ALT The Fe Kα fluorescence line emitted when cold gas is irradiated by high-energy X-rays has long been used as a powerful diagnostic to probe the distribution and physical state of matter surrounding X-ray sources such as black holes and neutron stars. Using XRISM, we have revealed for the first time that the Fe Kα line detected from the accretion-powered pulsar Centaurus X-3 originates not from neutral iron atoms, but from low-ionization iron ions in which approximately five electrons have been removed. This discovery was made possible by the exceptional spectroscopic capability of the X-ray microcalorimeter Resolve onboard XRISM, which enabled the detection of both the Fe Kα and Fe Kβ lines and the measurement of their energies with an accuracy of about 1 eV. This study demonstrates a new approach in which macroscopic astrophysical phenomena serve as a “cosmic laboratory” to diagnose the microscopic electronic states of atoms.