This thesis studies type Ia supernovae (SNe Ia) discovered by the Palomar Transient Factory (PTF). We use this large, high-quality imaging and spectroscopic dataset to analyse the very earliest part of the SN Ia light curve, probing the ejecta structure, the postmaximum light curve, the 56Ni distribution, and the utility of SNe Ia as standard candles at different wavelengths. The high cadence of PTF makes it capable of studying SNe Ia at very early times, as they rise just after explosion. In this thesis we use 18 SNe Ia from PTF and the La Silla-QUEST variability survey (LSQ) to measure the time between first light, t0, and maximum light (t_rise), and rise index (n), where f = (t - t0)n. We find that n shows significant variation (1:48 ≤ n ≤ 3:70), and has a mean value n̄ = 2.44 ± 0.13, inconsistent with a simple fireball model (n = 2) at 3σ. We calculate an average t_rise of our sample t_rise = 18:98 ± 0:54 days, and find that the variation seen in t_rise and n is principally driven at the very earliest epochs. We use a further sample of 31 PTF SNe Ia to explore the diversity of the post-maximum light curve in R-band, as the emission at these phases is linked to the same driver of variation - the distribution of 56Ni through the ejecta. By modelling the shoulder on the light curve we quantify the strength and timing of this feature. We find that the timing of the additional emission is correlated with light curve width, but that the duration is not. We also find that the prominence of the gaussian, relative to its peak, is inversely correlated with the light-curve width. We interpret this as the impact of intermediate mass element spectral features dominating over those of iron peak elements. Having explored the physics that governs these objects, we turn to the primary motivation for studying SNe Ia - their use as standardisable candles. We consider 60 SNe Ia in three filters, B, R and i, constructing hubble diagrams for each. We find that the magnitude of the corrections for light curve width and SN colour decreases as a function of wavelength, and that SNe Ia in i-band do not need correction for light curve width. We also find that the colour law observed for our sample is not consistent with the standard Milky-Way extinction law, with RV = 3.1. Finally, we find that SNe Ia in i are superior standardisable candles to B-band, and that they are overall the best in R. This sample is well placed to anchor future high-redshift rest-frame i-band cosmological studies.

A multiwavelength study of the circumstellar matter distribution of massive young stellar objects (MYSOs) was conducted. First, the potential of the new Herschel 70 micron data to resolve MYSOs in the Hi-GAL survey was analysed. These data have the highest resolution achieved at far infrared wavelengths where the spectral energy distribution of MYSOs peaks. These data showed that relatively isolated sources with high L^0.5/d, where L is the luminosity and d the distance of the source, are resolved at 70 micron. The analysis of these data and 1-D spherically symmetric radiative transfer modelling of three sources in the l=30 deg and 59 deg fields showed that they have a shallower density power law index than expected for infalling material. This suggests that the far-IR emission may be dominated by warm dust from the outflow cavity walls rather than rotational flattening as suggested by earlier studies. In order to explain the 70 micron observations, the circumstellar matter of the proto-typical MYSO AFGL 2591 was studied by utilising and modelling full resolution Herschel data from the HOBYS survey and other multi-wavelength dust continuum observations including high-resolution near-IR and mm interferometric data. A 2-D axi-symmetric radiative transfer model was used to find the density and temperature distributions that better reproduce the observations. The model that best fits the continuum observations has a rotationally flattened envelope, paraboloidal outflow cavities and a flared disc with a mass of 1 solar mass. As a result it was found that the extended emission observed at 70 micron can be explained in part by dust emission from the envelope outflow cavity walls. The modelling was able to reproduce most of the other multi-wavelength observations. Finally, the velocity structure of gas in the envelope of AFGL 2591 was studied by modelling methyl cyanide observations in the CH3CN J=12-11 transition at 1.3 mm. The transition K-ladder was fitted assuming a constant density and isothermal distribution of gas, and an excitation temperature ranging between 100-300 K was found. In addition, the first moment (velocity) maps are consistent with rotation of the inner envelope, and its linear velocity gradient is slower than the one observed at smaller scales. The radiative transfer modelling of the methyl cyanide data with a velocity structure of a rotating and infalling envelope suggests that rotation is faster than predicted by the model. This may be solved by magnetic fields transporting angular momentum from the accretion disc.

Circumstellar discs of pre-main sequence stars undergo different processes depending on the nature of the circumstellar environment, which is governed by stellar mass. I have performed numerical simulations of the circumstellar regions of classical T~Tauri stars (CTTs) and Herbig~Ae (HAe) stars using the radiative transfer code TORUS in order to test the paradigm of magnetospheric accretion in CTTs, and to ascertain the nature of the material in the inner regions of HAe discs. The process of magnetospheric accretion (MA) involves disc material attaching to stellar magnetic field lines and impacting the photosphere, producing accretion shocks. When the magnetic field is inclined to the star, disc warps form which periodically occult the photosphere. With specific reference to the CTTs AA Tau I perform three-dimensional MA models to study this variability. By comparing synthetic photometry with observational data I show that the geometry of the system can be constrained. I go on to study Balmer line profiles in the context of MA and disc wind outflows. I present three-dimensional models of a system comprising the star, magnetosphere, disc, and disc wind, producing synthetic line profiles and images. Using these profiles I perform time-series fitting to observational data and demonstrate that the mass accretion rate, mass loss rate, and magnetosphere temperature can be constrained. I show that there is a degeneracy between wind temperature and wind acceleration which require alternative methods to constrain further. While an outflow model alternative to a sole disc wind may produce better fits to observations, MA models reproduce various observational features well. Finally I test the hypothesis that refractory grains produce the innermost emission in HAe discs. Focussing on the HAe stars MWC 275 and AB Aur, I perform radiative equilibrium modelling to create synthetic images of these objects from which interferometric visibility profiles are produced. I show that the temperatures at which these refractory grains are required to survive are too high to be physically plausible. I also find that silicate dust is shielded when sufficiently high mass fractions of refractory grains are used, enabling the silicates to survive interior to the classical sublimation radius. While refractory dust may provide a significant contribution to the emission observed in these inner regions, this alone is not sufficient.

Starbursts and starburst-driven outflows play a central role in the evolution of galaxies. However, the paucity of detailed observations of superwinds limits our current understanding of these complex systems. To this end we have undertaken two intensive ground- and space-based observing campaigns aimed at studying the ionized gas conditions in two nearby starburst galaxies, M82 and NGC 1569. These two systems host starbursts on different scales: M82 contains densely-packed star cluster complexes that drive a large-scale bipolar superwind, whereas NGC 1569 exhibits a set of discrete superbubbles powered by only a handful of young massive clusters. We have used long-slit spectra, obtained with the Hubble Space Telescope (HST), together with HST and ground-based imaging from the WIYN 3.5 m telescope, to observe M82 at optical wavelengths. The high quality HST spectroscopy obtained with the Space Telescope Imaging Spectrograph (STIS), have allowed us to investigate the properties of the gas across the starburst core. By combining high-resolution HST imaging with deep WIYN observations, we have created the most comprehensive image of the M82 superwind to date, and used it to characterise the outflow morphology. We also observed the centre of NGC 1569 with the Integral Field Unit (IFU) of the Gemini Multi-Object Spectrograph (GMOS) on the Gemini-North telescope, and M82 with the WIYN/DensePak and SparsePak IFUs. We decomposed the observed emission-line profile shapes, and identified an underlying broad ( > 100 kms-1) component across the starburst cores of both galaxies. By mapping the spatial variation of each individual line component, we have developed a new model to explain the broad emission and the state of the interstellar medium (ISM) in the central starbursts. We have also observed the outer-wind environment of NGC 1569 with the WIYN SparsePak instrument. We find that the broad line is only found within 500-700 pc of the centre, and speculate that the boundary of this region may indicate the point at which bulk motions begin to dominate over turbulence.

Stellar feedback refers to the injection of energy, momentum and mass into the interstellar medium (ISM) by massive stars. This feedback owes to a combination of ionising radiation, radiation pressure, stellar winds and supernovae and is likely responsible both for the inefficiency of star formation in galaxies, and the observed super-sonic turbulence of the ISM. In this thesis, I study how stellar feedback shapes the ISM thereby regulating galaxy evolution. In particular, I focus on three key questions: (i) How does stellar feedback shape the gas density distribution of the ISM? (ii) How does feedback change or influence the distribution of the kinetic energy in the ISM? and (iii) What role does feedback play in determining the star formation efficiency of giant molecular clouds (GMCs)? To answer these questions, I run high resolution (dx~4.6 pc) numerical simulations of three isolated galaxies, both with and without stellar feedback. I compare these simulations to observations of six galaxies from The HI Nearby Galaxy Survey (THINGS) using power spectra, and I use clump finding techniques to identify GMCs in my simulations and calculate their properties. I find that the kinetic energy power spectra in stellar feedback- regulated galaxies, regardless of the galaxy's mass and size, show scalings in excellent agreement with supersonic turbulence on scales below the thickness of the HI layer. I show that feedback influences the gas density field, and drives gas turbulence, up to large (kiloparsec) scales. This is in stark contrast to the density fields generated by large-scale gravity-only driven turbulence (i.e. without stellar feedback). Simulations with stellar feedback are able to reproduce the internal properties of GMCs such as: mass, size and velocity dispersion. Finally, I demonstrate that my simulations naturally reproduce the observed scatter (3.5-4 dex) in the star formation efficiency per free-fall time of GMCs, despite only employing a simple Schmidt star formation law. I conclude that the neutral gas content of galaxies carries signatures of stellar feedback on all scales and that stellar feedback is, therefore, key to regulating the evolution of galaxies over cosmic time.

Recent observations of ultra-cool objects suggest that the gas in their uppermost atmospheres is heated, ionised and magnetised to levels that radio and X-ray emission is possible. The aim of this work is to identify which low mass objects are most susceptible to plasma and magnetic processes by the effect of thermal ionisation and Lyman continuum photoionisation from the irradiation of external sources in different environments. I focus my work on very low mass, ultra-cool objects (late M-dwarfs, brown dwarfs and giant gas planets: T[sub]eff = 1000 . . . 3000 K) to analyse the electrostatic and magnetic character of these atmosphere. This work has been carried out in two distinct parts: a) A reference study to identify which ultra-cool objects are most susceptible to plasma and magnetic processes considering the thermal ionisation as the only mechanism to ionise the atmospheric gas. This presents a theoretical framework using a set of fundamental parameters to analyse the ionisation and magnetic coupling state of objects with ultra-cool atmospheres. The DRIFT-PHOENIX model atmosphere simulations are used to determine the local gas properties T[sub]gas [K], p[sub]gas [bar], pₑ [bar] of the atmospheres structures from the global parameters T[sub]eff [K], log(g) [cm s[super]-2] and [M/H]. Electrostatics interactions dominance over electron-neutral interactions increases as T[sub]eff increases: throughout the M-dwarf atmospheres, almost for the whole brown dwarf atmospheres and only giant gas planets with T[sub]eff =1200 K, log(g)=3.0, [M/H]=+0.3 fulfil that criterion at the most deeper atmosphere. A magnetised gas is found for M-dwarfs, brown dwarfs and giant gas planet atmospheres even for those with a small degree of ionisation except to the most deeper atmospheric regions. Hence, the upper atmosphere of all of studied objects can be magnetically coupled. A large fraction of the atmospheric volume of M-dwarfs and of early spectral subtypes of brown dwarfs is found to occur plasma processes and magnetised gas, and are therefore the best candidates to emit in Hα and radio wavelengths. b) An analysis of how the Lyman continuum external irradiation effect the plasma and magnetic state of an ultra-cool atmospheres in additional thermal ionisation. The Monte Carlo radiative transfer code simulations provide the atmospheric ionisation structure due to photoionisation. Three different cases are studied to see the effect of the Lyman continuum irradiation given by an external source: free-floating ultra-cool objects irradiated by Lyman continuum photons from the interstellar medium (dominated by Galactic O and B stars), brown dwarfs in star forming regions irradiated by a nearby (few parsecs) O star and binary systems where a brown dwarf atmosphere is photoionised by a companion white dwarf. The effect of Lyman continuum irradiation from external sources greatly increases the level of ionisation in the uppermost atmospheric regions. Brown dwarfs in star forming regions and brown dwarfs in binary systems with a companion white dwarf have upper atmospheres that are close to being fully ionised. Adopting temperatures typical for a chromosphere or a corona, the resulting free-free X-ray luminosities are found to be comparable with those observed from brown dwarfs in star forming regions. The results of this study show that it is not unreasonable to expect powerful energy emissions from brown dwarf atmospheres. The conclusions of this study are that the thermal ionisation allows the establishment of a magnetised plasma in brown dwarf atmospheres as in particular the rarefied upper parts of the atmospheres despite having low degrees of thermal gas ionisation and that the Lyman continuum photoionisation allows to argue for a chromosphere/corona on brown dwarfs and for X-ray emissions from ultra-cool objects. However, other mechanisms, like Alfvén wave heating, are needed to occur which then lead to rise the local gas temperature in the chromosphere/corona on brown dwarfs.

This thesis presents the first detailed analysis of three extragalactic fields observed by the infrared satellite, AKARI. AKARI is the only telescope able to observe deeply in the Spitzer/IRAC - Spitzer/MIPS band gap of 8-24 microns. The first analysis of these extragalactic fields, was to perform galaxy number counts, the most basic statistical property of galaxy populations. Presented are the counts at 3, 4, 7, 11, 15 and 18 microns. These number counts were compared with published results and galaxy evolution models. These models are dependent on both the Spectral Energy Distribution (SED) templates and evolution of the galaxy types. The phenomenological backwards evolution model of Pearson 2005 appeared to be consistent with the number counts. To probe this, number counts were extracted from below the survey limit by performing Probability of Deflection (P(D)) fluctuation analysis. The Pearson model was found to be consistent with the 11 microns P(D) analysis but not with the 15 microns. The results from the 15 microns P(D) analysis indicates that the Pearson model under predicts the evolution of star forming galaxies. Multi-wavelength band-merged catalogues for the two deep extragalactic fields were created using AKARI data from the work of this thesis and ancillary data. The separation of galaxy type using AKARI/IRC filter colours was explored. For z < 2 a separation criteria was found for Active Galactic Nuclei (AGN), but to accurately classify galaxy type, SED fitting is required. Submm-selected galaxies detected by AKARI were found to resemble scaled-up normal local spiral galaxies, rather than starbursts, consistent with them lying on the high-redshift `main sequence' unlike local ultraluminous infrared galaxies. The thesis concludes by measuring the percentage of the far-infrared Cosmic Infrared Background (CIB) resolved by AKARI mid-infrared galaxies. By performing stacking analysis on a Herschel/SPIRE 250 microns image of AKARI galaxy positions, it was found that AKARI mid-infrared galaxies resolve ~10% of the 250 micron CIB.

In this thesis, I present analysis and interpretation of the multiwavelength spectra and variability of Active Galactic Nuclei (AGN). The most luminous sustained sources in the Universe, these powerful objects are consistent with being the result of gas accretion on to central galactic supermassive black holes. Due to their compact sizes, the inner regions of AGN cannot be spatially resolved by conventional means, so we must instead use spectroscopy and temporal monitoring to determine their composition and structure. I undertake a number of studies of the spectral energy distributions (SEDs) of AGN, using data from infrared-X-ray bands and employing a range of numerical models. Results from SED modelling of 11 moderate redshift (1.5 < z < 2.2) AGN are presented, in which there is a selection bias towards nuclei with cooler accretion discs. I find that the peak of the SED is sampled by our data for 5/11 objects, thereby breaking several of the model degeneracies that affected previous studies. This results in stronger constraints on the physical processes at work in these AGN, and provides a powerful tool with which I examine and discuss the relationships between the various radiating components, including those of the emission line regions, dusty torus and host galaxy. I then explore the nature of four 'hypervariable' AGN, for which the origin of their extreme variability is currently unknown. Through an investigation of their SEDs, I find that either an accretion rate change, or gravitational microlensing by a star in a foreground galaxy, are energetically consistent with the data. The new insights provided by this work lead me to suggest several worthwhile routes for the future development of research in these areas. With the next generation of telescopes, satellites and surveys on the horizon, it will be possible to build on my results, to further our understanding of AGN.

Active galactic nuclei (AGNs), the sites of mass accretion onto supermassive black holes, have been hosted by most galaxies at some point in their lifetime. X-rays are a direct and efficient means of identifying AGNs and measuring their intrinsic properties reliably. A recent breakthrough in this regard is the Nuclear Spectroscopic Telescope Array (NuSTAR), the first space satellite observatory with the ability to focus high-energy (i.e., ``hard''; >10 keV) X-ray photons. In this thesis I use NuSTAR to study the distant hard X-ray emitting AGN population, with a view to improving the cosmic census of AGNs as well as understanding their demography and evolution. In addition to these broad goals, a more specific focus is to identify elusive Compton-thick (CT) AGNs, which may represent an important phase of hidden black hole growth. Two overall approaches are taken: (1) optically selected Type 2 quasars suspected to be CT (i.e., candidate CTQSO2s) are deliberately targetted with NuSTAR; and (2) a large and unbiased serendipitous survey of ~500 X-ray sources is performed using almost all of the science data taken with the NuSTAR observatory over a 40-month period. For both of these complementary samples, the broad-band X-ray and multiwavelength properties are studied. For the candidate CTQSO2s, the addition of > 10 keV NuSTAR data provides an improvement compared to constraints with Chandra and XMM alone (i.e., with the most sensitive observatories at < 10 keV), generally allowing significantly higher column densities (Nh) and intrinsic AGN luminosities (Lx) to be constrained, and providing strong evidence for CT absorption in some cases. Implications for the Nh distribution of Type 2 quasars are discussed. For the NuSTAR serendipitous survey, an extensive ground-based followup program has been undertaken, which was crucial to obtain spectroscopic redshifts and classifications for the bulk of the sample. The serendipitous survey AGNs cover a redshift range of z=0.002 to 3.4 (median of < z > = 0.56) and a hard X-ray luminosity range of log(L_10-40keV [erg/s]) ~ 39 to 46. Singling out the most extreme likely-CT sources in the serendipitous survey gives an insight into the prevalence of such extreme systems within the general AGN population.

I present the first two- and three-dimensional simulations of accretion disc-driven outbursts in cataclysmic variables and X-ray transients. A Smoothed Particle Hydrodynamics scheme is used to model the accretion disc, which is subject to the full tidal potential of a binary system. The scheme, which includes a time-varying viscosity, is applied to cataclysmic variables and X-ray transients with differing mass ratios and orbital periods. Simulations of the dwarf nova SS Cygni, with mass ratio q = 0.6, produce coherent outbursts and a wide range of other observationally testable results. When applied to the tidally unstable system Z Chamaeleontis (q = 0.15), the scheme reproduces superoutbursts and superhumps. The tidal and thermal instabilities are shown to be entirely decoupled: a superoutburst is simply a normal outburst in the presence of a 3:1 eccentric inner Lindblad resonance. The model is refined for X-ray transients to include a simple treatment of irradiation of the disc by the X-rays generated at the primary. In short periods systems with correspondingly small discs, if q is small tidal effects can influence the outburst behaviour. If a small region at the outer edge of the disc remains shielded from the X-rays, tidal forces can drive this gas inwards through the disc, producing the rebrightening that has been observed in several X-ray light curves of soft X-ray transient outbursts. At longer periods, the disc cannot ever entirely be heated by the X-rays and more complex outburst behaviour is possible. Simulations show that this behaviour is determined by competition between the variable size of the irradiated region and the thermal instability operating in the cool region.