A white dwarf star close to Earth may be the result of a powerful cosmic collision where two stars merged into one, according to a University of Warwick team’s review of ultraviolet data from the Hubble Space Telescope.
By analyzing the white dwarf’s atmosphere, researchers made the unusual discovery of carbon emissions, which are rarely observed in such stellar remnants. Investigating how the element arrived there led the team to uncover a rare process occurring within the white dwarf, suggesting a highly unusual origin.
White Dwarf Stars
Typically about half the mass of our Sun, white dwarfs form when stars exhaust their nuclear fuel and collapse. What remains is generally a carbon-oxygen core surrounded by layers of helium and hydrogen. However, some white dwarfs defy this norm—ultra-massive and structurally atypical.
The white dwarf at the center of this study, WD 0525+526, is one such anomaly, weighing in at 20% more massive than the Sun. That makes it “ultra-massive” by white dwarf standards. The mechanism by which such an object could form has remained unclear. While the collapse of a single, exceptionally massive star is one possible explanation, ultraviolet data from Hubble suggested another origin.
Using ultraviolet spectroscopy, the researchers observed carbon from the white dwarf’s core mixing into a hydrogen-rich atmosphere—indicating that the single-star theory is likely incorrect.
“In optical light (the kind of light we see with our eyes), WD 0525+526 looks like a heavy but otherwise ordinary white dwarf,” said first author Dr Snehalata Sahu, Research Fellow at the University of Warwick. “However, through ultraviolet observations obtained with Hubble, we were able to detect faint carbon signatures that were not visible to optical telescopes.
Strange Carbon Behavior
“Finding small amounts of carbon in the atmosphere is a telltale sign that this massive white dwarf is likely to be … the remnant of a merger between two stars colliding,” Sahu said. “It also tells us there may be many more merger remnants like this masquerading as common pure-hydrogen atmosphere white dwarfs. Only ultraviolet observations would be able to reveal them to us.”
In a typical white dwarf, thick layers of hydrogen and helium act as a barrier, preventing carbon from surfacing. But in this case, the carbon’s presence suggests those outer layers were largely burned off during a stellar collision—leaving too little to suppress mixing between layers.
“We measured the hydrogen and helium layers to be ten billion times thinner than in typical white dwarfs,” said co-first author Antoine Bédard, Warwick Prize Fellow in the Astronomy and Astrophysics group at Warwick. “We think these layers were stripped away in the merger, and this is what now allows carbon to appear on the surface.”
“But this remnant is also unusual: it has about 100,000 times less carbon on its surface compared to other merger remnants,” Bédard added. “The low carbon level, together with the star’s high temperature (nearly four times hotter than the Sun), tells us WD 0525+526 is much earlier in its post-merger evolution than those previously found.
“This discovery helps us build a better [understanding of] the fate of binary star systems, which is critical for related phenomena like supernova explosions,” Bédard said.
A Previously Unseen Process
Even in the absence of a barrier, the carbon’s behavior remains strange. Older, cooler white dwarfs can push carbon to the surface through convection—but WD 0525+526 is still far too hot for that mechanism to be active. Upon closer examination, the team discovered another clue: an unusual semi-convective process, never before observed in a white dwarf.
“Finding clear evidence of mergers in individual white dwarfs is rare,” added co-author Professor Boris Gänsicke, Department of Physics, University of Warwick. “But ultraviolet spectroscopy gives us the ability to detect these signs early, when the carbon is still invisible at optical wavelengths.”
“Because the Earth’s atmosphere blocks ultraviolet light, these observations must be carried out from space, and currently only Hubble can do this job,” Gänsicke said.
The ultraviolet glow emitted by WD 0525+526 offers a unique view into the early aftermath of a stellar merger. Instruments like Hubble and the James Webb Space Telescope remain essential to modern astronomy, enabling scientists to study the composition of distant stars through spectroscopic analysis without the need for probes.
“Hubble just turned 35 years old, and while still going strong, it is very important that we start planning for a new space telescope that will eventually replace it,” Gänsicke concluded.
The paper “A Hot White Dwarf Merger Remnant Revealed by an Ultraviolet Detection of Carbon” appeared on August 6, 2025, in Nature Astronomy.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.