Astronomers Just Found a “Zombie Star” With a Shocking Backstory

Astronomers using NASA’s Hubble Space Telescope have uncovered a stellar oddity: a massive white dwarf star born from the violent merger of two stars, rather than the quiet death of a single one.

This white dwarf, WD 0525+526, hides carbon traces in its atmosphere that only ultraviolet light could reveal — a telltale sign of its explosive past.

A Rare Cosmic Discovery

An international group of astronomers has identified a rare stellar object: an ultra-massive white dwarf created when a white dwarf combined with another star, rather than forming from the life cycle of a single star. Using the ultraviolet sensitivity of NASA’s Hubble Space Telescope, the team uncovered this unusual case, hinting that such merger-born white dwarfs may occur more often than scientists once believed.

“It’s a discovery that underlines things may be different from what they appear to us at first glance,” explained Boris Gaensicke of the University of Warwick in the United Kingdom, principal investigator of the Hubble program. “Until now, this appeared as a normal white dwarf, but Hubble’s ultraviolet vision revealed that it had a very different history from what we would have guessed.”

Astronomers using NASA’s Hubble Space Telescope have found a rare ultra-massive white dwarf formed from a stellar merger. The discovery was made possible by Hubble’s sensitive ultraviolet observations and suggests these unusual white dwarfs may be more common than once thought.

The Life Cycle of Stars

A white dwarf is an extremely dense stellar remnant about the size of Earth, marking the final stage for stars that are not large enough to end their lives in core-collapse supernovae. In roughly 5 billion years, our own Sun will follow this path and shrink into a white dwarf.

Although a white dwarf can theoretically reach up to 1.4 times the Sun’s mass, most are less massive. The heavier examples, known as ultra-massive white dwarfs, can arise in two ways: either from the evolution of a single massive star or from the merger of a white dwarf with another star, such as a close binary companion.

First Ultraviolet Evidence of Stellar Collisions

This new discovery, published in the journal Nature Astronomy, 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.

Astronomers used Hubble’s Cosmic Origins Spectrograph to investigate a white dwarf called WD 0525+526. Located 128 light-years away, it 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.

Why Carbon Matters in White Dwarfs

White dwarfs that form through the evolution of a single star have atmospheres composed of hydrogen and helium. The core of the white dwarf is typically composed mostly of carbon and oxygen or oxygen and neon, but a thick atmosphere usually prevents these elements from appearing in the white dwarf’s 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.

Solving the Hot-Carbon Puzzle

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 (37,000 degrees Fahrenheit) 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 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.

Why Only Hubble Could See It

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.

Many More Hidden Collisions Await

Because WD 0525+526’s 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 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 among similar white dwarfs, and how many stellar mergers are hiding among the normal white dwarf family,” said study co-leader 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.”

Reference: “A hot white dwarf merger remnant revealed by an ultraviolet detection of carbon” by Snehalata Sahu, Antoine Bédard, Boris T. Gänsicke, Pier-Emmanuel Tremblay, Detlev Koester, Jay Farihi, J. J. Hermes, Mark A. Hollands, Tim Cunningham and Seth Redfield, 6 August 2025, Nature Astronomy.
DOI: 10.1038/s41550-025-02590-y

The Hubble Space Telescope, a joint project of NASA and the European Space Agency (ESA), has been orbiting Earth for more than three decades, transforming our understanding of the universe. Managed by NASA’s Goddard Space Flight Center in Greenbelt, Maryland, with operational support from Lockheed Martin Space in Denver, Hubble continues to deliver groundbreaking discoveries that shape modern astronomy. Science operations are carried out by the Space Telescope Science Institute in Baltimore, run by the Association of Universities for Research in Astronomy. From revealing distant galaxies to uncovering hidden details of stellar evolution, Hubble remains one of the most powerful and productive scientific instruments ever built.

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