WD 0525 +526 lies roughly 130 light-years away in the constellation of Auriga.

With a mass 20% bigger than our Sun, this white dwarf is thought about ultramassive, and how this star happened is not completely comprehended.

Such a white dwarf might form from the collapse of an enormous star. Ultraviolet information from Hubble exposed WD 0525 +526 to have little quantities of carbon increasing from its core into its hydrogen-rich environment– recommending this white dwarf did not stem from a single huge star.

“In optical light, WD 0525 +526 appear like a heavy however otherwise normal white dwarf,” stated University of Warwick astronomer Snehalata Sahu.

“However, through ultraviolet observations gotten with Hubble, we had the ability to discover faint carbon signatures that were not noticeable to optical telescopes.”

“Finding percentages of carbon in the environment is an indicator that this huge white dwarf is most likely to be a residue of a merger in between 2 stars.”

“It likewise informs us there might be much more merger residues like this masquerading as typical pure-hydrogen environment white overshadows.”

“Only ultraviolet observations would have the ability to expose them to us.”

Generally, hydrogen and helium form a thick barrier-like envelope around a white dwarf core, keeping components like carbon concealed.

In a merger of 2 stars, the hydrogen and helium layers can burn practically entirely as the stars integrate.

The resulting single star has an extremely thin envelope that no longer avoids carbon from reaching the surface area– this is precisely what is discovered on WD 0525 +526.

“We determined the hydrogen and helium layers to be ten-billion times thinner than in common white overshadows,” stated University of Warwick astronomer Antoine Bédard.

“We believe these layers were removed away in the merger, and this is what now permits carbon to appear on the surface area.”

“But this residue is likewise uncommon: it has about 100,000 times less carbon on its surface area compared to other merger residues.”

“The low carbon level, together with the star’s heat (almost 4 times hotter than the Sun), informs us WD 0525 +526 is much previously in its post-merger advancement than those formerly discovered.”

“This discovery assists us construct a much better comprehend the fate of binary star systems, which is important for associated phenomena like supernova surges.”

Contributing to the secret is how carbon reaches the surface area at all in this much hotter star.

The other merger residues are later on in their advancement and cool enough for convection to bring carbon to the surface area. WD 0525 +526 is far too hot for that procedure.

Rather, the authors determined a subtler kind of blending called semi-convection, seen here for the very first time in a white dwarf.

This procedure permits percentages of carbon to gradually increase into the star’s hydrogen-rich environment.

“Finding clear proof of mergers in private white overshadows is uncommon,” stated University of Warwick’s Professor Boris Gänsicke.

“But ultraviolet spectroscopy offers us the capability to find these indications early, when the carbon is still undetectable at optical wavelengths.”

“Because the Earth’s environment obstructs ultraviolet light, these observations should be performed from area, and presently just Hubble can do this task.”

“As WD 0525 +526 continues to develop and cool, it is anticipated that more carbon will emerge at its surface area in time.”

“For now, its ultraviolet radiance uses an uncommon glance into the earliest phase of an excellent merger’s after-effects– and a brand-new criteria for how binary stars end their lives.”

The findings appear today in the journal Nature Astronomy

_____

S. Sahu et alA hot white dwarf merger residue exposed by an ultraviolet detection of carbon. Nat Astronreleased online August 6, 2025; doi: 10.1038/ s41550-025-02590-y