Baron, EdwardDerKacy, James M2022-07-262022-07-262022https://hdl.handle.net/11244/335962Type Ia Supernovae (SNe Ia) are important astrophysical objects. They produce roughly half of the iron group elements found in the universe, the energy they release drives the evolution of galaxies, and their high intrinsic luminosities allow them to be seen across cosmological distances. Through the width-luminosity relation (often called the Phillips relation), they can be used as ``standardizable candles" to serve as cosmological distance indicators and were instrumental in the discovery of the accelerating expansion rate of the universe. However, despite decades of detailed study, many fundamental questions about these objects remain; including the exact nature of their progenitor systems and the mechanism(s) by which they explode. UV spectra are unique probe of SNe Ia physics and their evolution in throughout the history of the universe, as much of the information about the properties of the progenitor star and the explosion mechanism are encoded in the outer layers of the ejecta; a region that ultraviolet spectra probe at later times than the optical. The ultraviolet properties of SNe Ia are much more diverse than in the optical and near infrared, and may vary with redshift. The ultraviolet properties of SNe Ia have the ability to help us unlock their true nature; but have historically been under-studied due to difficulties in obtaining observations in this wavelength regime. However, recent growth in the data sets of SNe Ia observed with the Hubble Space Telescope and the Neil Gehrels Swift Observatory (Swift), have illuminated the need for detailed modeling of this region in order to perform the differential comparisons necessary to further our understanding of these objects and improve their use as cosmological distance indicators. In Part I, I discuss the theoretical foundations of this work. I briefly review the important aspects of spectral formation in the ultraviolet of SNe Ia, and introduce the two codes SYNOW and PHOENIX) used to generate synthetic spectra of SNe Ia. I apply the spectra generated from these codes to the UV spectra of SN~2011fe and for the first time make line identifications in all the major ultraviolet features near maximum light, including the first ever identifications of C IV and Si IV in a SNe Ia spectrum. Then, using the suite of PHOENIX models, I explore the impact of luminosity variations on the ultraviolet spectra and discuss the connections I find between the ultraviolet and other wavelength regimes. In Part II, I shift focus and discuss how differential comparisons in observational studies can further our understanding of SNe Ia. Chapter 4 details the science cases behind nearly five years of observations using the Astrophysical Research Consortium 3.5-m telescope at Apache Point Observatory, including highlighting instances where my observations contributed to the advancement of our understanding of the underlying physics of all types of supernovae and their progenitors. Chapter 5 focuses on SN 2021fxy, a Type Ia supernovae observed extensively in multiple wavelength regimes by the Precision Observations of Infant Supernovae (POISE) collaboration, for which ultraviolet spectra were obtained with the Hubble Space Telescope. In comparing SN 2021fxy to the broader sample of spectroscopically normal SNe Ia with ultraviolet spectra from HST, I show that mid-ultraviolet flux suppression is a common feature of SNe Ia and discuss possible mechanisms that cuase this flux suppression and how they may be connected to different progenitors and explosion mechanisms. Additionally, I show that SN 2021fxy is substantially similar to another SN Ia with mid-ultraviolet suppression, SN 2017erp, and illustrate how luminosity variations between the two SNe Ia may be responsible for the observed flux differences between them.Physics, Astronomy and Astrophysics.SupernovaeUltravioletRadiative Transfer