Corinne Charbonnel, President of IAU Division G Stars and Stellar Physics, for IAU Strategic Plan 2020-2030
Stars are the fundamental bricks of the baryonic Universe. They produce energy, radiation, and chemical elements that shape the structures of the Universe from small to very large scales in both space and time. Understanding how they form, evolve, die, and interact with their environments is a fundamental challenge with deep implications to probe and describe properly the assembly and the evolution of exoplanetary systems, of galaxies over cosmic time, and of the Universe as a whole. This requires multi-disciplinary developments in fundamental physics, including nuclear and particle physics, and cosmology.
Stellar astrophysics is in the middle of an extraordinary revolution caused by large amounts of unprecedented detailed data gathered for the first time with complementary multi-messenger techniques. Enormous high-resolution spectroscopic and photometric surveys coupled to ultra-high precision astrometry are providing chrono-chemo-kinematical information on the different stellar populations in the Milky Way and beyond. The maturing field of asteroseismology brings unrivaled information on stellar interiors together with constraints on fundamental stellar parameters and evolutionary clocks that are independent of the classical methods for very large samples of stars over the entire Hertzsprung-Russell diagram. Interferometry and adaptative optics provide details on stellar surfaces that were until now only possible for the sun, and allow exquisite mapping of close circumstellar environments including disks and jets, while spectropolarimetric observations coupled to tomographic techniques reveal the large-scale magnetic topologies of stars. Last but not least, the recent detection of gravitational waves opens a new avenue to study stars and their remnants using observational clues beyond the electromagnetic spectrum. The coming decade will see the explosion of these techniques thanks to the advent of extremely large telescopes and of dedicated instruments and space missions.
On the theoretical side, the challenges are immense at the frontiers of fundamental physics. Magnetohydrodynamics, as typically manifested through convection, mixing, rotation, magnetic activity, winds, interactions in multiple systems and interplay with interstellar surroundings, remains the largest uncertainty in modern stellar astrophysics, with crucial implications ranging from cosmic reionisation and distance ladders to galactic evolution and exoplanet studies including habitability. The growth of computational resources to simulate stellar magnetohydrodynamics from the formation of stars to the explosions of supernovae holds promise for substantial improvements in the domain in the next decade, in synergy with astrostatistics and big data science to exploit the extraordinary observational developments.