A major goal of the field of Galactic archeology is to understand the formation and evolution of the Milky Way disk. Stars migrate to different Galactic radii throughout their lifetimes, often leaving little dynamical signature of their initial orbits. Therefore, we need to look at the archaeological record preserved in stellar chemical compositions, which is indicative of their birth environment. In this thesis, we use the measurable properties of stars (chemical compositions and ages) to reconstruct the Milky Way disk's past. First, using hydrodynamical simulations, we find that a star's birth radius and age are linked to its chemical abundances. Subsequently, we learn that even with current-day measurement uncertainty and sample sizes, chemical abundances of Milky Way stars provide a route to reconstructing its formation over time. Extending the insights from hydrodynamical simulations to 30,000 stars observed across the Milky Way disk in the APOGEE survey reveals the importance of using the high-dimensional chemical abundance space. Specifically, we determine that we can use groups of chemically similar stars with 19 measured abundances to trace different underlying formation conditions. Using the high-dimensional abundance data for 10,000 stars from two spectroscopic surveys, APOGEE and GALAH, we empirically describe the chemical abundance trends across a vast radial extent of the Milky Way disk. To do this, we employ a novel approach of quantifying radial variations for individual abundances conditioned on supernovae enrichment history. This enables us to assess the information content in each of the 15 abundances examined and capture the fine-grained signatures in the disk's chemical evolution history. This thesis outlines the potential of using stellar chemistry to trace different evolutionary events of the Milky Way disk, particularly in a time where survey data sample size and precision are growing rapidly.

Galactic Archaeology aims to understand the formation history of the Milky Way (MW). Observations from large spectroscopic and photometric surveys in the recent over the last decade have revolutionized this field. Many substructures and stellar populations have been discovered thanks to full sky surveys, such as Gaia, suggesting the MW is out of equilibrium. However, with numerous missions providing high-quality spectra and photometric time series for billions of stars, it has become increasingly difficult to interpret multidimensional data. One way to address the challenge of large data ensembles is to convey multidimensional information in a more compact way. This can be done by constructing a set of key summary statistics. In my thesis, I use photometric and abundance data to obtain the ages and birth radii of stars in the MW. These two physical quantities of stars, along with stellar abundances and kinematic measurements provide a ``Galactic timetable'' that marks the locations and times of occurrence of different events including mergers and enhancements in the star formation rate. To infer stellar ages, I use gyrochronology, one of the only viable methods to age-date main-sequence (especially for low-mass K/M dwarfs) stars. This technique uses stellar rotation periods and temperature measurements as age indicators. Due to the complexity of magnetic fields in stars, no purely theoretical gyrochronology model currently exists. As a result, gyrochronology relies strongly on empirical calibrations to known stellar ages using other methods. However, none of the age-dating methods for single field stars are suitable for low-mass main sequence stars, as they are faint and their physical properties evolve slowly. To get ages for these stars, I apply the simple assumption that the velocity dispersion of stars increases over time and adopt an age--velocity--dispersion relation (AVR) to estimate average stellar ages, which we called gyro-kinematic ages, for groupings of stars with similar period, temperature, absolute G magnitude, and Rossby number values. Since calculating gyro-kinematic ages requires a large number of stars with period and kinematic measurements, I measured rotation periods for K and M dwarfs using the Zwicky Transient Facility (ZTF). With conservative vetting criteria, I created the largest rotation period catalog (~ 40,000) for low-mass dwarf stars. By combining open cluster ages from literature and gyro-kinematic ages inferred from stars with 6-D kinematic from Gaia DR3 and rotation periods from Kepler and ZTF, I calibrated a fully empirical gyrochronology relation using Gaussian Processes. This approach is suitable for age-dating dwarf stars between 0.67 - 14 Gyr. Using this newly calibrated relation, I provide the community with the largest and most precise stellar ages for ~ 100,000 low-mass dwarf stars. This is the first time that the approach of gyrochronology has been used to date stars older than 4 Gyr. This sample can be used to study exoplanet evolution and the kinematic sub-structure in the solar neighborhood. Stars move away from their birthplaces over time via a process known as radial migration, which blurs chemo-kinematic relations used for reconstructing the MW formation history. One of the ultimate goals of Galactic Archaeology, therefore, is to understand stars’ birth locations. In my thesis, I first tested the reliability and limitation of the only method \cite{Minchev2018} that is able to infer star by star birth radius. I do so by testing the underlying assumption --- the metallicity gradient is linear at all times --- using the cosmology zoomed-in simulation NIHAO-UHD. This analysis concluded that for the MW, we can infer birth radii with an uncertainty of ~0.5 kpc if the metallicity gradient evolution is known and after the rotationally supported stellar disk has started to form. I then developed a method to recover the time evolution of the stellar birth metallicity gradient, d[Fe/H](R, t)/dR, through its inverse relation to the metallicity range as a function of age today. This allows me to place any star with age and metallicity measurements back to its birthplace, Rb. Applying this method to a high-precision large data set of MW disk subgiant stars, I find a steepening of the birth metallicity gradient from 11 to 8 Gyr ago, which coincides with the time of the last major merger, Gaia-Sausage-Enceladus (GSE). By dissecting the disk into mono-Rb populations, clumps in the low-[alpha/Fe] sequence appear, which are not seen in the total sample and coincide in time with known star-formation bursts.

We have observed our Universe in many different wavelengths, from gamma ray to radiowaves and have observed countless stars, galaxies, and everything else in between. These measurements slowly add to our understanding of what our Universe is, how it formed, and where it is heading next. In Chapter 2 we introduce the Ultraviolet GAlactic Plane Survey (UVGAPS), which produced a high resolution map of the Milky Way’s Galactic plane in the NUV using the Galaxy Evolution Explorer (GALEX), an orbiting ultraviolet space telescope operated by NASA and Caltech between 2003-2013. Of the many astrophysical phenomena observable in ultraviolet wavelengths, we choose to focus on a few interesting objects: red clump stars and OB type stars, and the Galactic dust that impacts them. We use an image source extractor to obtain the NUV photometry and apply several cuts to clean the data. We present a catalog of 2,843,399 objects with GALEX NUV band mea- surements. Despite the difference in observing strategy and analysis pipeline, we find good agreement between previously targeted GALEX observations and the UVGAPS catalog in overlapping regions. The data were cross matched to Gaia DR2 and Pan-STARRS DR2, two visible-band surveys that have considerable coverage of the Galactic Plane. We characterize matched objects in color-magnitude and color-color space to highlight a range of objects, from main sequence stars to binaries detected with these data. Although core helium-burning red clump (RC) stars are faint at ultraviolet wavelengths, their ultraviolet-optical color is a unique and accessible probe of their physical properties. In Chapter 3, using data from the GALEX All Sky Imaging Survey, Gaia DR2, and the SDSS APOGEE DR14 survey, we find that spectroscopic metallicity is strongly correlated with the location of a RC star in the UV-optical color magnitude diagram. The RC has a wide spread in (NUV - G)0 color of over 4 magnitudes compared to a 0.7-magnitude range in (G BP - G RP )0 . We propose a photometric, dust-corrected, ultraviolet-optical (NUV - G)0 - [Fe/H] color-metallicity relation using a sample of 5,175 RC stars from APOGEE. We show that this relation has a scatter of 0.16 dex and is easier to obtain for large, wide-field samples than spectroscopic metallicities. Importantly, the effect may be comparable to the spread in RC color attributed to extinction in other studies. In Chapter 4 we extend our RC analysis to UVGAPS and include new age and mass measurements. We find that RC stars separate into two distinct populations in the thin and thick disk. Thick disk stars tend to be old, low mass stars while thin disk stars are more heterogeneous, containing mostly young, massive stars but also some old, low-mass stars typical of the thick disk. These last two chapters are just two of many projects that UVGAPS enables.