4D-STAR · Lifting stellar structure and evolution to higher dimensions in the era of space asteroseismology

Stars are the source of radiation, chemistry, and life in the Universe. Models of how stars live are key ingredients in planetary, astrophysical, and cosmological research. A star is a hot plasma rotating around an axis. Small stars like the Sun rotate slowly but bigger ones with more mass rotate faster, shaping them as flattened spheroids. Yet, current stellar models simplify the flattening or treat stars as spheres during their lives, using 1 spatial and 1 time dimension (1+1D). Rotation and magnetism induce transport processes in 3 spatial dimensions, which change over time, requiring a 3+1D treatment. Current age-dating of stars is done from 1+1D models, with uncertainties up to 1000%. Accurate ages of stars are the dominant missing ingredient to understand stellar and planetary evolution, the emergence of life, and the chemistry in our Universe. 4D-STAR will answer the fundamental question of how rotating spheroids evolve in time and build up their chemistry during their lives. We will develop a new 3+1D theory of stellar rotation for flattened spheroids evolving over millions to billions of years, from birth to death. Lifting stellar models to 3+1D can only be done now, using asteroseismic data of thousands of stars in all life phases. Such data reveal nonradial oscillations, or starquakes, allowing us to infer internal stellar rotation, magnetism, chemistry, and the ages of stars with 10% accuracy. 4D-STAR will provide open-source modules to compute the evolution of rotating magnetic stars in 3+1D, calibrated to asteroseismic observables of single stars and stars in binaries and clusters. 4D-STAR brings a paradigm shift based on mathematical modelling, astrophysics, and computational science. Its breadth, challenges, and goals require a transdisciplinary integration of four teams led by an asteroseismologist, a theoretician specialised in transport, a hydrodynamicist, and a stellar evolution software developer, each with proven track records.