HAYDN — High-precision AsteroseismologY of DeNse fields

Astrophysics is now entering an era of unprecedented large-scale, multi-messenger surveys: Gaia, JWST, Roman, Euclid, Rubin/LSST, PLATO and LISA from space, and large complementary efforts from the ground (e.g., ELT, large spectroscopic surveys) are giving us the potential for a step change in our understanding of properties and processes of the physical universe. The interpretation of this wealth of data relies, however, on our understanding of the building blocks of cosmic structures: stars and star clusters. In this poster, we discuss the scientific rationale and current concept of the HAYDN ESA M-class mission candidate, a space mission specifically dedicated to gathering long photometric series of large samples of coeval and initially chemically-homogeneous stars in open and globular clusters. Such a mission would transform stars into laboratories, enabling us to test stellar physics and catalysing the development of next-generation stellar models, thus reinforcing the foundations of astrophysical inference.