Stellar astrophysics in the ultraviolet: Setting the scene

Abstract

The ultraviolet (UV) spectral domain occupies a unique position in stellar astrophysics, serving as the bridge between the thermal continuum of photospheres and the high-energy, non-thermal processes of stellar coronae and winds. This article provides a review of stellar physics in the UV, addressing both the theoretical framework and observational applications across the Hertzsprung-Russell diagram. We explicitly structure our discussion around key scientific questions, demonstrating that accurate spectral synthesis in this regime demands Non-LTE radiative transfer codes, which in turn rely on precise atomic collision and recombination rates. We highlight how a critical scarcity of modern laboratory astrophysics data limits these models, particularly for complex ions. Moving to observational diagnostics, we review how UV spectroscopy constrains diffusion and radiatively driven winds in hot sub- luminous stars, and traces shock dynamics and abundance patterns in Planetary Nebulae and Supernova Remnants. In the context of star clusters, we illustrate how UV sensitivity to light-element variations (C, N, O) allows us to disentangle multiple stellar populations that appear degenerate in optical bands. We conclude that future progress depends on facilities capable of high-resolution spectroscopy, time-domain monitoring, and polarimetry to recover these diagnostic tracers and resolve the physics of stellar feedback.