The White Dwarf represents the compact core that forms when stars exhaust their fuel and collapse. These remnants are the ashes of Earth-sized stars, typically about half the mass of the Sun, composed of carbon-oxygen cores surrounded by layers of helium and hydrogen. Utilizing far-ultraviolet data from the NASA/ESA Hubble Space Telescope, astronomers have identified carbon in the atmosphere of the famously large white dwarf WD 0525+526. They also determined that the overall mass of hydrogen and helium in the star’s atmosphere was significantly lower than anticipated based on single-star evolution.
An illustration of a merger with a white dwarf sub-huge star (size without scale) that would have occurred in the past. Image credit: Snehalata Sahu/Warwick University.
WD 0525+526 is located approximately 130 light years away in the constellation Auriga.
With a mass exceeding that of our Sun by 20%, this white dwarf is classified as a super-genocide, and its formation process remains poorly understood.
Typically, such white dwarfs form from the collapse of massive stars. However, Hubble’s UV data indicates that WD 0525+526 has a hydrogen-rich atmosphere originating from its core.
“In optical light, WD 0525+526 appears to be a massive yet typical white dwarf,” remarked Sneharata Saff, an astronomer at the University of Warwick.
“However, the ultraviolet observations from Hubble allowed us to detect faint carbon signatures that optical telescopes could not observe.”
“The presence of a small amount of carbon in the atmosphere suggests that this massive white dwarf is likely the product of a merger between two stars.”
“We also believe that many similar merged remnants may pose as white dwarfs in a predominantly hydrogen atmosphere.”
“Only ultraviolet observations can reveal them to us.”
Typically, hydrogen and helium create dense, barrier-like layers around the white dwarf core, concealing carbon-rich elements.
In a stellar merger, the hydrogen and helium enveloping layers can burn away almost entirely as the stars combine.
The resulting single star possesses a very thin envelope that does not prevent carbon from surfacing, which is precisely what is observed in WD 0525+526.
“We found that the hydrogen and helium layers are around one billion times thinner than those typical of a white dwarf,” noted Antoine Bedard, an astronomer at Warwick University.
“We believe these layers were stripped away during the merger, allowing carbon to manifest on the surface.”
“However, this phenomenon is also unusual, as the carbon present is about 100,000 times less than that found on the surfaces of other merged remnants.”
“Coupled with the star’s elevated temperatures—nearly four times hotter than the Sun—the diminished carbon levels suggest that WD 0525+526 evolves at a much faster pace than previously observed.”
This discovery will aid in understanding the destiny of binary star systems, which are crucial for related phenomena such as supernova explosions.
Alongside the enigma, this significantly hotter star’s carbon migrates to the surface.
Other merged remnants later cool enough for convection to bring carbon to the surface; however, WD 0525+526 remains too hot for this process.
Instead, the author identified a subtle mixing process known as semiconvection, uniquely observed in this White Dwarf.
This mechanism permits small amounts of carbon to gradually ascend into the star’s hydrogen-rich atmosphere.
“Finding conclusive proof of individual white dwarf mergers is rare,” remarked Professor Boris Gensick from Warwick University.
“Yet, ultraviolet spectroscopy enables us to detect these signals early, while carbon remains invisible at optical wavelengths.”
“Because the Earth’s atmosphere filters out UV rays, such observations must be conducted from space—currently, only Hubble is capable of this.”
“As WD 0525+526 continues to evolve and cool, we anticipate more carbon will emerge at the surface over time.”
“For now, this ultraviolet illumination offers rare insights into the early aftermath of stellar mergers.
Survey results are published today in the journal Nature Astronomy.
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S. Saff et al. The remnants of Hot White Dwarfs revealed by ultraviolet detection of carbon. Nature Astronomy Published online on August 6th, 2025. doi:10.1038/s41550-025-02590-y
Source: www.sci.news
