Hubble Finds Remnant of White Dwarf Merger 130 Light-Years Away

The white dwarfs are the dense nuclei left behind when the stars exhaust their fuel and collapse. These are stellar embers of the size of the earth generally weighing half that the sun, composed of carbon-oxygen cores with helium and hydrogen surface layers. Using extraviolet data from the Hubble NASA / ESA Hubble space telescope, astronomers have discovered atmospheric carbon in the ultramassive dwarf long known WD 0525 + 526, and also found that the total hydrogen and helium masses in the atmosphere of the star are considerably lower than those expected from the merger, which is resisting.

Illustration of a fusion of a white dwarf with a sub-giant star (size not on a scale) which would have occurred in the past. Image credit: Snehalata Sahu / University of Warwick.

WD 0525 + 526 is located about 130 light years in the Auriga constellation.

With a mass of 20% larger than our sun, this white dwarf is considered ultramassive, and the way this star has become is not fully understood.

Such a white dwarf could form from the collapse of a massive star. However, Hubble’s ultraviolet data revealed that WD 0525 + 526 had small quantities of carbon which rose from its nucleus in its atmosphere rich in hydrogen – suggesting that this white dwarf did not come from a single massive star.

“In the optical light, WD 0525 + 526 looks like a heavy but otherwise ordinary white dwarf,” said the astronomer of the University of Warwick, Snehalata Sahu.

“However, thanks to ultraviolet observations obtained with Hubble, we were able to detect low carbon signatures which were not visible for optical telescopes.”

“Finding small quantities of carbon in the atmosphere is a revealing sign that this massive white dwarf is probably a remaining of a fusion between two stars.”

“He also tells us that there may be many more fusion remains like this passing as white dwarfs in common pur hydrogen.”

“Only ultraviolet observations could reveal them to us.”

Normally, hydrogen and helium form a thick barrier -shaped envelope around a white dwarf nucleus, keeping elements like hidden carbon.

In a fusion of two stars, layers of hydrogen and helium can burn almost completely as the stars combine.

The single star resulting has a very fine envelope which no longer prevents carbon from reaching the surface – this is exactly what is on WD 0525 + 526.

“We have measured the layers of hydrogen and helium as ten billion times thinner than in typical white dwarfs,” said astronomer at Warwick University Antoine Bédard.

“We believe that these layers have been deleted in the fusion, and this is what now allows Carbon to appear on the surface.”

“But this rest is also unusual: it has about 100,000 times less carbon on its surface compared to the other remains of merger.”

“The low carbon level, with the high temperature of the star (almost four times warmer than the sun), tells us that WD 0525 + 526 is much earlier in its post-fusion evolution than those previously found.”

“This discovery helps us to better understand the fate of binary star systems, which is essential for related phenomena such as supernova explosions.”

Adding to the mystery is the way carbon reaches the surface in this much warmer star.

The other remains of fusion are later in their evolution and fresh enough for convection to bring carbon to the surface. But WD 0525 + 526 is far too hot for this process.

Instead, the authors identified a more subtle form of mixture called semi-convection, seen here for the first time in a white dwarf.

This process allows small quantities of carbon to rise slowly in the atmosphere rich in hydrogen of the star.

“It is rare to find clear evidence of mergers in individual white dwarfs,” said professor of the University of Warwick, Boris Gänsicke.

“But ultraviolet spectroscopy gives us the opportunity to detect these signs early, when carbon is always invisible to optical wavelengths.”

“Because the earth’s atmosphere blocks ultraviolet light, these observations must be made of space, and currently only Hubble can do this work.”

“While WD 0525 + 526 continues to evolve and cool, it is expected that more carbon emerges on its surface over time.”

“For the moment, its ultraviolet glow offers a rare overview of the first stage of the consequences of a stellar fusion – and a new reference on the way in which binary stars end their lives.”

The results appear today in the newspaper Natural astronomy.

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S. Sahu and al. A warm white dwarf fusion revealed by an ultraviolet carbon detection. Nat Astronpublished online on August 6, 2025; DOI: 10.1038 / S41550-025-02590-Y