Abstract

Rest-frame far-ultraviolet (FUV) spectra are fundamental to our understanding of star-forming galaxies, providing a unique window on massive stellar populations, chemical evolution, feedback processes, and reionization. The launch of JWST and construction of the ELTs will soon usher in a new era, pushing the FUV spectroscopic frontier to z=15-20. The success of these future endeavors hinges on a comprehensive understanding of the massive star populations and interstellar medium (ISM) gas conditions that power the observed FUV spectral features. I will present the COS Legacy Archive Spectroscopic SurveY (CLASSY) Treasury as a powerful and promising solution. CLASSY is a large HST program creating the first high-quality, high-resolution FUV spectral catalog of star-forming galaxies at z=0, but which span similar properties as seen at high-z. The spectra contain a suite of emission and absorption lines that characterize the massive stellar populations that populate metal poor galaxies, the physical properties of large-scale outflows that regulate star formation, and the chemical abundance patterns of the gas and stars. CLASSY will improve the diagnostic power of the FUV lines for future JWST /ELT surveys, providing a long-lasting legacy to the astronomical community for decades to come.