Dartmouth Events

Abstract: This thesis uses the oldest stars in the universe to examine several important questions in astronomy. I use large groups of old stars to study the formation mechanisms of globular clusters, thought to be the first objects in the Universe to form stars. I found that five out of six globular clusters examined in the Large Magellanic Cloud, a dwarf satellite galaxy of the Milky Way, contain multiple stellar populations whose origins are still unknown. I test stellar models in the old and therefore metal-poor star regime by obtaining high resolution spectroscopic abundances of 86 single, main sequence stars which have excellent parallaxes from Gaia, finding 19 new metal-poor stars. Stellar models have traditionally been calibrated to match the Sun. However, it is becoming apparent that this calibration should not be used for metal-poor stars. I use these metal-poor main sequence stars to calibrate these stellar models so that they can be more accurately used in many different areas of astronomy, including determining the age of old stars and in the study of exoplanets and their host stars. Finally, I obtained high resolution spectra of 49 local metal-poor variable stars (RR Lyrae stars) to determine their iron content. I combined my sample with 95 RR Lyrae whose iron content had previously been determined from high resolution spectra, along with Gaia parallaxes to calibrate the period-luminosity-metallicity (PLZ) relation in the infrared. The uncertainty in my observational PLZ relations is a factor of ~4 smaller than previous observational calibrations, and can be used as the first rung to independently determine the expansion rate of the Universe, the Hubble constant (H_0). Astronomers disagree on the local and global value of Hubble constant If this discrepancy remains, then physics beyond the canonical model of the Universe (LambdaCDM) needs to be explored.