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Sects. 12, 13. 89 sequence and that subgiant and fainter stars in globular clusters have ultraviolet excesses. When dealing with stars whose physical properties are imperfectly under stood, such as in globular cluster stars, we cannot rely too heavily on the empiri cal calibration by the kinds of stars used to define Fig. 5, to determine their true, unreddened U-B, B-V curve. But if by a combination of arguments, principally the reddening in the region of the stars we do known about, we can assign a fairly probable unreddened U-B, B-V curve to a group of stars about which we know little, the argument may be turned around. In this case some information may be gained about the energy envelope of the stars by examining the differences between the normal two-color index curves for the unknown group of stars compared to the known. In general there seem to be two possible causes for different stars defining different normal sequences in the U-B, B-V plane. One, the relative energy distribution in the continuum in the U, B and V photometry bands are different. An example of this is the effect of the Balmer depression in supergiants. This, of course, requires deviation from black body radiation curves for one or both groups of stars. This cause seems to be the dominant effect for very blue, hot stars where the depression of the continuum by absorption lines is at a minimum.
This thesis addresses two very different but equally important topics in the very broad fields of astrophysics and cosmology: (I) the generation of cosmological magnetic fields and (II) gravitational fragmentation of the Cosmic Web. All mathematical developments are completed by illuminating physical interpretations, and the thesis, which is guided by existing observations, is purely theoretical. In part I, the author further develops a magnetogenesis model proposed in the literature, providing an unprecedented level of physical understanding. He demonstrates that the physics of photoionisation is very likely to have premagnetised, at a relevant level, the entire Universe at the early epoch of the formation of the first luminous sources. In part II, the author adapts the tools of plasma spectral theory to the context of gravitational instability of the baryonic gas within the stratified structures of the Cosmic Web. He skillfully derives the wave equation governing the growth of perturbations and explores various equilibrium configurations, in planar and cylindrical geometries characteristic of cosmic walls and filaments, for isothermal and polytropic conditions, with or without an external gravitational background. Clearly structured and written in pedagogical style, this outstanding thesis puts the results into perspective and highlights the merits and limitations of the various approaches explored.
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