An invisible companion consuming material from the famous naked-eye star gamma- Cassiopeia has been revealed as the culprit for curious X-rays coming from the stellar system. This closes the case on a mystery that has puzzled astronomers for more than fifty years. Unique high-resolution observations made by the X-Ray Imaging and Spectroscopy Mission (XRISM) revealed that the X-rays are linked to the orbital motion of a companion white dwarf star, enabling astronomers to finally solve the mystery. The star gamma-Cas (γ-Cas) is visible to observers in the northern hemisphere. It makes up the central ‘point’ of the distinctive ‘W’-shaped constellation Cassiopeia.