“An artist’s impression of the star Gamma-Cas being drawn upon by its smaller companion star. (Image credit: ESA, Y. Nazé)
Astronomers have finally cracked one of space science’s longest-running mysteries. And the answer is a bit dramatic: a star is slowly being eaten by its hidden companion.
Using the XRISM spacecraft, scientists discovered that the well-known star Gamma Cassiopeiae is being stripped of its material by a nearby “dead” star, likely a white dwarf.
Gamma-Cas sits about 550 light-years away in the constellation Cassiopeia and is actually visible to the naked eye under clear skies. But despite being easy to spot, it has puzzled astronomers since 1866. For one, it showed an unusual hydrogen signature unlike stars like our Sun. Then in the 1970s, things got even stranger—it was found blasting out powerful X-rays from superheated gas reaching around 150 million degrees, far more intense than expected.
For decades, scientists debated what could be causing this. One idea was that the star’s fast spin created a swirling disk of gas that somehow generated the X-rays. Another theory suggested something more extreme: that an unseen companion was pulling in material from the star, creating the high-energy emissions.
Now, XRISM’s ultra-sensitive instruments have tipped the scales. The data reveal a compact companion—most likely a white dwarf—feeding on Gamma-Cas by siphoning off its outer layers. As this stolen material crashes onto the white dwarf, it heats up and produces the intense X-rays astronomers have been seeing.
“It took decades of work across many teams, but we’ve finally solved it,” said Yaël Nazé, who led the study.
Gamma-Cas also gave rise to a whole class of stars known as Be stars—hot, massive stars with distinctive hydrogen signatures. Over time, astronomers have found more than 20 similar stars showing the same kind of mysterious X-ray behavior.
This new finding confirms that, at least for some of these stars, the culprit isn’t just stellar activity—it’s cosmic cannibalism.
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