2025ApJS..280....5H

STARBURST99 is a population synthesis code tailored to predict the integrated properties or observational characteristics of star-forming galaxies. Here, we present an update to STARBURST99 where we port the code to Python and include new evolutionary tracks, both rotating and nonrotating, at a range of low-metallicity environments. We complement these tracks with a corresponding grid of new synthetic spectral energy distributions (SEDs). Additionally, we include both evolutionary and spectral models of stars up to 300-500 M_⊙. Synthesis models made with the Python version of the code and new input stellar models are labeled pySTARBURST99. We make new predictions for many properties, such as ionizing flux, SED, bolometric luminosity, wind power, hydrogen line equivalent widths (EWs), and the UV β-slope. These properties are all assessed over wider coverage in metallicity, mass, and resolution than in previous versions of STARBURST99. A notable finding from these updates is an increase in H I ionizing flux of 0.3 dex in the first 2 Myr when increasing the upper mass limit from 120 to 300M_⊙. Changing metallicity has little impact on H I in the first 2 Myr (range of 0.015 dex from Z = 0.02 to 0.0), but lower metallicities have higher H I by 1 dex (compared to Z = 0.02-0.0004) at later times, with Z = 0.0 having even higher H I at later times. Rotating models have significantly higher H I than their equivalent nonrotating models at any time after 2 Myr. Similar trends are found for He I and He II, bolometric luminosity and wind momentum, with more complex relations found for hydrogen line EWs and UV β-slopes.