Solar and stellar flares are explosive phenomena in which magnetic energy stored around
starspots is suddenly released through magnetic reconnection. The radiation emitted
during flares covers a broad range of wavelengths from radio to X-rays, each tracing
different aspects of the flare process. In X-rays, the emission arises from hot thermal
plasma heated by nonthermal electrons that travel upward from the chromosphere into the
corona.

RS CVn-type binaries and protostars exhibit giant flares that are several orders of
magnitude more energetic than those on the Sun, and it remains unclear whether their
underlying physical processes are fundamentally the same as in solar
flares. Furthermore, the impact of high-energy radiation from flares on exoplanetary
environments has recently attracted increasing attention. In particular, X-ray emission
from protostars has recently drawn significant interest from the star and planet
formation community in the context of X-ray–driven chemistry, as it may strongly affect
the surrounding protoplanetary disks. Consequently, studies of protostellar flares have
the potential to broaden the impact of X-ray astronomy into new fields such as planetary
formation, where it has rarely ventured before.

In this talk, I focus on the Fe K-shell emission lines, which are covered by many X-ray
satellites, and introduce the physics of stellar flares that can be inferred from their
line intensities and structures, based on observations with NICER and XRISM.