heic2510 — Science Release
Hubble uncovers rare white dwarf merger remnant
13 August 2025
An international team of astronomers using the NASA/ESA Hubble Space Telescope have discovered a stellar rarity: an ultra-massive white dwarf that formed when a white dwarf merged with another star, rather than through the evolution of a single star. This discovery, which was made possible by Hubble’s sensitive ultraviolet observations, suggests that these rare white dwarfs may be more common than previously suspected.
A white dwarf is the end state for a star that is not massive enough to explode as a core-collapse supernova. The transition to a white dwarf begins when a star exhausts the supply of hydrogen in its core. The changes in and around the star’s core cause the star to expel its outer layers in a massive stellar sigh, revealing the star’s dense, Earth-sized core, which evolves into a white dwarf. The cores of white dwarfs are mostly composed of either carbon and oxygen or oxygen and neon, depending on the mass of the progenitor star. The Sun will become a white dwarf in about 5 billion years.
White dwarfs can theoretically have masses up to about 1.4 times the mass of the Sun, but white dwarfs that are more massive than the Sun are rare. These objects, which astronomers call ultra-massive white dwarfs, can form either through the evolution of a single massive star or through the merger of a white dwarf with another star.
Recently, astronomers used Hubble’s Cosmic Origins Spectrograph to investigate one such ultra-massive white dwarf, WD 0525+526. WD 0525+526 is just 128 light-years away and is 20% more massive than the Sun.
In visible light, the spectrum of WD 0525+526’s atmosphere resembled that of a typical white dwarf. However, Hubble’s ultraviolet spectrum revealed something unusual: evidence of carbon in the white dwarf’s atmosphere.
White dwarfs that form through the evolution of a single star have atmospheres composed of hydrogen and helium. These thick atmospheres blanket the carbon–oxygen or oxygen–neon surface of the white dwarf, usually preventing these elements from appearing in its spectrum.
When carbon appears in the spectrum of a white dwarf, it can signal a more violent origin than the typical single-star scenario: the collision of two white dwarfs, or of a white dwarf and a subgiant star. Such a collision can burn away the hydrogen and helium atmospheres of the colliding stars, leaving behind a scant layer of hydrogen and helium around the merger remnant that allows carbon from the white dwarf’s core to float upward, where it can be detected.
“It's a discovery that underlines that things may be different from what they appear to us at first glance,” said the principal investigator of the Hubble programme, Boris Gaensicke, of the University of Warwick in the United Kingdom. “Until now, this appeared as a normal white dwarf, but Hubble's ultraviolet eyes revealed that it had a very different history from what we would have guessed. It's like asking a person you think you know well a different kind of question.”
This discovery marks the first time that a white dwarf born from colliding stars has been identified by its ultraviolet spectrum. Prior to this study, six white dwarf merger products were discovered via carbon lines in their visible-light spectra. All seven of these are part of a larger group that were found to be bluer than expected for their masses and ages from a study with ESA’s Gaia mission in 2019, with the evidence of mergers providing new insights into their formation history.
WD 0525+526 is remarkable even within the small group of white dwarfs known to be the product of merging stars. With a temperature of almost 21 000 kelvins and a mass of 1.2 solar masses, WD 0525+526 is hotter and more massive than the other white dwarfs in this group.
WD 0525+526’s extreme temperature posed something of a mystery for the team. For cooler white dwarfs, such as the six previously discovered merger products, a process called convection can mix carbon into the thin hydrogen–helium atmosphere. WD 0525+526 is too hot for convection to take place, however. Instead, the team determined that a more subtle process called semi-convection brings a small amount of carbon up into WD 0525+526’s atmosphere. WD 0525+526 has the smallest amount of atmospheric carbon of any white dwarf known to result from a merger, about 100 000 times less than other merger remnants.
The high temperature and low carbon abundance mean that identifying this white dwarf as the product of a merger would have been impossible without Hubble’s sensitivity to ultraviolet light; spectral lines from elements heavier than helium, like carbon, become fainter at visible wavelengths for hotter white dwarfs, but these spectral signals remain bright in the ultraviolet, where Hubble is uniquely positioned to spot them.
“Hubble's Cosmic Origins Spectrograph is the only instrument that can obtain the superb quality ultraviolet spectroscopy that was required to detect the carbon in the atmosphere of this white dwarf,” said study lead Snehalata Sahu from the University of Warwick.
Because WD 0525+526’s unusual origin was revealed only once astronomers glimpsed its ultraviolet spectrum, it’s likely that other seemingly ‘normal’ white dwarfs are actually the result of cosmic collisions — a possibility that the team is excited to explore in the future.
“We would like to extend our research on this topic by exploring how common carbon white dwarfs are, and how many stellar mergers are hiding among the normal white dwarf family,” said study co-lead Antoine Bedrad from the University of Warwick. “That will be an important contribution to our understanding of white dwarf binaries, and the pathways to supernova explosions.”
The team’s paper has been published in Nature Astronomy.
More information
The Hubble Space Telescope is a project of international cooperation between ESA and NASA.
Image Credit: NASA, ESA, R. Crawford (STScI)
Links
Contacts
Snehalata Sahu
University of Warwick
Bethany Downer
ESA/Hubble Chief Science Communications Officer
Email: [email protected]
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