We present results from high-precision photometric monitoring of several bright early-type stars. The data cover five years and are collected by the STEREO satellite. Stellar types covered are a sample of CP, δ Scuti, and Be-stars. We describe STEREO data and data pipeline developed to produce final light curves. We studied long-term variations in the rotational periods of 14 CP stars. With our pipeline, we were able to accurately determine their periods, and hence we investigated any period changes. The second study concerned Be-stars and looked at the connection between disk structures and non-radial pulsations. STEREO photometric data were supported by spectroscopic information on the Hα line. As a result, we found that, in some cases, variations in emission line properties were connected with changes in pulsation characteristics, and we discussed the consequences of this situation. In the final study, which involved a sample of δ Scuti stars, STEREO data were used to investigate the stars' pulsational properties and to look for any possible changes in their pulsations. Pulsation constants and modes of the main frequencies were identified, and regular spacings in frequencies were examined. Several cases in which pulsational characteristics changed were also determined, and their consequences were interpreted.

It has been known for some time that many early–type galaxies possess halos of X–ray emitting gas. However, the properties of these halos and the processes by which they form are as yet not well understood. We have compiled the largest catalogue of X–ray luminosities to date, and use it to examine the relationships between X–ray luminosity and galaxy age, environment and optical luminosity. It is shown that the mass of the X–ray halo increases with galaxy age, and that there is no trend in LX/LBwith environment. Group dominant galaxies are shown to be more X–ray luminous than galaxies in other environments, and the effect of this on the LX:LBrelation is explored. A smaller sample of highly X–ray luminous galaxies is studied in more detail, and their properties compared to those of galaxy groups and clusters. It is shown that while galaxy halos are similar to those of larger structures they differ in that their surface brightness profiles do not vary with system temperature. It is also shown that early–type galaxies have lower gas fractions than groups and clusters, probably owing to gas loss from the system through galaxy winds.

The thesis focuses on the use of asteroseismology to probe the structure of stellar sources. By observing pulsational variability, a detailed description of the interior of stars, and the determination of stellar parameters can be obtained. An extensive sample of bright K and M class giant stars were surveyed to detect the presence of any noticeable variability. Three giant stars have been studied in detail. In Arcturus (alpha Boo), no oscillations at a significant amplitude were detected. In beta UMi (Kochab) multiperiodic oscillations have been observed for the first time, allowing an asteroseismic estimate for the mass. In gamma Crucis (Gacrux) previously observed frequencies have been confirmed, and the modes shown to be sinusoidal in nature. Two main-sequence stars have been studied in detail. In the first, gamma Dor, prototype star of the class of gamma Doradus variable stars, three oscillations further to those previously known were unambiguously detected in the star. In the second, nu Eri, a beta Cephei class variable star, modes detected by an extensive previous campaign were confirmed, with determined frequencies in excellent agreement between the two studies. All results gathered should provide a valuable input to future models of these stars, and provide an interesting starting point for further, detailed studies.

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.

For longer than a century, analysis of a quasi-periodic variability of the Sun on various time scales has been attracting great interest among the research community. These signals are continuously detected throughout the whole electromagnetic spectrum, and often have non-stationary oscillation periods and irregular profile shapes. The physical mechanisms responsible for such variations are usually hidden, and their revealing always require an advanced combination of powerful spectral techniques and strong theoretical foundations. This thesis considers oscillatory phenomena occurring in the solar atmosphere from these two perspectives. For the spectral analysis of solar periodicities the self-adaptive Hilbert–Huang transform (HHT) method is employed. With the statistics of coloured noises incorporated, it allowed for the detection of a long-period oscillation of a small-scale photospheric magnetic structure, whose period grows from 80 to 230 min and positively correlates with the amplitude. A multi-modal nature of the solar flare quasiperiodic pulsations was also revealed with HHT. The detected intrinsic modes have mean periods of 15, 45, and 100 s, and can be interpreted as the kink and sausage magnetohydrodynamic oscillations of a flaring loop. Analysis of much longer solar periodicities associated with the magnetic activity cycles 22, 23, and 24 was also successfully performed with HHT, revealing a broad range of intrinsic modes with periods from about a month to several years (including the 11 yr cycle). From the theoretical point of view, the one-dimensional equilibrium and dynamical models of current sheets in the corona have been developed. The equilibrium model provides an inhomogeneous distribution of macroscopic plasma parameters across the current sheet, as found for realistic physical conditions. The dynamical model describes nonlinear oscillations of the current sheet formed by the coalescence of two magnetic flux ropes. The oscillation period is found to be about the ion plasma period or longer, and is prescribed by the current sheet thickness, the plasma parameter β, and the oscillation amplitude. Analytical modelling of finite amplitude transverse oscillations in quiescent prominences situated in a magnetic field dip, is also performed in the thesis. The model is based on the line-current concept and accounts for the interaction of the prominence current with the electrically conductive photosphere. In the linear regime, the horizontal and vertical motions are considered independently, and the mechanical stability of the system is analysed. The oscillation periods are determined by the prominence current, its mass and height above the photosphere, and the properties of the magnetic dip. In the case of finite amplitudes, the horizontal and vertical modes were found to be nonlinearly coupled with each other, and a metastable equilibrium state of the prominence was revealed. The periods of nonlinear oscillations are shown to depend upon the oscillation amplitude.

The extent of dynamical processes in the lower atmosphere of the Sun during solar flares is not fully understood. While it is widely accepted that the majority of the associated flare energy is deposited in the Sun's chromosphere, it is less clear how this energy is transported and how it influences the configuration of material flows. Current models of chromospheric evaporation and condensation assume an upwards expulsion of high-temperature plasma, with an accompanying downwards flow at cooler temperatures. In this thesis, the validity of these assumptions are tested using a combination of observations and modelling, with particular focus given to the Lyman lines of hydrogen.

This thesis explores possible solutions to the dwarf galaxy problem. This is a discrepancy between the number of dwarf galaxies we observe, and the number predicted from cosmological computer simulations. Simulations predict around ten times more dwarf galaxy satellites than are currently observed. I have investigated two possible solutions: dark galaxies and the low surface brightness universe. Dark galaxies are dark matter halos which contain gas, but few or no stars, hence are optically dark. As part of the Arecibo Galaxy Environment Survey I surveyed the neutral hydrogen gas around the nearby galaxy M33. I found 32 gas clouds, 11 of which are new detections. Amongst these there was one particularly interesting cloud. AGESM33-32 is ring shaped and larger than M33 itself, if at the same distance. It has a velocity width which is similar to the velocity dispersion of gas in a disk galaxy, as well as having a clear velocity gradient across it which may be due to rotation. The fact that it also currently has no observed associated stars means it is a dark galaxy candidate. Optically, dwarf galaxies may be out there, but too faint for us to detect. This means that with newer, deeper, images we may be able to unveil a large, low surface brightness, population of dwarf galaxies. However, the question remains as to how these can be distinguished from background galaxies. I have used Next Generation Virgo Survey (NGVS) data to carry out photometry on 852 Virgo galaxies in four bands. I also measured the photometric properties of galaxies on a background (non-cluster) NGVS frame. I discovered that a combination of colour magnitude and surface brightness information could be used to identify cluster dwarf galaxies from background field galaxies. The most effective method is to use the surface brightness-magnitude relation.

The nature of superluminous supernovae (SLSNe), supernovae whose radiated luminosities are a hundred times greater than normal core collapse supernova events, remains an outstanding question in the transient field. Many models for their production have been postulated, although placing constraints upon these models via the properties of the explosions themselves remains challenging. The potential to unlock their progenitor types may be contained within the properties of their host galaxies. Prior studies have shown SLSNe to preferentially occur within faint, star forming galaxies, highly suggestive of a strong connection between progenitor production and environment conditions. Within this thesis I study the photometric characteristics of a sample of SLSN host galaxies, with a particular focus upon their stellar masses, metallicities and star forming properties. To do this I utilise high resolution imaging of a sample of SLSN host galaxies obtained with the Wide Field Camera 3 on the Hubble Space Telescope to study the global, and sub-galactic environments of SLSN events. By considering the photometric properties of these host galaxies within the near infrared and at rest-frame UV wavelengths, I am effectively able to probe the stellar mass and star forming properties of these environments. When compared to the host galaxies of other well-known core collapse transients, such as long gamma ray bursts and core collapse supernovae, constraints may be placed upon the likely progenitors of SLSNe relative to other transient progenitors. I show that on a global scale, the host galaxies of SLSNe are fainter, more compact, lower mass and less star forming than other core collapse transient host galaxies, which is highly indicative of low metallicity environments. I also highlight the diversity in environments exhibited between different spectral subclasses of SLSNe, which itself is reflective of the likely different progenitor routes for the two different subclasses of event. When considered on a sub-galactic scale, SLSNe events are associated with star forming regions within their galaxies, although at present it remains unclear whether these events are linked with the strongest regions of star formation (which would imply younger, more massive progenitors). Finally, I consider the issue of progenitor metallicity threshold estimations, and the consequences of using both global spectroscopic measurements and mass metallicity relation proxies to determine upper limits to progenitor chemical enrichment. I present a robust model for estimating this, incorporating the key sources of scatter in metallicity estimation which may be applied to a host galaxy populations to determine the presence of a threshold within the progenitor population.

Light reflected by planetary atmospheres and/or surfaces is polarised, and the degree and direction of polarisation can yield information that cannot always be gleaned from flux measurements alone. Polarimetric studies of solar system planets can reveal more details about the seasonal variations in their atmospheres, and the variation with orbital geometry can place constraints on the properties of cloud particles. With the advent of extremely large telescopes, and potentially the most accurate instruments ever realised thus far, polarimetry has great potential for both detecting and characterising exoplanets. A key difference when observing exoplanets with respect to the planets of our solar system is that despite the much lower signal-to-noise than solar system planets, we can access them at wider phase angle ranges, thus enabling us to probe their scattering properties more extensively, especially at geometries where the degree of polarisation is highest. This can result in an easier interpretation of the atmospheric characteristics through theoretical modelling. My original contribution to the field that is presented in this thesis is the observation, data reduction and analysis of polarimetric data along with model interpretation of the six most outer solar system planets and Titan. In addition, model results for exoplanets of varying types are shown and discussed. The overall goal is to show that polarimetry is necessary for giving a full description of light reflected by planetary atmospheres and surfaces, and to demonstrate its worth as a diagnostic tool for atmospheric characterisation, from both ground-based observations of solar system and exoplanets, and from in-situ missions to the outer planets, such as a potential atmospheric probe into any of the outer planets.

Over the last few decades it has become clear that metals present within the atmospheres of more than one quarter of white dwarfs signify recent accretion of minor bodies from their planetary systems. Spectral analysis of these metal-polluted white dwarfs allows determination of the accreted body composition, providing the most direct method for measuring the makeup of exoplanetary material. So far, most detailed abundance analyses have mostly been limited to a few systems at a time. In this thesis, I present a sample of 231 cool DZ white dwarfs identified from SDSS spectroscopy. These stars exhibit strong metal lines from multiple elements, permitting detailed abundance analyses of each. Furthermore their low effective temperatures of 9000{4400 K imply corresponding cooling ages of 1{8 Gyr, allowing me to examine some of the oldest planetary systems in orbit of stellar remnants. Across the sample, I found a huge diversity in the metal abundance ratios, with Fe/Ca varying by a factor 100. I developed a simple method for interpreting the rocky geology of the accreted parent bodies, indicating that some were composed of > 80 % crust material, and with > 80 % core material for others. Using the calculated white dwarf ages, I identified a downwards trend of the highest levels of metal pollution for the oldest systems, suggesting their mass reservoirs of exoplanetesimals become depleted on a ' 1 Gyr time scale. Finally, Zeeman split metal lines are found in the spectra of 33 of these systems, with surface magnetic fields in the range 0:25{30 MG. Investigation of this rare combination of metals and magnetism has consequences for the formation of white dwarf magnetic fields, and motivates new research in atomic physics.