NASA’s Hubble Telescope has uncovered WD 0525+526, a rare ultra-massive white dwarf formed in a violent star merger. Published in Nature Astronomy, the discovery reveals hidden carbon and shows stellar collisions may be more common than once believed
At first glance, the star WD 0525+526 looked like any other white dwarf. But thanks to NASA’s Hubble Space Telescope, astronomers have uncovered its hidden, violent history — one that began with a stellar smash-up.
A cosmic rarity
An international team of researchers has discovered that this white dwarf is not the quiet remnant of a single star, as most white dwarfs are. Instead, it was forged in a catastrophic merger between stars. The finding, published on September 10, 2025, in Nature Astronomy, suggests that such dramatic stellar collisions may be far more common than previously believed.
“This appeared to be a normal white dwarf,” said Boris Gaensicke of the University of Warwick, who led the Hubble program. “But Hubble’s ultraviolet vision revealed it had a very different story.” The discovery, published in the journal Nature Astronomy on September 10, 2025, marks the first time a white dwarf born from colliding stars has been identified through its ultraviolet spectrum.
What makes WD 0525+526 special
White dwarfs are dense stellar remnants about the size of Earth, formed when smaller stars run out of fuel. While they can weigh up to 1.4 times the Sun’s mass, ultra-massive white dwarfs are rare. WD 0525+526, located just 128 light-years away, is one of them: it is 20% heavier than the Sun and much hotter than typical white dwarfs.
In visible light, it looked unremarkable. But Hubble’s Cosmic Origins Spectrograph detected faint carbon lines in its ultraviolet spectrum — a telltale sign of a violent origin. Normally, white dwarfs formed from single stars have atmospheres dominated by hydrogen and helium, which block carbon from view. But in merger scenarios, violent collisions strip away those outer layers, allowing carbon from the core to rise to the surface.
A puzzle for astronomers
Even among merger-born white dwarfs, WD 0525+526 is exceptional. Its atmosphere contains far less carbon than similar stars — about 100,000 times less — and its scorching temperature (21,000 kelvins) rules out the usual mixing processes. Researchers believe a subtle mechanism called semi-convection explains how just traces of carbon leak into its atmosphere.
“Hubble’s ultraviolet capabilities were essential here,” said Snehalata Sahu, lead author of the study. “Without it, this star would have looked completely ordinary.”
Why it matters
The discovery hints that many white dwarfs previously thought to be “normal” may in fact be the products of stellar collisions. This could reshape how astronomers understand stellar evolution, binary star systems, and even the pathways to supernova explosions.
As Antoine Bedrad of the University of Warwick, a co-author of the study, explained: “We want to know how many stellar mergers are hiding among what look like normal white dwarfs. That will tell us a lot about how stars live and die.”
Hubble’s continuing legacy
More than 30 years after launch, the Hubble Space Telescope continues to rewrite the story of the cosmos. With every discovery, it reminds us that even the most ordinary-looking stars can hold explosive secrets.