Understanding the stars is the bedrock of modern astrophysics. Stars are the source of life. The chemical enrichment of our Milky Way and of the Universe with all elements heavier than lithium originates in the interiors of stars. Stars arethe tracersof the dynamics of the Universe,gravitationallyimplying much more than meets the eye. Stars ionize the interstellar medium and re-ionized the early intergalactic medium. Understanding stellar structure and evolution is fundamental. While stellar structure and evolution are understood in general terms, we lack important physical ingredients, despite extensive research during recent decades. Classical spectroscopy, photometry, astrometry and interferometry of stars have traditionally been used as observational constraints to deduce the internal stellar physics. Unfortunately, these types of observations only allow the tuning of the basic common physics laws under stellar conditions with relatively poor precision. The situation is even more worrisome for unknownaspects of the physics and dynamics in stars. These are usually dealt with by using parameterised descriptions of, e.g., the treatments of convection, rotation, angular momentum transport, the equation of state, atomic diffusion and settling of elements, magneto-hydrodynamical processes, and more. There is a dearth of observational constraints on these processes, thus solar values are of ten assigned to the m. Yetit is hard to imagine that oneset of parameters is appropriate for the vast range of stars.