Yue-Hong Chen, AIP Leibniz Institut für Astrophysik, Potsdam — October 26, 2026

What would it be like to live around a young Sun? Magnetic fields generated inside the star emerge at its surface and heat a thin outer atmosphere, the corona, to millions of degrees, so that it shines in X-rays. The same fields drive a steady outflow of plasma into space, the stellar wind. From time to time they also throw out coronal mass ejections (CMEs). The shock waves in front of CMEs can accelerate particles to very high energies. Young, fast-rotating stars are far more active than today's Sun, so their planets grow up in a much harsher "space weather" environment. This environment can strip planetary atmospheres and affect whether a planet can become habitable.

I will begin with an introduction to stellar magnetic activity, using the Sun, the one star we can observe in detail, as a guide. I will then explain how we build 3D models of stellar coronae and winds. We applied these models to four young Sun-like stars and the present-day Sun, with rotation periods from 1 to 27 days. The results show that stars that are brighter in X-rays also lose more mass through their winds, following a scaling relation. Finally, I will present the first 3D simulation of stellar energetic particles accelerated by CMEs on a young Sun-like star, EK Draconis. Its CMEs carry 100 to 10,000 times more energy than solar CMEs. Yet the flux of high-energy protons reaching its habitable zone varies a lot, and mostly stays within the range produced by the Sun. This finding suggests that simply scaling up solar observations may misjudge the particle radiation around young stars.