UWEC CERCA 2025

Aim: To explore the use of various definitions for the exact moment a collapsing cloud of gas and dust becomes a star, often called the zero-age main sequence (ZAMS).Context: In stellar models, the definition of when a collapsing cloud of gas becomes a fully-fledged (hydrogen burning) star, can be characterized in many ways. To obtain data, we stop a computational star model when each of the various “definitions” is met, graph, and analyze to determine how well the data represents a newly matured star.Methods: This study uses MESA (Modules for Experiments in Stellar Astrophysics) to generate computational star models for 30 stars of initial masses between 0.126 solar mass and 100.0 solar mass. The stopping conditions explored involve the central hydrogen abundance, the ratio of power created by fusion to total power leaving the star, and, most notably, a quantity that compares the local luminosity gradient with the nuclear energy generation rate, which should balance for a star that is powered primarily by nuclear fusion.Results: We present Hertzsprung Russell diagrams depicting the isochrones of star models using the different stopping conditions. While the investigated stopping conditions typically generate similar star populations, some are able to catch stars slightly earlier in their evolution. We make a subjective argument for favoring a luminosity gradient vs. nuclear energy generation rate definition of the ZAMS.