentally, fundamental effects occurring around vortex flows and rotating black holes. In particular, we will focus on light-bending, superradiance scattering, and quasi-normal modes emission. The experimental nature of this work will lead us to study these processes in the presence of dispersive effects. After a general and historical discussion of the field of analogue gravity, we will first present a well-established technique, the gradient expansion method, to obtain approximate solutions of dynamical equations. This method will be used to generalise the notion of light-rings around black holes to vortex flows. Secondly, we will present a wave-vortex scattering experiment in which the superradiance process was observed. Finally, we will relate the properties of the light-rings to the characteristic modes emitted during the relaxation phase of a perturbed vortex flow. We will show that these characteristic modes can be used to develop a flow measurement technique that we call ‘Analogue Black Hole Spectroscopy’. We will then report on an experiment in which these characteristic modes were observed and the analogue black hole spectroscopy technique was applied successfully. Our results strengthen the link between vortices and rotating black holes and open up new challenges to be addressed in the future.

Black holes are among the simplest objects in the Universe, yet they are also the perfect probes of the strong-field regime of gravity, where alternative theories of gravity are expected to yield new predictions for the physics of compact astrophysical objects. In the era of gravitational wave astronomy, some of the predictions can finally be put to a test. We start by investigating the Horndeski class of scalar-tensor theories. It incorporates many alternative theories of gravity that feature a scalar degree of freedom in addition to the metric, either explicitly or through a specific representation. Such scalars have been employed to explain the puzzles of cosmology and they often arise in the effective field theories of more fundamental physics. With the additional field comes the possibility of black-hole solutions that differ from those of general relativity. We investigate the dynamics of a theory that admits only solutions of this kind and find evidence suggesting that such black holes form through gravitational collapse. Moreover, we study the causal structure of any solution with a non-trivial scalar, as the equations permit superluminal propagation for scalar perturbations. We find that the notion of a horizon that bounds all field excitations persists. This is not the case in theories with a preferred frame where black holes have multiple nested horizons. We address the claim that superluminal modes, the hallmark of Lorentz-violating theories, allow for processes that violate the generalized second law of black hole thermodynamics. We derive the necessary conditions for such a violation and find those unlikely to arise from a theory where gravity is attractive. Finally, we sketch a path to advance the research programme featured in this thesis in view of the recent progress in gravitational wave astronomy.

Massive binary stars have been associated with X-ray binaries, Galactic double neutron stars, short gamma-ray bursts, luminous red novae, gravitational waves and other energetic astronomical phenomena. In this thesis we study their evolution and fate. Rapid population synthesis is used to study double neutron stars and binary black holes. Synthesised populations are compared with observations of double compact objects. Our method reproduces the observed Galactic double neutron star period and eccentricity distributions as well as the binary black hole mass distribution. Intermediate phases in the evolution of binaries leading to double neutron stars are also studied. We focus on mass transfer leading to common-envelope events. It is found that most double neutron stars come from two formation channels with very distinctive common-envelope events. It is also found that a significant fraction of binaries will not be circular by the time the common-envelope phase begins. Finally, detailed stellar evolution is used to model massive stellar mergers. Merger products evolve differently than their single star counterparts. A novel formation scenario involving stellar mergers is proposed as a progenitor of (pulsational) pair-instability supernovae. This scenario is suggested in the context of hydrogen-rich long-lasting multi-peaked transients like iPTF14hls.

Mass-loss prior to core collapse is arguably the most important factor affecting the evolution of a massive star across the Hertzsprung-Russel (HR) diagram, making it the key to understanding what mass-range of stars produce supernova (SN), and how these explosions will appear. It is thought that most of the mass-loss occurs during the red supergiant (RSG) phase, when strong winds dictate the onward evolutionary path of the star and potentially remove the entire H-rich envelope. Uncertainty in the driving mechanism for RSG winds means the mass-loss rate (Ṁ) cannot be determined from first principles, and instead, stellar evolution models rely on empirical recipes to inform their calculations. At present, the most commonly used Ṁ-prescription comes from a literature study, whereby many measurements of mass-loss were compiled. The sample sizes are small (< 10 stars), highly heterogeneous in terms of mass and metallicity, and have very uncertain distances from observations and analysis techniques that at best provide order-of-magnitude estimates compared to what is possible today. The relation itself contains large internal scatter, which could be the difference between a star losing its entire H-envelope, or none of it at all. More modern efforts to update the RSG mass-loss rate prescription rely on samples which suffer from statistical biases, for example by selecting objects based on mid-IR brightness or circumstellar maser emission, and hence are inevitably biased towards higher mass-loss rate objects. It is the aim of this thesis to overhaul our understanding of RSG mass-loss. By selecting RSGs in clusters, where the initial mass and metallicity are known, I will be able to observe how mass-loss changes as the star approaches SN and compare this to what is currently implemented in stellar evolutionary models. Ultimately, I will measure Ṁ values and luminosities for RSGs in 5 different clusters of varying ages, thus targeting RSGs of different initial masses. I will then combine these mass-loss rate-luminosity relations to derive a new initial mass-dependent mass-loss rate, which can be implemented into stellar evolutionary models.

Hyper-velocity stars are stars that have been accelerated to speeds in excess of the escape velocity of the galaxy. The first hyper-velocity star observed was found in 2005 by Brown et. al. (2005) and to date we can confirm 25 stars as being hyper-velocity. The small number of these stars discovered is due to the relatively low rate of acceleration events as well as observational methods limiting observations to B type stars. The leading processes by which these stars may be accelerated are binary tidal disruption events in the center of our galaxy. A binary tidal disruption event occurs when a binary star system orbits close to a massive compact object, such as a massive black hole. When this occurs the binary members are separated and one of the binary members gains the orbital energy of its partner, ejecting it at high speeds leaving the partner bound to the black hole. These bound stars, known as S-stars, have been observed and can be used to calculate the mass of the central massive black hole in our galaxy. In this thesis, I present the result of a series of simulations of binary tidal disruption. These simulations provide insight into the required parameters to trigger a binary tidal disruption event as well as the properties of stars ejected via these events. My simulations utilize a restricted three body model to reduce the parameter space that I need to explore to simply the binary orientation, binary phase angle and the penetration depth D, that being the ratio of the closest distance the binary center of mass approaches to the black hole and the tidal radius. This approximation models binaries as having zero energy parabolic orbits but also allows me to model the binary orbit as radial for binaries that have orbits bringing them extremely close to the massive black hole. I will firstly show how the disruption rate behaves as a function of the individual initial conditions of the binary. As the penetration depth shrinks the majority of binaries will be disrupted, however as the penetration depth approaches zero there are still approximately 12\% of binaries that survive their orbit around the black hole. The magnitude of velocity gained by the ejected stars is relatively independent of penetration depth with ejected stars having energies of the order of $E \approx (Gm^2/a)(M/m)^{1/3}$ where $m$ is the mass of the binary, $M$ is the black hole mass and $a$ is the binary separation. Hyper-velocity stars seen in my simulations are only rarely ejected with energies greater or less than this order of magnitude. Because of this the distribution of binary properties (binary and black hole masses, and binary separation) is more important to the spectrum of hyper-velocity stars than the distribution of injection parameters (penetration depth, binary orientation binary phase, and eccentricity). While the average disruption rate can be well defined with penetration depth for a given orientation the disruption dependency is more complex with prograde orbits being favoured for disruption in shallow penetrations $D\approx 1\sim 2$, while binaries oriented with their angular momentum vector toward the black hole are favored in deep penetrations $D\ll 1$. Secondly I consider the disruption rate in terms of the binary phase at the periapsis of its orbit and attempt to constrain the critical criteria that determine the fate of the binary. By looking at binary disruption in terms of free solutions in the radial approximation I can approximate the range of binary phase angles that will survive their encounter with the black hole. Thirdly I explore the properties of the former partners of hyper-velocity stars that remain bound to the black hole in terms of their eccentricity. I find that post disruption bound stars have eccentricities close to unity. This is significantly higher than observed S-stars in our galaxy, suggesting stars in the center of our galaxy have undergone relaxation over time. Finally I discuss the implications of binary interactions for gravitational wave astronomy. Binaries that survive these orbits will have their own internal orbits shifted, gaining significant eccentricity and potentially having their semi-major axis changed. As surviving binaries have their orbits deformed, the inspiral time for binary compact objects can be decreased significantly.

According to Einstein's theory of General Relativity, the acceleration of matter can cause ripples in the curvature of spacetime, given the name gravitational waves. Such ripples are negligible in magnitude for all but the most energetic astrophysical events, such as the coalescence of compact binary stars. In 2015, gravitational waves were first directly detected from a binary black hole (BBH) coalescence [19]. This was achieved using two independent laser interferometers which each measured the fluctuations caused by the gravitational waves as they passed by. Matched filtering and other data analysis techniques were then employed to identify the properties of the source and measure the likelihood that the detection is a false alarm. The efficacy of these matched filtering techniques is pivotal to not only detecting gravitational waves, but drawing as much information about their sources as possible. The methods for detecting a BBH involve the construction of a template bank; a group of synthesised waveforms which each represent a detectable series of gravitational waves that a BBH could produce. The characteristics of a BBH template are governed by the two masses and how they spin, the distance to the source, its orientation and its sky location. Current template banks do not include templates for sources where the spins are misaligned with the orbital momentum, which is the cause of precession in BBH. Thus, the algorithms are effective for detecting a non-precessing BBH, but much less sensitive towards precessing sources. Creating a template bank which includes all possible precessing waveforms is computationally infeasible and would induce enough statistical noise to negate any extra sensitivity gained. However, many precessing signals would be undetectable or indistinguishable from non-precessing signals. Including such signals in a bank would result in no gain in its sensitivity. This thesis attempts to locate areas of precessing parameter space where waveforms are distinguishable from non-precessing sources, and begins work on forming a function which maps observable precession through parameter space.

Far-infrared observations have detected dusty star-forming galaxies, a subset of galaxies which is extremely dust-extincted from the ultraviolet down to near-infrared colours. Recent studies show that this population of sources contributes significantly to the history of star formation, especially out to very high redshift. Recent surveys with the Herschel Space Observatory have uncovered around half a million of these sources, with the largest of these surveys, the H-ATLAS, covering 616 square degrees. One of the most exciting discoveries is the lensing nature of the brightest of these sources, where the gravitational potential of a foreground galaxy lenses and amplifies the signal. The applications of gravitational lensing range from studying individual sources down to unprecidented resolution at high redshift in sub-mm wavelengths with ALMA, to cosmological studies by analysing the distribution of groups of lenses. In this thesis, I explore the effect of applying a more inclusive selection criterion for lensed sources, and study the properties of the sources that are selected. Whereas the first attempts at finding lensed sources use a strict S500μm > 100 mJy flux density cut, the sample I study is selected with a flux cut at 80 mJy: The Herschel Bright Sources (HerBS) sample. A photometric redshift cut of zphot > 2 is also taken, as most lensing takes place out at higher redshift. This redshift is calculated by fitting a spectral template to the 250, 350 and 500 μm observations from the Herschel SPIRE instrument. I push down the selection flux in order to select more lensed sources from the sub-mm surveys, whilst potentially including several unlensed sources. These unlensed sources could be among the most intrinsically luminous and star-forming objects in the Universe. Only less than five of such objects are known to exist, while our HerBS sample could contain up to 35 of these sources, which could teach us about the upper-limits of star-formation and their contribution to forming the most massive galaxies in the Universe. I use 850 μm SCUBA-2 observations on the James Clerk Maxwell Telescope (JCMT) to remove blazar interlopers, which results in 209 sources in the HerBS sample, after removing 14 blazar sources. At the time I wrote the paper upon which Chapter 2 is based, 24 sources had a spectroscopic redshift. I use this sub-sample to fit a two-temperature modified blackbody, and find a cold-body temperature of 21.3 K, a warm-body temperature of 45.8 K, a mass ratio of 26.7 and a dustemissivity index of 1.8. These values do not challenge the current knowledge of sub-mm galaxies, but the quality of the fit suggests a large diversity among the galaxies in the sub-sample, and that they are poorly fitted by a single template. This diversity is also found by the spectroscopic observations with the IRAM 30m-telescope observations on eight of the highest-redshift (zphot > 4) sources of the HerBS sample. We found five spectroscopic redshifts, with one of the sources at the highest known HerBS redshift at zspec = 4.8. The spectrum fitted in Chapter 2 shows a poor agreement with the photometric data points. The spatial resolution of the SPIRE instrument on Herschel is not fine enough to resolve the structure of these high-redshift sources. Worse still, the beam width is so large, ranging from 18 to 36 arcseconds, that it is unsure whether we observe a single galaxy, or perhaps observe multiple galaxies together. The beam width of the SCUBA-2 instrument at 850 μm is only 13 arcseconds. In the case the sample would be dominated by blended sources, one would expect to resolve several of the sources into their individual components. This is not seen in any of the continuum images, although the blended sources might be blended on scales smaller than 13 arcseconds. The IRAM-observations of two sources have detected multiple, contradicting spectral lines, suggesting we might be observing multiple sources, instead of a single source, that are aligned along the line-of sight. Unfortunately, only single spectral lines have been observed per source, and we are awaiting more observations verifying the blending nature of these sources, which are still expected to lie at high redshift. The hypothesis that our sample consists for a significant portion out of blended sources is in contradiction with multi-wavelength observations. When I look at the positions of these sources at different wavelengths, I find that most sources have a counterpart in these multi-wavelength observations, also when chance-encounters are considered. Considering the high redshift nature of our sources, together with the possibility of lensing, these counterpart sources are most likely foreground, lensing galaxies. I compare the positions of the HerBS sources to both the Sloan Digital Sky Survey (SDSS), which covers 121 out of the 209 sources, and the VISTA Kilo-Degree Infrared Galaxy (VIKING) survey, which covers 98 HerBS sources. For the SDSS counterparts, I use the H-ATLAS catalogue of counterpart sources, which was done by using a statistical estimator. This statistical estimator assumes a certain angular distribution between the sources in the Herschel position, and the opticalor near-infrared observations. I expect the majority of my sources to be lensed, and therefore I adjust the original angular distribution by including the effect of gravitational lensing. The adjustment is based on 15 ALMA observations of lensed, bright H-ATLAS sources. The revised analysis finds 41 counterparts, instead of the 31 that were found by the initial analysis. This catalogue is not available for VIKING counterparts, and therefore I had to do the entire analysis for the VIKING counterparts, starting from the VIKING fields. I use the sextractor package to extract the potential counterparts, and then derive the necessary estimators for the statistical method. I find a significantly different angular distribution, even than the one derived from the 15 ALMA observations of lensed H-ATLAS sources. The angular distribution extends to much larger angular scales, potentially suggesting a stronger contribution to galaxy-cluster lensing, which produces larger angular offsets due to the larger masses and different mass profiles associated with galaxy clusters. In total, I find 60 counterparts with a reliability greater than 80% to the 98 HerBS sources covered by VIKING. Possibly, not all counterparts could be positively identified, as the analysis showed 88% of sources has a source within 10 arcseconds when taking chance encounters into account. This is mostly due to ambiguity between several nearby sources, which causes a low reliability of the counterpart identification, but it does allow us to state that an counterpart could be present. A cosmological model suggest that 76% of our sources are gravitationally lensed. This model assumes a certain distribution of halo masses, and lensing magnification based on mass density profiles. The validity of these models has been shown with the 15 ALMA observations of lensed H-ATLAS sources, and also agree with the SMA observations from Bussmann et al. (2013). The IRAM observations provide me with both line luminosities and line velocity widths. Larger galaxies are expected to be brighter, and have larger line velocity widths. The five sources with confirmed redshift (and therefore line luminosity) have a luminosity-to-velocity width ratio agreeing with a magnification of around 10, when compared to unlensed, and known lensed sources. I show that the SDSS is not deep enough to observe all the foreground galaxies, while the VIKING observations agree with the results from the simulation, with 60 sources actually cross-compared, and 88% of sources have a source nearby, when accounting for random chance.

Star formation is a chaotic process, involving the evolution and interaction of a wide variety of structures. The interstellar medium exhibits substructure over a range of scales, and the clusters which form from the densest parts of this material may be imprinted with this clumpy distribution. In this thesis, we describe and evaluate statistical tools for quantifying structures that are important to the star formation process, in order to constrain the underlying physics and robustly compare observations, simulations and synthetic observations. We describe the basic theory and some common applications of fractal theory in astronomy. We show that some common measures of fractal structure are inconsistent and that comparing values derived from different data types (e.g. continuum data of molecular cloud maps and discrete data of star distributions) can lead to confusion. We introduce the Q+ algorithm which quantities the substructure in star clusters in terms of a fractal distribution. We describe the derivation and validation of this method and apply it to observed and simulated data sets. We examine the possibility of applying this same analysis to continuum data by converting the greyscale image into a statistically representative distribution of points. We introduce the J plots algorithm which uses the principal moments of inertia of a two-dimensional pixelated structure to quantify its shape. We show that this can be used to identify the shapes of structures extracted from astrophysical images using dendrograms. We apply this method (i) to data from the Hi-GAL survey to demonstrate the identification of ring-like shapes, and (ii) to simulations of _lament formation to quantify the differences in structure resulting from the nature of turbulence in the accreting material.

The study of ancient stellar systems in the vicinity of the Milky Way provides a wealth of information on the conditions, in the early Universe, that led to the properties we observe today in galaxies and in their constituent components. Resolved stellar populations enable us to gain detailed insights on the age and chemical composition of such stellar systems, tracing their properties on a fine spatial scale. The deep investigation of Local Group objects revealed that even very old, low mass, stellar systems host unexpected complexities in their stellar populations. Such complexities remain largely unexplained, our understanding limited by observational and theoretical limitations. Here I present work aimed at a deeper characterisation of the complex stellar popula- tions in dwarf spheroidal galaxies and Galactic globular clusters. I use a combination of observational and modelling techniques to shed light on the detailed stellar properties of these objects. Part of my investigation focuses on the horizontal branch of dwarf spheroidal galaxies. By careful modelling of the horizontal branch in the galaxy Carina, which has well known star formation history, I demonstrate that the horizontal branch contains precious information, that can be used to refine age measurements in nearby galaxies. To this aim, I develop a new modelling method that, for the first time, combines constraints from the main sequence turn-off and the horizontal branch to provide very precise measurements of the star formation history in resolved galaxies. The combined information from different regions of the colour-magnitude diagram permits to recover the value of mass loss experienced by red giant branch stars with very high precision. I test this technique on a range of synthetic populations and on the well studied galaxy Sculptor, demonstrating the increased age resolution that this approach provides. I apply my modelling tool to the distant galaxy Tucana, determining a very detailed star formation history, where multiple events of star formation can be clearly distinguished. The identification of the different stellar populations on the horizontal branch permits us to characterise the spatial distribution of the star formation events in this galaxy. I also perform a photometric study of the massive globular cluster M13, focussing on the multiple stellar populations present in this object. I identify and trace the different stellar populations out to most external regions of this cluster. The spatial distribution of these populations, which shows no sign of radial segregation, reveal the very advanced dynamical evolutionary stage of the cluster. The work presented in this manuscript constitutes a step forward to understand the formation of low mass ancient stellar systems and paves the way for deeper studies of large samples of stellar systems in the Milky Way vicinity.

Understanding the formation and evolution of our galaxy, the Milky Way, has been an ongoing process, which with the development of large-scale surveys has picked up considerable pace. Together with these new surveys, pipelines have been constructed which allow for the rapid and automatic processing of this wealth of new data. These codes are able to turn raw data files into tables of stellar parameters and chemical abundances in far less time than if they were analysed by hand. The results from these surveys open new windows on to the history of our galaxy and other disk galaxies. In this thesis, we present the development of a new pipeline, the STellAR Parameter AND Abundances pipeline (STARPANDA), which is able to rapidly derive stellar parameters, CNO abundances and other elemental abundances by utilising measurements of spectral features in both observed and synthetic spectra. We take the observed spectra, synthetic spectra and line lists employed by the APOGEE survey and produce new values for the stellar parameters, CNO abundances and Al abundances of the APOGEE stars. We then compare our results with those achieved by the APOGEE pipeline.

Stripped-envelope supernovae (SE-SNe) are a subset of core-collapse supernovae; the explosive death of a massive star. Their defining characteristic is the lack of promi- nent He and/or H envelope suggesting significant mass loss prior to explosion. Their progenitors may be high mass single stars (> 30 M⊙) or lower mass stars that are stripped via binary interaction. Since their discovery as a separate population in 1983, and until recently, the data on these objects steadily increased. SN discoveries have increased year on year since the early 2000s with the advent of targeted and untargeted surveys looking at the skies for transient objects. As a result, some of these surveys have amassed photometric and spectroscopic data on a large number of SE-SNe. The last few years has seen this data made available, dramatically increasing the number of objects with data. I present an investigation into the bulk properties of SE-SNe, using a large database accumulated from public sources, the Palomar Transient Factory, the Public ESO Spectroscopic Survey of Transient Objects, and my own observations. I begin the investigation by constructing and analysing the largest set of bolometric light curves of SE-SNe to date – 85 objects. The light curves are analysed to derive temporal characteristics and peak luminosity Lp, enabling the construction of a lumi- nosity function. Subsequently, the mass of 56Ni synthesized in the explosion, along with the ratio of ejecta mass to ejecta kinetic energy, are calculated. It is found that broad-lined SNe Ic (SNe Ic-BL) and gamma-ray burst SNe are the most luminous sub- types with a combined median Lp, in erg/s, of log10 (Lp)= 43.00 compared to 42.51 for SNe Ic, 42.50 for SNe Ib, and 42.36 for SNe IIb. It is also found that SNe Ic-BL synthesize approximately twice the amount of 56Ni compared with SNe Ic, Ib, and IIb, with median MNi = 0.34, 0.16, 0.14, and 0.11 M⊙, respectively. SNe Ic-BL, and to a lesser extent SNe Ic, typically rise quicker than SNe Ib/IIb; consequently, their light curves are not as broad. Next I examine the spectroscopic properties of these SNe using analytical methods. For He-rich SNe, the presence of H becomes the focus. The strength, velocity, and ratio between absorption and emission of H are measured, along with additional analysis of He I lines, in order to categorize the SNe. The He-poor SNe are ordered according to the number of absorption features N present in the spectra, which is a measure of the degree of line blending. The kinetic energy per unit mass Ek/Mej is strongly affected by mass at high velocity, and such situations principally occur when the outer density profile of the ejecta is shallow, leading to the blending of lines. Using the results, the existing SE-SN taxonomic scheme is adapted I then present the data and analysis of 19 SE-SNe observed since 2012. These SNe are analysed within the context of the earlier findings in this work, as well as exam- ining the ejecta mass distributions as derived from an analytical light curve model. The results support the assertion that SE-SNe reside in a parameter space which is still under-sampled as approximately 20 – 25 percent of these objects have properties that deviate significantly from that of the bulk population. The statistics of the ejecta mass distributions also provide evidence that these SNe arise from relatively low mass progenitors (< 25 M⊙) as the mean ejecta mass for all SN types is 2 – 4 M⊙. Furthermore, distribution of ejecta mass appears unimodal, which suggests that SE-SNe are preferentially arising from one channel; stars that undergo binary interaction. Understanding SE-SNe is important as their stripped pre-explosion progenitor stars are hot, making them sources of ionizing radiation. Their explosions influence their local environment by injecting energy, both radiative and kinetic, and seeding the ISM with the ashes of nucleosynthesis. Finally, they are a source of neutron stars and stellar mass black holes in the universe, which gives rise to other astrophysical events such as X-ray binaries, pulsars, and strong gravitational wave events.

Stars are of fundamental importance to the entire field of astronomy. The conversion of elements and the distribution of energy throughout the lifetime of stars drives the evolution of the Universe. Despite this, we do not have a unified understanding of the formation process for all stars. This thesis attempts to move forward this understanding, by focussing on the question: How do the initial conditions of star-forming regions vary across environments, and do these influence the process of star formation? To investigate the initial conditions of star formation, regions on the verge of forming stars have to be first identified and analysed. These regions have to be untouched by the disruptive effects of stellar feedback, such that the natal conditions of the gas – e.g. kinematics and chemistry – are not destroyed. Quiescent regions that are expected to form low-mass stars have been well studied over the past few decades, and the general process of low-mass star formation is well understood. Only relatively recently, however, has a group of objects being identified as being potential hosts of these initial stages of high-mass star formation: Infrared Dark Clouds (IRDCs). The study of these objects is difficult, due to both their rarity and complexity. An end-to-end understanding of high-mass star formation is, therefore, much less developed compared to their lower mass counterparts. This thesis presents the study of a sample of IRDCs within the Disc and Centre of the Milky Way; two very different environments. Several key aspects of the star formation process within IRDCs from these environments are investigated. Firstly, a chemical signpost – the deuterium fraction of N2H+ – is used to identify the regions of dense and cold gas on the verge of forming high-mass stars within a quiescent Disc IRDC, which can be used to study the initial conditions for star formation. Omitting potential beam dilution effects, chemical modelling suggests that the cloud could have reached a global chemical equilibrium, and, if so, would also be dynamically old (survived for several free-fall times). This timescale, with estimates of the embedded stellar mass, is used to determine star formation rates and efficiencies. Secondly, the kinematic structures within two apparently similar Disc IRDCs are identified using dense gas tracers – C18O and N2H+. The properties of these structures appear to be very similar, hinting at a similar formation scenario for both clouds, or, potentially, that these may be inherent to the larger Disc IRDC population. The dynamics of these filaments also show that they may be merging, which would suggest a compressive mode of turbulence driving. These structures are then linked to the larger kinematic structures – identified using a lower density tracing molecule, 13CO – and found to show good coherence with the brightest, most extended structures. These are then placed in the context of the previously identified Galactic scale structures, and in doing so show that IRDCs could be the densest parts of the much larger arm or inter-arm filamentary structures. Thirdly, the level of star formation within the Galactic Centre is investigated on both global (∼ 100 pc) and local (∼ 1 pc) scales. On a global scale, the star formation rate has been determined from all the available observational star formation diagnostics – i.e. direct counting of young stellar objects and integrated light measurements – and found to be in agreement with previous studies; i.e. around one-to-two orders of magnitude lower than predicted by the star formation models. On individual cloud scales, the star formation efficiency per free-fall time is in better agreement with the model predictions. However, uncertainties on the properties of these regions, such as the mode of turbulence driving, limit the further verification or falsification of the star formation theories. Lastly, the investigation of the local scale star formation within the Galactic Centre highlighted a particular part of the parameter space as the most promising to further test the star formation theories. In light of this, high-spatial resolution ALMA observations have been taken of two Galactic Centre clouds within this regime. Early results show that they have a complex structure, similar to that seen within Disc IRDCs, containing both filamentary and core-like features. Investigation of the brightest, most compact core region shows that it contains a very rich chemistry, and, of particular interest, is the rigorous detection of the pre-biotic molecule formamide (NH2CHO). When placing the results of this thesis in the bigger context of star formation theory, they appear to show interesting implications for the initially posed question – what is the influence of environment on the process of star formation? It is found here that despite the very different cloud scale properties of these regions, the star formation efficiency per free-fall time is surprisingly similar. To investigate this, the properties of the individual sites of high-mass star formation, the high-mass star-forming cores, are compared. Interestingly, despite the different environmental conditions, several key properties of the cores, such as their size and mass distribution, are also found to be very similar. The similarity of high-mass core properties and star formation rate per free fall time implies that once a region has produced high-mass cores, the evolution of these cores towards star formation must be similar. The difference in the global/environmental properties of the gas must then be setting the total star formation rate within these regions, by limiting the number of cores that can form. In particular, the mode of turbulence driving may play a major role in governing the fraction of gas that can be converted into stars per free-fall time within these two environments.

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

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

This thesis focuses on the application of asteroseismology to red giants observed with Kepler alongside searching for solar g-modes using the Birmingham Solar Oscillations Network (BiSON). In the case of the Sun, solar gravity modes are highly sought after because they can shed light on the inner rotation profile of the Sun. This thesis contains work showing how the low frequency regime of BiSON data has been cleaned enabling the search to be made in BiSON data without instrumental artefacts. Moving onwards along the stars evolution, thanks to space mission such as Kepler and CoRoT tens of thousands of red giant stars have been observed allowing huge ensemble investigations. The ability to use high-quality, long datasets as constraints to shorter and noiser datasets has been investigated through fitting the background power of 6000 Kepler red giants. Red giants also offer the opportunity to study the inclination angle distribution of stars to confirm that the distribution conforms to the expected isotropy used in many simulations. This can be extended to inferring the obliquity through asteroseismology, as applied to a red-giant, M-dwarf eclipsing binary. This offering a means to probe obliquity distributions in in a different regime to that using traditional spectroscopic techniques.

First, we investigated the ν max scaling relation, a widely-used equation that states that the frequency of maximum amplitude in a power spectrum scales with a combination of surface gravity and effective temperature. We tested how well the oscillations of cool main-sequence and sub-giant stars follow this relation, using a ensemble of asteroseismic targets observed by Kepler. We then tested seismic scaling relations in a small group of 10 bright red-giant stars observed by Kepler. These giants, some of the brightest observed in the Kepler field, have precise values of parallaxes. We compared the measured distances with inferences made using asteroseismic parameters. We also combined high-quality spectroscopic data with seismic constraints to determine their evolutionary phase. We compared the observed surface abundances of lithium and carbon with models that account for additional mixing processes in redgiants. Finally, we analyzed a group of 13 stars observed by Kepler, and use asteroseismic tools to extract modelindependent information about their internal regions. Our objective is to detect the so-called acoustic glitches, characterized as departures from the uniform frequency spacings predicted by the asymptotic relation. Such departures originate in regions where there is an abrupt change in the stratification of the star.

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

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

This work uses data from the Herschel Space Observatory and complementary surveys to study how the properties of star forming galaxies have changed over a large redshift range. Using the likelihood ratio technique, infrared counterparts from the VIKING survey are found for a large sample of Herschel ATLAS galaxies over the three GAMA fields. I find that approximately half of all Hershcel ATLAS galaxies have a reliable VIKING counterpart. I find that the fraction of Herschel sources with a reliable VIKING counterparts remains above 30% for Herschel sources at redshifts above z = 3, beyond the VIKING detection limit. We propose that this is the result of observing a large number of gravitationally lensed sources in the Herschel ATLAS survey. I match a sample of Herschel ATLAS sources to the optical Subaru Deep Field (SDF) catalogue, using radio data as an intermediary step to attain accurate positions. I compare this technique with matching Herschel ATLAS sources directly to the SDF catalogue and find that of the common matched sources, 6% of Herschel ATLAS sources are matched to two different SDF sources. I study the star forming properties of Herschel ATLAS and Herschel-GOODS galaxies. I study the two galaxy samples in terms of the galaxy main sequence, Kennicutt-Schmidt relation and the K magnitude-redshift relation. I find that the Herschel galaxies are relatively homogeneous. There is little evidence that the starburst phase of galaxies plays a large role in the star formation history of Herschel galaxies. I find that the star formation efficiency of our starburst galaxies is on average the same as main sequence galaxies, implying that starburst galaxies form more stars due an increased gas mass. The optical images of the Herschel-GOODS sample of galaxies are decomposed in to their bulge and disc components. I find that 67% feature a bulge which is best fit with a low Sersic index profile, suggesting the majority of the population feature a disc-like or pseudo-bulge. I see little evidence of a correlation between the properties of the bulge and the overall star forming properties of the galaxy.

This thesis is structured in two parts. In part 1 we summarise the necessary theoretical background and the numerical tools in order to conduct the research in part 2. In chapter 2 we summarise the 3 + 1 approach to numerical relativity, with a focus on the initial data formalism in the weighted transverse and conformal thin-sandwich decompositions. We then move on to discuss initial data for black hole simulations in chapter 3, where we focus on the moving puncture approach. We note that the standard gauge evolution equations force the initial numerical wormhole slices to evolve into trumpet slices, which suggests the construction of a priori trumpet initial data. We summarise the literature on trumpet research, with a focus on maximal Schwarzschild trumpet initial data in its analytical form. In chapter 4 we provide the necessary numerical tools and techniques, which we use in part 2 to solve the constraint equations numerically. We focus on finite difference methods in connection with the Thomas algorithm and successive over-relaxation to solve boundary value problems in one and two variables. Part 2 contains the novel research of this thesis. In chapter 5 we discuss Kerr in quasi-isotropic coordinates and point out that it represents a maximal trumpet foliation for extreme Kerr, however a wormhole foliation for slow Kerr. We lay out our approach to numerically derive maximal Kerr trumpet initial data. The approach is based on the proposition that two nontrivial impositions on the constraints suffice to construct the data numerically. We propose that these relations can be generalised from Schwarzschild and extreme Kerr to slow Kerr. The main motivation for this stems from the observation that the square of the extrinsic curvature for Bowen-York shows the same behaviour as for Kerr for small radii, and Bowen-York trumpets have been constructed successfully. Our main results are then presented in chapters 6 and 7 in which we test our approach for the special cases of zero and maximal spin, ie for Schwarzschild and extreme Kerr. We succeed with Schwarzschild and present the first ever purely numerical derivation of maximal Schwarzschild trumpet initial data in the weighted transverse decomposition – our first main result. Because of the complexity of the problem for extreme Kerr, we proceed in steps and start out by using more information of the analytical solution to treat the constraints separately. For instance, we provide the conformal metric to the Hamiltonian and momentum constraints, and solve them successfully for the trumpet solution – our second main result. Finally we elaborate on how to relax the assumptions. In particular, we introduce an additional equation which we can solve successfully for the function which describes the deviation of the conformal metric from being flat – our third main result.

The process of converting gas into stars underpins much of astrophysics, yet many fundamental questions surrounding this process remain unanswered. For example -- how sensitive is star formation to the local environmental conditions? How do massive and dense stellar clusters form, and how does this crowded environment influence the stars that form within it? How do the most massive stars form and is there an upper limit to the stellar initial mass function (IMF)? Answering questions such as these is crucial if we are to construct an end-to-end model of how stars form across the full range of conditions found throughout the Universe. The research described in this thesis presents a study that utilises a multi-scale approach to identifying and characterising the early precursors to young massive clusters and high-mass proto-stars, with a specific focus on the extreme environment in the inner few hundred parsecs of the Milky Way -- the Central Molecular Zone (CMZ). The primary sources of interest that are studied in detail belong to the Galactic centre dust ridge -- a group of six high-mass (M ~ 10^(4-5) Msun), dense (R ~ 1-3 pc, n > 10^(4) cm^(-3)), and quiescent molecular clouds. These properties make these clouds ideal candidates for representing the earliest stages of high-mass star and cluster formation. The research presented makes use of single-dish and interferometric far-infrared and (sub-)millimetre observations to study their global and small-scale properties. A comparison of the known young massive clusters (YMCs) and their likely progenitors (the dust ridge clouds) in the CMZ shows that the stellar content of YMCs is much more dense and centrally concentrated than the gas in the clouds. If these clouds are truly precursors to massive clusters, the resultant stellar population would have to undergo significant dynamical evolution to reach central densities that are typical of YMCs. This suggests that YMCs in the CMZ are unlikely to form monolithically. Extending this study to include YMCs in the Galactic disc again shows that the known population of YMC precursor clouds throughout the Galaxy are not sufficiently dense or central concentrated that they could form a cluster that then expands due to gas expulsion. The data also reveal an evolutionary trend, in which clouds contract and accrete gas towards their central regions along with concurrent star formation. This is argued to favour a conveyor-belt mode of YMC formation and is again not consistent with a monolithic formation event. High angular resolution observations of the dust ridge clouds with the Submillimeter Array are presented. They reveal an embedded population of compact and massive cores, ranging from ~ 50 - 2150 Msun within radii of ~ 0.1 - 0.25 pc. These are likely formation sites of high-mass stars and clusters, and are strong candidates for representing the initial conditions of extremely massive stars. Two of these cores are found to be young, high-mass proto-stars, while the remaining 13 are quiescent. Comparing these cores with high-mass proto-stars in the Galactic disc, along with models in which star formation is regulated by turbulence, shows that these cores are consistent with the idea that the critical density threshold for star formation is greater in the turbulent environment at the Galactic centre.

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

Globular clusters were among the first luminous objects to form in the Universe. They are dense collections of hundreds of thousands of stars. Globular cluster formation is a major unsolved problem in astrophysics. A new constraint on the problem came from the discovery of unexpected star-to-star variations in the abundances of some light elements. These abundance variations (or multiple stellar populations) are ubiquitous to all globular clusters studied to date. The pursuit to explain this longstanding prob- lem using these new constraints (i.e. the abundance variations), has reinvigorated the study of globular clusters, and at the same time has challenged our understanding of nucleosynthesis and stellar evolution. Several scenarios have been put forward to explain the presence of multiple stellar populations in globular clusters, nearly all requiring multiple generations of stars. The basic hypothesis in these models is that a second generation of stars is born during the early life of the globular cluster from the chemically-processed ejecta of some first generation stars in order to account for the signature multiple stellar populations observed in old globular clusters today. Many of these scenarios are mutually exclusive. Therefore, to determine which of them fits the current evidence the best became the priority of globular cluster studies. Modern observational facilities cannot resolve the globular cluster formation process in the early Universe. However, none of the scenarios for the origin of globular cluster and their multiple stellar populations make any distinctions between star/cluster formation at the present day and earlier epochs of the universe. Accordingly, the processes invoked in these scenarios can, in principle, be constrained by studies of the formation of young massive star clusters in the local Universe, which have similar sizes and masses as present-day globular clusters, but are significantly younger. In this work, I present some of the strongest constraints from such studies coming from the gas content of young massive clusters and their star formation histories. These studies showed that: 1) young massive clusters are consistent with a single star formation burst, and 2) there is no significant cool gas reservoirs left within young massive clusters that can fuel future star-formation events. These results are in stark contrast with the predictions of nearly all the scenarios that have been proposed to explain the origin of abundance variations in globular cluster stars, which require that young massive clusters should host multiple star formation events.

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

It has been long known that galaxy properties are strongly connected to their environment; however, a complete picture is still missing. This work's aim is to better understand the role of environment in shaping the galaxy properties, using a sample of 25 redshift-selected galaxy groups at 0.060 < z < 0.063, for which 30 multi-wavelength parameters are available. Given the wide variety of group dynamical states, it was fundamental to try and identify different classes of groups performing a statistical clustering analysis using all the available parameters independently of their physical meaning, which resulted in two classes distinct by their mass. To move beyond mass driven correlations, a new clustering analysis was performed removing the mass dependent properties, this approach provided a categorisation in four classes with distinctive group properties. Based on this, the galaxy properties were investigated and the classes interpreted as follows: a class of field-like galaxies in the early stage of structure formation; a class of low-mass groups either still in formation phase, or evolved, but small because they are isolated; a class of massive groups with no, or very little, ongoing star formation, likely in a more evolved stage of structure formation; and a class of massive groups possibly experiencing merger events. The result obtained have shown that it is possible to distinguish between classes of groups and thus be able to study the property of galaxies in systems with homogeneous properties. The method developed applied to data sets with larger statistics and good data quality could be a powerful tool to study galaxy evolution in galaxy groups.

Following the first detection of gravitational waves from a binary coalescence, the study of the formation and evolution of these gravitational-wave sources and the recovery and analysis of any detected event will be crucial for the newly realised field of observational gravitational wave astrophysics. This thesis covers a wide range of these topics including simulating the dense environments where compact binaries are likely to form, focusing on binaries containing an intermediate mass black hole (IMBH). It is shown that such binaries do form, are able to merge within a ∼ 100 Myr simulation, and that the careful treatment of the orbital evolution (including post-Newtonian effects) implemented here was crucial for correctly describing the binary evolution. The later part of the thesis covers the analysis of the gravitational waves emitted by such a binary, and shows it is possible to identify the IMBH with high confidence, together with most other parameters of the binary, despite the short-duration signals and assumed uncertainties in the available waveform models. Finally a method for rapid parameter estimation of gravitational wave sources is presented and shown to recover source parameters with comparable accuracy using only a small fraction ∼ 0.1% of the computational resources required by conventional methods.

The aim of this thesis is to study stellar evolution and asteroseimology of red-giant stars mainly from a modelling point of view, in particular the impact on core-convective-burning stars of adopting different mixing schemes. Thanks to NASA space telescope Kepler, asteroseismology of thousands of giants provided us new information related to their internal structure, that can be used for finding constraints on their cores. I used several stellar evolution codes (MESA, BaSTI, and PARSEC) to investigate the effect of different mixing schemes in the helium-core-burning stars. Comparing them with observed stars, I concluded that standard stellar models, largely used in literature, cannot describe the combined observed distribution of luminosity and period spacing. I then proposed as solution a penetrative convection model with moderate overshooting parameter. Additional tests on Kepler's open clusters (NGC6791 and NGC6819) and secondary clump stars, allowed me to revised to my mixing model.

Feedback from massive stars is thought to be very important in regulating star formation on a range of scales. However, it is not clear if this feedback acts in a positive way by triggering star formation, or negatively by terminating it. In this thesis we investigate what role feedback plays in determining both the structure of molecular clouds and the rate of star formation, using Smoothed Particle Hydrodynamics. We begin by looking at how the evolution of an HII region is dependent on the amount of ionising radiation the exciting star produces. We then go on to explore the stellar populations created by cloud-cloud collisions and assess their ability to form high mass stars capable of producing large amounts of feedback. We then model the HII regions of these stars and determine what impact these have on star and structure formation. We find that there is a minimum stellar mass required to produce enough feedback to maintain an HII region. Below this value an HII region will either not form, or form and then implode. Above this value the HII region will act as a traditional HII region, and expand. When two clouds collide we �nd that they produce a shock compressed layer which forms �lamentary structures. The arrangement of these �laments is highly dependent on the collision velocity. Low velocity collisions produce a hub and spoke system in which competitive accretion dominates and produces a few very massive stars and a plethora of low mass stars. High velocity collisions produce lamentary networks that resemble a spider's web. In these spider's webs the stars form at nodes where multiple �laments meet. These nodes act as small local sites for star formation and form either a single, or small collection of stars. As a result stars formed in these systems tend to have a characteristic mass and there is less low mass or high mass star formation. However, we do find that eventually stars capable of producing signi�cant feedback form in all simulations. We model the HII regions of these stars and �find that they very quickly terminate star formation. They also produce very interesting bi-polar HII regions that are diffi�cult to interpret when viewed from some directions.

The beginning of gravitational wave astronomy started on September 14th 2015 [13]. The event, GW150914, was so loud that the distinct morphological features indicative of the merger of two inspiraling black holes was di�cult to deny. The estimation of source parameters and parameterised tests of general relativity in the strong �eld regime require the use of gravitational waveform models that predict the inspiral, merger and ringdown of binary black holes according to general relativity. This thesis is focused on providing the gravitational wave community with an accurate model for the gravitational wave signal emitted by coalescing, non-precessing binary black holes covering the inspiral, merger and ringdown. The solutions to the Einstein equations for the late inspiral, merger and ringdown of binary black holes can only be obtained by using numerical relativity. However, the computational cost of a single simulation is on the order of weeks to months and prohibits a dense sampling of the parameter space. Our method is founded on the phenomenological modelling program, which was speci�cally designed to directly incorporate results from numerical relativity and analytic approximations to construct global models across the parameter space for gravitational wave searches. We have re�ned the phenomenological method and developed a new waveform model, IMRPhenomD, which is suitable not only for gravitational wave searches but is also su�ciently accurate to be in used to estimate the parameters of gravitational wave candidate events without incurring large systematic uncertainties due to waveform modelling errors. Subsequent to the work presented here our waveform model was also extended to include the e�ects of precession, which was used in the analysis of advanced LIGO data during its �rst observing run (2015-2016), including the analysis of GW150914. We evaluate the current state of the �eld of waveform modelling by performing numerous comparisons between leading inspiral, merger and ringdown waveform models and �nd that independently developed models are largely in agreement. This builds con�dence in our models when we use them outside of their respective calibration regions. However, there are still large regions of parameter space where the models are in disagreement and we highlight these regions as urgent targets for new numerical relativity simulations.

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

While stars are relatively well understood, the timescales on which they form are still debated. The young cluster NGC 2264 is an ideal region in which to test hypotheses about the timescale of star and cluster formation. Co-eval stars at any given effective temperature are expected to have similar luminosities and radii, but previous research on clusters, including NGC 2264, has found that this may not be the case. In this thesis, fibre spectroscopy from the FLAMES spectrograph is used to find radial and projected equatorial velocities for many low-mass pre-main sequence stars in NGC 2264. Projected radii are estimated by combining these data with published rotation periods. The projected radius distribution is compared with models incorporating radius and age dispersions. These methods circumvent many uncertainties that arise when using luminosities to infer ages from the Hertzsprung-Russell diagram (HRD). Comparisons of models and data favour a spread of radii that is inconsistent with a coeval population but consistent with the spread of ages seen in the HRD. Model-dependent, but distance-independent, ages of 1 - 2.6 Myrs are found, and agreement with ages determined from the HRD is found for a cluster distance of 770± 46 pc. The cluster velocity dispersion is well resolved, and a connection between spatial and kinematic substructure is established. The substructure is unlikely to be responsible for the observed age dispersion. A catalogue of 547 spectroscopic observations of stars in NGC 2264 is presented, with measurements of radial and projected equatorial velocities.

Massive stars have very strong stellar winds which interact with their environment. This work has involved the study of these interactive winds at radio and other wavelengths. Radio observations have been made of the massive runaway star BD+43◦ 3654 and its bow shock which is interacting with the inter-stellar medium. These observations, together with archive data at other wavelengths, have revealed stratified dust and turbulent gas in this interaction zone. Further radio studies have been undertaken of the interaction zones of the colliding winds of massive binary systems. Observations of the colliding wind binary WR 147 at 5GHz have revealed a curved collision zone, suggestive of simple interactive models. Measurements of the flux from the Wolf-Rayet component of this massive binary system has allowed a mass-loss rate to be derived and though the companion O-star is not detected, an upper flux limit has allowed upper limits on the mass-loss rate and limits on the terminal velocity to be inferred. Also revealed is a curious ’bridge’ feature previously observed in WR 147 which occurs between the two binary components. One mechanism is suggested to explain this anomalous feature, the ionising flux of one binary component, the O-star, may be ionising the wind of the other, the WR component. Modelling of the ionisation structure of the stellar winds has been undertaken to verify that this may be occurring. Radio observations of massive stars made at low-frequency have produced detections of WR 147 and the brighter colliding wind binary, WR 146. These detections have allowed modelling of the non-thermal emission in order to deduce where the non-thermal absorption turn-over occurs in these systems. The resultant modelling has illustrated that these colliding wind regions are complex, with multiple absorption regions best describing their nature.

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

The advent of large-area digital sky surveys marked a turning point for the entire field of astronomy. Today, with multi-band photometry for hundreds of millions of objects readily at hand, the ability to mine data for specific rare objects of interest has become of fundamental importance. The aim of this work was to study white dwarfs and planetary remnants by, first of all, developing efficient selection algorithms to identify these objects in large area surveys. Using SDSS DR7 we developed a routine which relies on colours and proper motion to calculate probabilities of being a white dwarf (PWD) which, in turn, enables a flexible selection of white dwarf candidates without recourse to spectroscopy. The application of this selection method to SDSS DR10 lead to the creation of a catalogue of ≃ 66, 000 bright (g ≤ 19) objects with calculated PWD from which it is possible to select ≃ 23, 000 high-confidence white dwarf candidates . The reliability of the method was further tested using a sample of spectroscopic objects from the LAMOST survey. This independent test confirmed the robustness of our algorithm and lead to the identification of 290 new white dwarfs. We also applied our selection routine to the recently released ATLAS DR2 to construct a preliminary catalogue of ≃ 9000 ATLAS white dwarf candidates. This catalogue represents the first sample of white dwarfs candidates in the southern hemisphere. We later exploited our catalogue in several science project. We developed a separate selection algorithm to identify variable white dwarfs in large area time-domain surveys. To test this method we carried out a pilot search for pulsating white dwarfs using 400 high-confidence white dwarfs candidates with available multi-epoch photometry in SDSS Stripe 82. This test proved the ability of our method to select different types of variable white dwarfs and allowed to identify 5 pulsating white dwarfs, 3 of which are new discoveries. During the development of our catalogue, we also identified 64 new metal polluted white dwarfs. Recent studies have shown that the metal pollution in these objects is the result of accretion of remnants of planetary systems. In a few cases these planetary remnants form a circustellar debris disc which can be detected as an infrared excess. Here we present the results of high-resolution spectroscopic follow-up of 15 of the newly identified metalpolluted white dwarfs. Using accurate spectral analyses of the atmospheres of these white dwarfs we determined chemical compositions and masses of the accreted bodies, and discuss the impact of these finding on the current knowledge of extra-solar planetary systems. Using optical and infrared photometric data from various large-area surveys we carried out a search for infrared excess around our newly identified metal polluted white dwarfs, and high-confidence white dwarf candidates. We identified four metal polluted white dwarfs with possible debris discs and compiled a list of ≃ 300 white dwarfs candidates with infrared excess ready for future spectroscopic follow-up.

Most stars in the Galaxy are found in multiple systems of two or more stars orbiting together. Two stars orbiting around their centre of mass are called binary stars. In close binary stars, the evolution of one star affects its companion and evolutionary expansion of one star allows for mass exchange between the components. In most cases, the material from the less massive star forms an accretion disc around the heavier companion that has evolved into a compact stellar remnant, the final state of stellar evolution. We call these systems compact binary stars (CBs). The study of CBs is key to the development of two fundamental phenomena: accretion and evolution of binary stars. Statistical information on CBs can be deduced by extracting common properties and characteristic system parameter distributions from observed data. But, despite being fundamental for a wide range of astronomical phenomena, our comprehension of their formation and evolution is still poor, mainly because of the limited knowledge of crucial orbital parameters. This lack of reliable orbital parameters estimation is mainly due to observational handicaps, namely, the accretion disc outshines the system components. Astronomers have developed different techniques to overcome this, but are often very dependant of the signal to noise ratio of the data or are only able to obtain via target of opportunity programs (wait until the target is brighter). The focus of this work is to test and develop techniques, based on indirect imaging methods, that can overcome the main observational handicaps to estimate orbital parameters of CBs. We combine these techniques with the exploitation of more “exotic” emission lines that trace the irradiated face of the donor star, namely Ca II NIR triplet and the Bowen blend. We made use of empirical properties of Doppler tomography to estimate the values of the phase zero Á0 and the velocity of the irradiated face of the secondary star (Kem). We then used synthetic models accounting for an irradiated secondary to fit our measured Kem and perform a K-correction to derive the radial velocity of the secondary K2. To derive K1, we used the centre of symmetry technique, testing its validity among several emission lines and the stability of the results depending on the selected area. Having strong constraints for K1 and K2, we find estimates for the mass ratio q. Furthermore, we developed a variation from the Doppler tomography secondary emission method to constrain the value of the systemic velocity ƴ. We derive meaningful uncertainties of these parameters with the bootstrap technique. Using these techniques, we have successfully set dynamical constraints on the radial velocities of the binary components of CBs and derived fundamental orbital parameters, including the mass ratio, using basic properties of Doppler tomography.

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

This thesis concerns inferring core collapse supernova physics using gravitational waves. The mechanism through which the supernova is re-energised is not well understood and there are many theories of the physical processes behind the so called supernova mechanism. Gravitational waves provide an opportunity to see through to the core of a collapsing star. This thesis provides an algorithm that will analyse a detected gravitational waveform from a core collapse supernova and identify the supernova mechanism. This is achieved through the use of Bayesian model selection and a nested sampling algorithm. This Bayesian data analysis algorithm is called the Supernova Model Evidence Extractor (SMEE). SMEE is designed to classify detected gravitational waveforms from core-collapse supernovae and 3 different versions which employ different types of data have been developed. These 3 versions utilise time domain (which has been fast Fourier transformed into the frequency domain), power spectrum domain and spectrogram data and the success of each version is investigated. Firstly, results for a simplified idealised version of SMEE are discussed. In this scenario only a single gravitational wave detector is considered and the effect of the sky position of the source are ignored. Next, techniques which can be employed to improve SMEE are investigated. Finally, SMEE is tested using 3 gravitational wave detectors and the full effect of the time delay between detectors and the antenna response on each detector is included. As well as this, recoloured detector noise from the Science runs from both LIGO and Virgo are utilised here. This thesis demonstrates that each version of SMEE is successful and are able to infer the supernova mechanism for a galactic supernova. The spectrogram version of SMEE is deemed the most accurate and it is recommended that this technique should be further explored in the future.

This thesis concerns the analysis of continuous gravitational waves from neutron stars with non-axisymmetric rotational motion using data from ground-based interferometric gravitational wave detectors, the development of a computationally efficient algorithm for analysis of this data and the use of this algorithm in follow up searches, which were previously too computationally expensive to consider.

Since the early 1990s, several groups of comoving, coeval stars younger than 100Myr and within 100 pc have been revealed. Studying and identifying members in these ‘Nearby Young Moving Groups’ (MGs) is vital because they provide: well characterised samples to test pre-main sequence evolution; ideal targets for direct imaging of exoplanets, discs and brown dwarfs; observational evidence for the birthsites of stars in the Solar neighbourhood. Spectroscopy is used to perform tests of membership for 24 M-dwarf candidates of both the Beta Pictoris MG (BPMG) and AB Doradus MG, confirming 8 and 6 new members, respectively. Measurements of lithium provide a precise age for BPMG, using the ‘Lithium Depletion Boundary’ (LDB) technique. This represents the most accurate age yet determined for this important MG and is about double what has been commonly assumed in the literature from other methods. A kinematically unbiased sample of 146 X-ray emitting FGK stars in the Northern hemisphere with short rotation periods chosen from the SuperWASP All-Sky Survey were spectroscopically investigated to assess their ages and kinematics. The search identified 26 stars younger than 200Myr based on their photospheric lithium. Whilst most of these were not associated with any MG, seven are comoving with the sparse (mostly Southern) Octans-Near MG. Infrared photometry is used to identify debris discs amongst M-dwarfs in MGs and their debris disc fractions are compared as a function of mass and age. Eight percent of the sample younger than 40Myr were identified as debris disc objects, although some may have remained undetected because the sensitivity limits for detecting debris discs around M-dwarfs is lower than for higher-mass stars. No debris discs were observed in MGs older than 40Myr, suggesting the timescale for disc removal is more rapid than for higher-mass stars.