Scientists finally solve century-old mystery of star with unexpected X-ray emissions

Astronomers have used the XRISM (X-Ray Imaging and Spectroscopy Mission) X-ray spacecraft to discover that a star is being slowly devoured by an elusive stellar companion, solving a mystery that has baffled scientists for over a century.

The star in question is named gamma-Cas, located around 550 light-years away and visible with the unaided eye over Europe on clear nights as the peak of a distinctive "W" in the constellation Cassiopeia. The star gamma-Cas, which is 19 times the size of the sun and up to 65,000 times brighter than our star, first became a puzzle in 1866 when it was found to possess a bright hydrogen "fingerprint" unlike other stars such as the sun.

In the 1970s, this puzzle turned into a full-fledged mystery as astronomers also discovered gamma-Cas shines brightly in high-energy X-rays. These were estimated to originate from plasma burning at an incredible 150 million degrees, shining 40 times brighter than a typical massive star.

Decades later, scientists would theorize that these strange emissions came from a rotating disk of stellar material that had been ejected by the rapidly spinning star. Over time, such disks can build up and then disperse, causing the brightness of the star to fluctuate. Now, thanks to the sensitivity of XRISM (pronounced "crism"), astronomers have discovered the presence of a compact companion, likely a white dwarf star, that is pulling material away from gamma-Cas. White dwarfs are often called "dead stars" because they represent the core remnant of a star that exhausted its fuel supply needed for its intrinsic nuclear fusion processes.

"There has been an intense effort to solve the mystery of gamma-Cas across many research groups for many decades. And now, thanks to the high-precision observations of XRISM, we have finally done it," team leader Yaël Nazé of the University of Liège, Belgium, said in a statement.

Two competing theories

The hydrogen feature of gamma-Cas resulted in the development of a new classification of stellar bodies, so-called Be stars ("B" representing massive, hot blue stars and "e" representing the unique hydrogen signal). Thanks to the proliferation of X-ray space telescopes like XRISM, XMM-Newton, Chandra and eROSITA, astronomers have now discovered over 20 stars with gamma-Cas-like X-ray emissions, resulting in a special family within the wider category of Be stars.

Scientists posited two explanations for the unusual X-ray emissions of stars like gamma-Cas. Either they are the result of the stars' magnetic fields interacting with surrounding plasma disks, or they are the result of stripped material falling onto the surface of an unseen companion white dwarf star. This research confirms the validity of that latter theory.

"The previous work using XMM-Newton really cleared the way for XRISM, enabling us to eliminate numerous theories and prove which of the last two competing theories was correct," Yaël said. "It's extremely satisfying to have direct evidence to solve this mystery at long last."

There are still some puzzles surrounding gamma-Cas and other high-energy X-ray emitting stars. Such pairings between stars and white dwarfs were once expected to be common, especially for low-mass stars. Recently, however, scientists have found that such pairings are rarer than previously estimated and occur for high-mass Be stars.

"We think the key is in understanding how exactly the interactions take place between the two stars," Yaël added. "Now that we know the true nature of gamma-Cas, we can create models specifically for this class of stellar systems, and update our understanding of binary evolution accordingly."

The team's research was published on Tuesday (March 24) in the journal Astronomy & Astrophysics.