Astronomers from the University of Warwick in the UK discovered the presence of carbon in the atmosphere of a nearby white dwarf star (WD 0525+526) through ultraviolet observations with the Hubble Space Telescope, confirming that it is the remnant of the merger of two stars. This discovery, published in Nature Astronomy, provides new clues for studying stellar evolution.
White dwarfs are the dense cores left behind after the death of a star, typically about half the mass of the Sun. However, WD 0525+526 is 20% more massive than the sun and is a rare "supergiant white dwarf". Traditional theory suggests that this type of white dwarf may have formed from the collapse of a single massive star, but Hubble's ultraviolet data revealed traces of carbon in its atmosphere, suggesting that it actually originated from the merger of binary stars.
The study points out that during stellar mergers, the hydrogen and helium layers are almost completely burned away, allowing carbon to break through the barrier and reach the surface. Compared with other merger remnants, WD 0525+526 has extremely low carbon content and is extremely hot (about four times that of the Sun), indicating that it is in the early stages of post-merger evolution. In addition, scientists have observed the "semi-convection" phenomenon in a white dwarf for the first time, explaining how carbon slowly rises into the atmosphere in a high-temperature environment.
As WD 0525+526 gradually cools, more carbon elements may appear on its surface in the future. This discovery provides a unique sample of the early evolution of stellar mergers and sets a new benchmark for the termination mechanism of binary star systems.