The existence of water (H2O) and acetonitrile (CH3CN) in various extraterrestrial environments attracts considerable attention as both molecules are crucial to understanding the genesis of life. This thesis involves investigations of the physical and chemical interactions of CH3CN and H2O ices condensed on an amorphous silica surface using ultrahigh vacuum-based surface science methods (reflection-absorption infrared spectroscopy, temperature programmed desorption, proton- and electron-irradiation experiments) in an effort to understand how such molecules will behave when they are processed by radiation and heat in space environments. The thermal behaviour of CH3CN on H2O and on the silica surface is governed by a balance of intermolecular forces and shows evidence for wetting of the silica surface (formation of a distinct first adsorption layer before multilayer growth) and of dewetting or island formation on the H2O surface (multilayer growth for all CH3CN exposures). This illustrates the importance of the substrate in determining the solid state growth morphology. Physical and chemical processes induced by 250 to 500 eV electrons have been investigated in the CH3CN and H2O systems. Such interactions promote only desorption from both CH3CN and H2O ice surfaces. 200 keV protons irradiation experiments of CH3CN ices, conducted by collaborators in Italy, in contrast showed evidence for chemical reactions in the solid CH3CN. Quantitative measurements of desorption and reaction cross-sections are used to reconcile this stark contrast. The impact of such processes in relation to H2O on astrophysics is investigated through simple numerical simulations using results derived from this work.

The interdependence of schemes of spectral classification and theories of stellar evolution is examined and discussed in the context of the work of major 19th and early 20th century astronomers, including A. Secchi, H. Vogel, W. Huggins, N. Lockyer, A. Ritter, F. McClean, W.H.S. Monck, A. Maury, W.P. Fleming, E.C. Pickering, E. Hertzsprung, and H.N. Russell. The nineteenth century concept of evolution through gravitational contraction is identified as a dominant theme, and is analysed within the historical context of the establishment and exploitation of the energy conservation laws during the mid-nineteenth and early twentieth centuries. The factors affecting studies of stellar evolution in the late nineteenth and early twentieth centuries, including developing techniques in astronomical spectroscopy, stellar kinematics, laboratory spectroscopy, astrometry, photography, and the application of physical theory to astronomical problems, form a necessary background to this theme. Finally, the early development of the Hertzsprung-Russell Diagram is critically examined.

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Most dynamo models agree that large-scale, weak poloidal magnetic field is wound up into strong toroidal field through differential rotation. Therefore, the strong radial gradients of angular velocity present in the solar tachocline are extremely likely to play a crucial role in the solar dynamo. Motivated by this, we focus on the instability mechanism thought to be responsible for the break up of a horizontal field in a stably stratified layer, namely magnetic buoyancy instability, and the effect a velocity shear has upon said instability. To study this interaction, we derive a new set of equations incorporating velocity shear and magnetic buoyancy into the Boussinesq approximation. These equations not only provide us with the ability to study the effects of a velocity shear on magnetic buoyancy instability, but also allow us to study magnetic buoyancy instability in the presence of a magnetic field varying on a short O(d) length scale, compared to the equations of Spiegel & Weiss (1982), which are restricted to field variations on a longer O(Hp) scale. Stability criteria for this new system is obtained through a linear analysis on the ideal (diffusionless) system. Motivated by the work of Mizerski et al. (2013) we use the newly derived equations to study the short-wavelength linear magnetic buoyancy instability. We first study this problem in the absence of a velocity shear and dissipation, deriving asymptotic results analogous to Mizerski et al. The governing set of equations are then solved numerically to verify the asymptotic results. A velocity shear is then added into the analysis; we derive new asymptotic results and use them to comment on the influence the velocity shear has on the instability. Upholding the short-wavelength limit, we individually introduce each diffusive parameter into the analysis, observing the role each has on the instability. Finally, we solve the newly derived system of linearised equations numerically, whilst including all diffusive parameters. We comment on the role of the diffusive parameters and investigate how they influence the instability.

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