Credit: ESA, Y. Nazé;
Resolving the Mystery of Gamma Cas
One of the brightest stars in the northern sky was, for a long time, one of the most mysterious. The star is called Gamma Cas, one of the bright stars in the well-known W-shaped asterism that marks the constellation of Cassiopeia. In 1866, Italian astronomer Angelo Secchi observed Gamma Cas and noticed that, unlike solar-type stars in which hydrogen atoms mostly absorb specific wavelengths of light, H atoms in Gamma Cas were producing light at these wavelengths. This made Gamma Cas the first of a new class of stars, the so-called "Be" stars. Be stars are stars hotter than the sun which show prominent emission lines from hydrogen and other elements. Subsequent observations and hard analysis showed that these emission lines originate in a rotating disk of gas that surrounds Gamma Cas. Deepening the mystery, however, about 100 years after Secchi's discovery, X-ray observations showed that Gamma Cas also emits an unusually large amount of high-energy X-rays. The strength of these X-rays was surprising, and many models were put forward to explain where this high-energy X-ray emission might be coming from. Observations with the Suzaku X-ray observatory revealed an important clue: sporadically, the amount of material between the observer near earth and the X-ray source would decrease, suggesting perhaps that accretion of a broken column of material onto the surface of a white dwarf companion star might play a role. If so, the X-ray emission should show evidence of orbital motion, but neither Suzaku nor other X-ray observatories had the number of high-precision observations to conclusively measure this motion. The origin of the high-energy X-rays from Gamma Cas was finally solved this year using the extraordinary precision provided by the Resolve X-ray spectrometer aboard the XRISM X-ray Observatory. Resolve observations of the X-ray emission from Gamma Cas confirmed the orbital motion of the X-ray emitting source, showing that the emission arises from a white dwarf companion star which accretes streams of matter from the disk of ejecta around the Be star, as seen in the illustration above. As the accreting stream of material nears the white dwarf star, it's funneled by loops of the white dwarf's magnetic field before it falls onto the white dwarf surface. So XRISM solves a fundamental Gamma Cas mystery, but another still remains: how did such a system arise in the first place?
Published: August 3, 2026
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Page Author: Dr. Michael F. Corcoran
Last modified Sunday, 09-Aug-2026 19:56:26 EDT