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An international team of astronomers has uncovered a stellar oddity — a white dwarf star heavier than our Sun, created not from the quiet evolution of a single star but from the dramatic merger of two stars.
This discovery, made possible by the NASA/ESA Hubble Space Telescope’s ultraviolet vision, challenges assumptions about the life stories of stars and hints that such rare merger products may be far more common than once believed.
“It’s a discovery that underlines things may be different from what they appear to us at first glance,” said Boris Gaensicke of the University of Warwick, 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.”
What is a White Dwarf ?
White dwarfs are the dense, Earth-sized remnants left behind when stars like our Sun reach the end of their lives. Our own Sun will take this path in about 5 billion years. Normally, a white dwarf is less massive than our Sun, and rarely do they grow beyond the so-called Chandrasekhar limit of 1.4 solar masses.
But ultra-massive white dwarfs exist, and they raise intriguing questions: How do they form? Astronomers know they can be forged either through the life of a single massive star or through the collision of stellar companions.
A Unique Discovery.
The star at the heart of this study, WD 0525+526, lies just 128 light-years away. It is 20% more massive than the Sun and burns at a searing 21,000 kelvins (37,000°F). In visible light, it looked like a typical white dwarf. But Hubble’s Cosmic Origins Spectrograph revealed a hidden fingerprint: traces of carbon in its atmosphere.
This carbon is the telltale signature of a violent stellar collision. Normally, the thick hydrogen and helium blanket around a white dwarf prevents elements like carbon and oxygen from surfacing. But in a merger event, the hydrogen and helium are stripped away, exposing the star’s deeper layers.
WD 0525+526 is extraordinary even among the handful of known merger remnants. It shows only the faintest traces of carbon — 100,000 times less than other merger white dwarfs — brought to the surface not by convection (which can’t occur at such high temperatures), but by a subtler process called semi-convection.
Why Hubble Made the Difference.
Without Hubble’s ultraviolet sensitivity, this white dwarf would have remained hidden in plain sight. In hotter white dwarfs like WD 0525+526, carbon lines fade in visible light but shine clearly in the ultraviolet — exactly where Hubble excels.
“Hubble’s Cosmic Origins Spectrograph is the only instrument that can obtain the superb quality ultraviolet spectroscopy required to detect the carbon in this white dwarf’s atmosphere,” said study lead Snehalata Sahu of the University of Warwick.
A New Window into Stellar Collisions.
This discovery adds a seventh known white dwarf merger product to the list, and astronomers suspect many more await detection. If so, the history of white dwarfs — and the pathways leading to supernova explosions — may need to be rewritten.
“We would like to extend our research by exploring how common carbon white dwarfs are, and how many stellar mergers are hiding among the ‘normal’ white dwarf family,” said study co-leader Antoine Bedrad, also of Warwick.
For more than three decades, the Hubble Space Telescope has been peeling back the layers of cosmic mysteries. With every ultraviolet spectrum it captures, Hubble reminds us that even the smallest, faintest stars can carry the most dramatic stories of the universe.