Astronomers have finally solved a five-decade mystery surrounding the famous star gamma-Cas (γ-Cas), visible as the central point of the “W”-shaped constellation Cassiopeia. The answer came from unique high-resolution observations by the X-Ray Imaging and Spectroscopy Mission (XRISM), a collaboration between Japanese, European, and American teams.
The star’s history of puzzles began in 1866 when Italian astronomer Angelo Secchi observed a peculiar bright hydrogen signature, leading to the classification of a new class of “Be” stars. By the mid-1970s, an even stranger mystery emerged: gamma-Cas shone brightly in high-energy X-rays, producing extremely hot plasma—some 40 times brighter than expected for such massive stars.
For decades, two competing theories sought to explain the X-rays. One suggested magnetic interactions between the star and its surrounding disc. The other proposed that the star’s invisible companion—a white dwarf—was consuming material from the disc, generating the X-rays in the process.
Previous observations by ESA’s XMM-Newton and NASA’s Chandra helped narrow the possibilities, but the case remained unsolved until XRISM’s high-resolution spectrometer, Resolve, made dedicated observations. The data revealed that the signatures of the hot plasma follow the orbital motion of the otherwise invisible white dwarf companion.
“Thanks to the high-precision observations of XRISM, we have finally done it,” said Yaël Nazé of the University of Liège, lead author of a new paper detailing the findings.
With the mystery solved, astronomers now confirm that gamma-Cas and its two dozen known counterparts are Be stars paired with accreting white dwarfs. However, this raises new questions. Such binary systems were expected to be common among low-mass stars, but they appear instead in high-mass Be stars—suggesting that current models of binary evolution need refinement.
“It’s incredible to see how this mystery has slowly unfolded over the years,” said Alice Borghese, ESA Research Fellow. “XMM-Newton did so much of the groundwork. And now with the next generation of instrumentation, XRISM has brought us over the finish line.”
