Observational Tests Of The Theoretical White Dwarf Mass-Radius Relation

The mass-radius relation is one of the keys to understanding the structure of white dwarf stars. It has a sound theoretical basis, but improved observational tests are required to confirm if it describes real white dwarfs accurately. This thesis presents tests of the mass-radius relation using two different techniques to measure the mass. The approaches used are the spectroscopic method of fitting models to the hydrogen lines in white dwarf spectra, and the gravitational redshift method first proposed by Einstein in 1916. The first spectroscopic test is a detailed study of white dwarfs in binaries, and makes use of the recently available parallaxes from Gaia DR2. The new data remove the primary source of uncertainty affecting similar previous studies and finds that most white dwarfs agree with the MRR within 2 σ. New results are obtained for several white dwarfs which have never previously been studied in detail. This study shows that the uncertainty remaining in the spectroscopic parameters is too large to test the detailed predictions of the MRR. The subsequent chapters detail results obtained using the gravitational redshift which provide further support for the MRR with greater precision than the spectroscopic results. A troubling discrepancy is found when comparing the mass of Sirius B measured using three different methods. The final chapter is a study of Sirius B using the gravitational redshift in a way specifically designed to remove the systematic uncertainties affecting previous studies. The new data confirm the validity of the gravitational redshift as a means of measuring white dwarf masses. Sirius B is found to be in agreement with both the MRR and the dynamical mass within 1 σ. General Relativity pases the 3rd classical test with flying colours.