A new class of stellar remnants has been identified with the discovery of two isolated ultra-massive and rapidly rotating white dwarfs, whose unique features have garnered equally unique fantasy-inspired names.
Dubbed “Gandalf” and “Moon-Sized,” this new class of stellar remnants, characterized by large masses, rapid rotation, x-ray emission, and strong magnetic fields, offers an efficient demonstration of how many exciting discoveries remain in the universe.
The findings, detailed in a pair of papers from researchers at the Institute of Science and Technology Austria (ISTA), demonstrate that, based on their similarity, Gandalf and Moon-Sized constitute a new class of white dwarf stars, which defy expectations by carrying markers associated with binary systems in a single object.
Gandalf: A Peculiar Stellar Remnant
White dwarfs are the burned-out remains of formerly active stars, a fate expected for our own Sun in 5 to 8 billion years. The stars are both like our own host star in another important way: they are isolated objects rather than forming a binary or larger system.
This is unusual, as both stars are emitting an X-ray signature previously associated exclusively with white dwarfs in binary systems, produced when one star siphons the material of another in a process known as accretion. Believing that the strange objects formed in cosmic collisions, the researchers have identified them as merger remnants.
Going back to her postdoctoral research, Ilaria Caiazzo, assistant professor at ISTA, first noted Gandalf as an interesting object years ago because of its strange signals, which she believed suggested it was surrounded by material. Caiazzo led the new work, which confirmed that the same X-ray signal is coming from both objects.
The Half Rings of Gandalf and Moon-Sized
Intriguingly, that material appears to comprise a half-ring shape, based on the two alternating peaks seen in Gandalf’s hydrogen emission spectra. Strangely, the material’s orbit is not synchronized to the white dwarf, with the half-ring orbiting not faster than 80 minutes per cycle, compared to Gandalf’s six-minute rotation.
“We initially thought it was a binary system,” said Andrei Cristea, a PhD student in the Caiazzo group and first author of the paper published in Astronomy & Astrophysics. “At the remnant’s extremely high level of magnetism, its spin should be synchronized with its companion’s orbit, similarly to Earth’s rotation with the Moon’s orbit.”
This asynchronous behavior would be extremely unusual even if the team had located a binary companion, which they did not. They then began to wonder where this material could have come from, a question that remains open.
Forming the Half-Ring and Naming Gandalf
For the material to form a half ring around the merger fragment, the object’s magnetic field must be both strong and asymmetric.
“To note, white dwarfs of similar age and evolutionary stage are typically nonmagnetic,” said Cristea. “While highly magnetic white dwarf remnants are already an exception, Gandalf is now one of only two known merger remnants to feature asymmetric magnetization.”
The strange puzzles inherent in the object reminded Cristea of Tolkien’s famous character Gandalf, who was often known to speak in riddles. For this reason, he chose to name the object after the iconic Lord of the Rings character.
And Moon-Sized Makes Two
Despite being unable to identify a binary companion for Gandalf, the researcher suspected that it may have a twin elsewhere in the universe, exhibiting similarly strange properties. In 2021, Caiazzo published her discovery of the object called “Moon-Sized,” named for its packing a mass similar to the sun into an object roughly the size of Earth’s moon.
The two objects do differ in some respects. Moon-sized doesn’t appear to be surrounded by material as Gandalf is. Moon-sized is also much older, at 500 million years, than Gandalf, at a mere 60 or 70 million. Finally, as an older and more evolved remnant, Moon-Sized’s X-ray emissions are much weaker and were outshone by Gandalf 100-fold.
“If we find one new object in the vastness of the Universe, what are the chances of it being the only one? Usually, one stellar object with new characteristics is more than enough for us to start looking for similar ones,” Caiazzo said. “But here, we actually found two objects with five overlapping features. This is plenty for a new class of star remnants!”
While the team is beginning to understand these objects themselves, how they influence their planetary systems remains a mystery so far.
“The two objects we identified so far have lots of similarities, but also differences,” explains Desai. “Finding more such remnants will help us exclude scenarios and perhaps find other explanations altogether.”
Forming the Merger Remnants
Exactly how these objects could have formed as they did remains a matter of speculation among the team, but they offer a few potential scenarios. For the first one, the researchers speculated that the objects may be the first observations in white dwarfs of an outflow scenario already known to occur in pulsars. In this scenario, the white dwarf is spinning so rapidly, with such strong magnetism, that it extracts material from itself.
“This is my favorite scenario because it only accounts for the white dwarf itself rather than material originating from outside the star remnant,” said lead author of the preprint study, Aayush Desai.
The second scenario involves a reminder of material from the merger event that failed to fully accrete onto the white dwarf. This ring of material would then orbit the remnant on a rotation lasting hundreds of millions of years.
A third scenario is also possible. Roughly one-third of white dwarfs are “polluted” with material from minor impacts such as asteroids. The problem is that, while Gandalf shows evidence of contamination by carbon- or silicon-rich material, Moon-Sized does not, suggesting this is not a complete explanation for the X-ray observations.
The team is still working to develop a better understanding of how to classify this new category, including which of the five shared parameters are essential for its identification.
The paper, “A Half Ring of Ionized Circumstellar Material Trapped in the Magnetosphere of a White Dwarf Merger Remnant,” appeared in Astronomy & Astrophysics on February 10, 2026; additionally, the paper “Revisiting the Most Compact White Dwarf Merger Remnant in the Light of New UV and X-ray Data” is available in preprint form on the arXiv.org preprint server.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.