## NASA's Hubble Uncovers Rare White Dwarf Merger Remnant **Report Provider:** Science@NASA **Author:** Andrea Gianopoulos **Publication Date:** August 13, 2025 **Source URL:** https://science.nasa.gov/missions/hubble/nasas-hubble-uncovers-rare-white-dwarf-merger-remnant/ **Keywords:** white dwarf, Hubble, Hubble Space Telescope, stars, Hubble observatory, Hubble observations, Hubble science --- ### Main Findings and Conclusions An international team of astronomers, utilizing NASA's Hubble Space Telescope, has discovered a rare **ultra-massive white dwarf star** that was formed through the **merger of two stars**, rather than the typical single-star evolution. This discovery suggests that such merger-formed white dwarfs may be more common than previously believed. The white dwarf in question, **WD 0525+526**, located 128 light-years away, presented as a "normal" white dwarf in visible light observations. However, Hubble's sensitive ultraviolet (UV) observations revealed the presence of **carbon in its atmosphere**, a key indicator of a more violent formation history. Typically, white dwarfs formed from single stars have atmospheres composed of hydrogen and helium, with heavier elements like carbon hidden deep within their core. The presence of carbon in the atmosphere of WD 0525+526 signals that its outer layers were stripped away during a stellar merger, exposing its carbon-rich interior. This finding is significant because it marks the **first time a white dwarf merger product has been identified by its ultraviolet spectrum**. Previously, six such merger products were identified through carbon lines in their visible-light spectra. These seven objects, along with others identified as bluer than expected for their mass and age from ESA's Gaia mission in 2019, provide new insights into the formation of these rare celestial objects. ### Key Statistics and Metrics * **WD 0525+526's Mass:** 1.2 solar masses (20% more massive than the Sun). * **WD 0525+526's Temperature:** Almost 21,000 kelvins (37,000 degrees Fahrenheit). * **WD 0525+526's Atmospheric Carbon Abundance:** Approximately 100,000 times less than other known merger remnants. * **Distance to WD 0525+526:** 128 light-years. * **Previous Merger Products Identified:** Six (via visible-light spectra). * **Total Merger Products Identified in this Study Group:** Seven (including WD 0525+526). ### Important Recommendations and Future Outlook The research team plans to **extend their research** to investigate: * How common carbon white dwarfs are among similar white dwarfs. * How many "normal" white dwarfs are actually the result of stellar mergers. This future work is expected to contribute significantly to the understanding of white dwarf binaries and their potential pathways to supernova explosions. ### Notable Trends or Changes The discovery highlights a significant trend: **what appears as a "normal" white dwarf in visible light may have a much more complex and violent origin when observed in ultraviolet light.** This underscores the importance of multi-wavelength observations in astronomical research. ### Significant Risks or Concerns The article does not explicitly mention any risks or concerns related to this discovery. ### Material Financial Data No financial data is presented in this news report. --- **Quote from Principal Investigator:** "It's a discovery that underlines things may be different from what they appear to us at first glance," said Boris Gaensicke, principal investigator of the Hubble program, of the University of Warwick. "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." **Instrumental Significance:** The Hubble Space Telescope's **Cosmic Origins Spectrograph** was crucial for this discovery, as it is the only instrument capable of obtaining the high-quality ultraviolet spectroscopy needed to detect the faint carbon spectral lines in the atmosphere of WD 0525+526, especially given its high temperature and low carbon abundance.
NASA’s Hubble Uncovers Rare White Dwarf Merger Remnant - NASA Science
Read original at Science@NASA →This is an illustration of a white dwarf star merging into a red giant star. A bow shock forms as the dwarf plunges through the star’s outer atmosphere. The passage strips down the white dwarf’s outer layers, exposing an interior carbon core.Artwork: NASA, ESA, STScI, Ralf Crawford (STScI)An international team of astronomers has discovered a cosmic rarity: an ultra-massive white dwarf star resulting from a white dwarf merging with another star, rather than through the evolution of a single star.
This discovery, made by NASA’s Hubble Space Telescope’s sensitive ultraviolet observations, suggests these rare white dwarfs may be more common than previously suspected.“It's a discovery that underlines things may be different from what they appear to us at first glance,” said the principal investigator of the Hubble program, Boris Gaensicke, of the University of Warwick in the United Kingdom.
“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.”A white dwarf is a dense object with the same diameter as Earth, and represents the end state for stars that are not massive enough to explode as core-collapse supernovae.
Our Sun will become a white dwarf in about 5 billion years. In theory, a white dwarf can have a mass of up to 1.4 times that of the Sun, but white dwarfs heavier 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, such as a binary companion.
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.
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.
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.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 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.”The Hubble Space Telescope has been operating for more than three decades and continues to make ground-breaking discoveries that shape our fundamental understanding of the universe.
Hubble is a project of international cooperation between NASA and ESA (European Space Agency). NASA’s Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope and mission operations. Lockheed Martin Space, based in Denver, also supports mission operations at Goddard. The Space Telescope Science Institute in Baltimore, which is operated by the Association of Universities for Research in Astronomy, conducts Hubble science operations for NASA.
To learn more about Hubble, visit: https://science.nasa.gov/hubble
