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.

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 e�ective 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, �bre spectroscopy from the FLAMES spectrograph is used to �nd 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. Modeldependent, 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.

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.

Stars light up galaxies that are the major building blocks of the Universe. Throughout this thesis we cover the analysis of stars and stellar populations to gain knowledge on the formation of galaxies and the evolution of the Universe. Understanding massive earlytype galaxies is key to understand mass assembly and the formation and evolution of galaxies in the Universe. The spectra of stellar populations carry a wealth of information regarding galaxy formation and evolution. Absorption lines are particularly useful as they are tracers of galaxy formation epoch and time-scale. Models of stellar populations are important tools for the analysis of galaxies. The accuracy of such models are crucial for the accuracy of the derived results. The first step in my thesis is therefore to improve upon current single stellar population models of absorption line indices. Calibration of the models with galactic globular cluster is crucial, since these are known to be close to single stellar populations. The main aim of the thesis is to apply the models to the analysis of earlytype galaxies and Type Ia supernovae (SN Ia) host galaxies. The main novelty of the stellar population models are new empirical calibrations of absorption line indices. These are based on the most comprehensive stellar library available to date MILES. The stellar spectra of this library have been carefully flux-calibrated. The models are therefore applicable to data without Lick index calibrations. Based on the new stellar population models we have developed a method for deriving element abundance ratios, including [O/Fe], [C/Fe], [N/Fe], [Mg/Fe], [Ca/Fe] and [Ti/Fe]. The method is applied to galactic globular clusters and we find the models to be well calibrated. The pattern of derived element abundance ratios show strong evidence for self-enrichment within globular clusters. The method for deriving element abundance ratios is then applied to �4000 SDSS early-type galaxies. The element abundance ratios [O/Fe], [Mg/Fe], [C/Fe] and [N/Fe] show strong correlations with stellar velocity dispersion. Using the derived trends of element abundance ratios we constrain the lower time-scale limit of star formation and star-burst components in massive early-type galaxies to �0.4 Gyr. Both in the globular cluster and early-type galaxy study we find that the heavy �-elements Ca and Ti scale with Fe rather than with the lighter �-elements O and Mg. This implies that a significant contribution from SN Ia to the enrichment of heavy �-elements is universally found and puts strong constraints on supernova nucleosynthesis and models of galactic chemical evolution. SN Ia as standard candles connect luminosity distance to redshift space to constrain cosmology. We derive stellar population parameters for a quality selected sample of 84 SN Ia host galaxies. We find that the stretch factor of SN Ia light-curves are mainly dependent on stellar population age, indicating that SN Ia progenitor mass is the main driver of the peak luminosities. We do not find any significant dependencies on host galaxy properties for the scatter in the luminosity-distance relationship after light-curve corrections. This implies that the derived cosmological parameters from SN Ia peak luminosities are robust.

In this work we present new, high-to-intermediate spectral resolution evolutionary population synthesismodels, complementing and extending the widely used Maraston models. The new models are based on four popular libraries of empirical stellar spectra, which has necessitated some modifications to the original code, while keeping much of the original ingredients – such as stellar energetics, treatment of the thermally pulsating asymptotic giant branch, and mass loss recipe – intact. In addition, we have computed models at very high resolution (R = 20000) based on the theoretical MARCS library, that extends far into the infrared region of the electromagnetic spectrum. A library-dependent, but modelindependent comparison is made, where both photometric and spectroscopic similarities and discrepancies are highlighted. We find that stellar population models employing empirical stellar spectra exhibit considerably bluer (B-V) colours compared to models adopting theoretical spectra synthesised from the Kurucz model atmospheres (such as the BaSeL library), but that some differences arise between the empirical libraries due to, in particular, the adopted temperature scale. Furthermore, the results obtained with the theoretical MARCS library are fully consistent with the empirical libraries in this respect. The same effect can be found also in other EPS models that are based on empirical stars. We show that this discovery, whose origin can be traced mainly to cool stars (Teff < 5000 K), leads to improved photometric agreement with both galaxy and Milky Way globular cluster data. Spectral energy distributions of the latter are also used for testing the models concerning their ability to reproduce, through full SED-fitting, the cluster ages and metallicities as derived through independent fitting in colour-magnitude diagrams. In general, the agreement is very promising, although the higher resolution of the new models cannot alleviate the age-metallicity degeneracy in the optical in any significant way. A comparison with models of absorption line indices with variable abundance ratios is also made, both for the full SED-fitting procedure and when measuring indices directly on the SED. We obtain satisfying agreement in the first case, but in the second case only when a subsample of indices are used, the combination of which is little sensitive to abundance ratio effects. As a side track we exploit for the first time for population synthesis purposes the vast stellar database of the Sloan Digital Sky Survey, but find that the metallicity and age range of the stars is currently too narrow for computing sensible population models. Finally, we re-invigorate the Ca K line for the purpose of absorption feature diagnostics, presenting a new version of the index which will help in separating solutions for a key case of the age-metallicity degeneracy.

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.

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.

A survey of A-type stars is conducted with the SuperWASP archive in the search for pulsationally variable stars. Over 1.5 million stars are selected based on their (J − H) colour. Periodograms are calculated for light curves which have been extracted from the archive and cleaned of spurious points. Peaks which have amplitudes greater than 0.5 millimagnitude are identified in the periodograms. In total, 202 656 stars are identified to show variability in the range 5 − 300 d−1. Spectroscopic follow-up was obtained for 38 stars which showed high-frequency pulsations between 60 and 235 d−1, and a further object with variability at 636 d−1. In this sample, 13 were identified to be normal A-type δ Sct stars, 14 to be pulsating metallic-lined Am stars, 11 to be rapidly oscillating Ap (roAp) stars, and one to be a subdwarf B variable star. The spectra were used not only to classify the stars, but to determine an effective temperature through Balmer line fitting. Hybrid stars have been identified in this study, which show pulsations in both the high- and low-overtone domains; an observation not predicted by theory. These stars are prime targets to perform follow-up observations, as a confirmed detection of this phenomenon will have significant impact on the theory of pulsations in A-type stars. The detected number of roAp stars has expanded the known number of this pulsator class by 22 per cent. Within these results both the hottest and coolest roAp star have been identified. Further to this, one object, KIC 7582608, was observed by the Kepler telescope for 4 yr, enabling a detailed frequency analysis. This analysis has identified significant frequency variations in this star, leading to the hypothesis that this is the first close binary star of its type. The observational results presented in this thesis are able to present new challenges to the theory of pulsations in A-type stars, with potentially having the effect of further delaying the full understanding of ‘so simple a thing as a star’.

Spectroscopic and photometric observations of six late-type contact and near-contact binaries are presented, and the results of the analyses discussed. Absolute dimensions have been deduced for three systems: YY Ceti. CX Aquarii and RV Corvi. YY Ceti, which has an orbital period of 0.79 days, is found to be composed of a normal main-sequence star of spectral type A8 and an evolved G-type star which completely fills its Roche lobe. We argue that the system will evolve into contact within approximately 10 9 years, either by the nuclear evolution of the primary component, or by the loss of angular momentum via magnetic braking. We also propose that this may represent a route to the A-type contact binaries which does not involve a passage through the W-type phase. The binary CX Aquarii, which has an orbital period of 0.56 days is found to have a similar configuration to YY Ceti, except that its primary component is a main-sequence star of spectral type F5 with an evolved companion of spectral type G9. We argue that the system should achieve contact by loss of angular momentum via magnetic braking within approximately 108 years if the atmosphere of the primary component is convective, or within approximately 5 x 108 years if the primary possesses a radiative atmosphere. RV Corvi is found to consist of an unevolved F2 primary component with a K-type companion which has a much larger radius and luminosity than expected for its mass. The system has an orbital period of 0.75 days. Although the binary may be semi-detached, with the primary component completely filling its Roche lobe, it is most probably in a state of marginal contact. In order to obtain a solution to the photometric data it was necessary to treat the secondary component albedo as a free parameter, yielding a value greater than unity. This result implies that there is an abnormal distribution of luminosity on the surface of the binary, which may be interpreted either as an excess of light on the facing hemisphere of one or both of the components, or as a deficit of light on the averted hemisphere of the secondary. The three remaining binaries, EZ Hydrae, AD Phoenicis and RS Columbae, were all identified as contact systems: EZ Hydrae as a W-type system of orbital period 0.45 days, AD Phoenicis as an A-type system of period 0.38 days, and RS Columbae as an A-type system with a period of 0 .67 days. Although a value for the mass ratio of EZ Hydrae had been obtained from spectroscopic observations, no photometric solution could found because of severe 'disturbances' in its light curve. From our sample of six systems, AD llioenicis and RS Columbae were the only two for which there were no spectroscopic observations. Although photometric solutions were obtained for both systems, these were very insensitive to the value of mass ratio, which tended toward the physically unrealistic value of unity. The results for YY Ceti, CX Aquarii and RV Corvi are combined with the published masses, radii and luminosities of 21 other contact and near-contact binaries. It is shown that the primary components of all contact and near-contact systems are normal main-sequence stars with radii and luminosities appropriate to their masses. The secondary components of the B-type systems and the W-type systems are all overluminous for their masses, due mostly to the radii being increased by a factor of ~1.7. However, these secondary components are segregated on the H-R diagram, where the W-type secondaries appear to the left of the main sequence band due to luminosity transfer from the primary components. The secondary components of the A-type contact systems all have radii and luminosities substantially larger than expected for their masses.

This work investigates the small scale structure of the Diffuse Interstellar Medium. To do this optical spectroscopy is used to obtain spectra of early type stars which are used as background targets with which the Diffuse Interstellar Medium (ISM) is probed. The spectra obtained contain the highly diagnostic Diffuse Interstellar Bands (DIBs), Na i D and Ca ii lines. The maps I present here are of the Local Bubble, the Small Magellanic Cloud and the Large Magellanic Cloud. These are the first DIB maps of the solar neighbourhood and large portions of external galaxies. The spectra were obtained with the New Technology Telecsope (NTT) at La Silla Observatory in Chile (Local Bubble survey) and at the Anglo-Australian Telescope (AAT) at Siding Spring Observatory, NSW, Australia. The NTT spectra are long slit spectra of 239 individual targets, whilst the AAT spectra were obtained with the multi-fibre spectrograph 2dF/AAOmega (about 350 targets in each of the Magellanic Clouds). I have successfully used the 5780 and 5797˚A DIBs to map the ISM in the Local Bubble and the Magellanic Clouds. The 5797˚A DIB traced neutral structures whereas the 5780˚A DIB traced warmer and/or more highly irradiated gas, possibly residing in the skins of those neutral clouds It showed a more highly structured Local Bubble than revealed by the sodium maps, on sub parsec scales; tracing the walls of the Bubble and clearly showing the Bubble opening out into the Halo. In the Magellanic Clouds the DIBs trace molecular clouds surrounding regions of active star formation; they are weak or absent in quiescent molecular cloud complexes and hot gas bubbles.