Symbiotic binary stars are a type of binary star system that consists of a compact star (such as a white dwarf) and a red giant star. In these systems, a compact star accretes materials from a red giant star and produces soft X-ray photons. However, some of them—known as hard X-ray-emitting symbiotic stars—also produce powerful X-ray photons, which may be due to the presence of massive white dwarfs.