Stars can generate powerful shock waves when gas and dust flowing from them collide with material in the surrounding space. But RXJ0528+2838 is a small stellar remnant, and based on known mechanisms, it should not be able to create the dramatic structures observed around it.

“We discovered something we had never seen before, and more importantly, something completely unexpected,” says Simone Scaringi, an associate professor at Durham University in the UK and co-lead author of the study published in Nature Astronomy.

“Our observations reveal a strong outflow that should not be there, according to our current understanding,” says study co-lead Christian Ilkiewicz, a postdoctoral fellow at the Nicolaus Copernicus Astronomical Center in Warsaw, Poland.

Astronomers use the term “outflow” to describe matter ejected from an object in space.

A mysterious bow shock around a white dwarf

RXJ0528+2838 is located approximately 730 light-years from Earth. Like the Sun and other stars, it moves around the center of the Milky Way. As it travels through space, it encounters interstellar gas, creating a type of shock wave known as a bow shock.

Noel Castro-Segura, a researcher at the University of Warwick in the UK and a co-investigator on the study, describes the structure as “a curved arc of material, similar to waves that accumulate in front of a ship.”

Bow shocks typically form when material flowing from a star collides with its surroundings. In the case of RXJ0528+2838, however, astronomers cannot identify any known process that fully explains the outflow and surrounding nebula.

Why this white dwarf is so unusual

RXJ0528+2838 is a white dwarf—the remaining core of a dying low-mass star. It is orbited by a companion star similar to the Sun.

In binary star systems like this one, material can be pulled from the companion and transferred to the white dwarf. The material often forms an accretion disk around the stellar remnant. That disk feeds the white dwarf, while some material may also be expelled into space as a powerful outflow.

RXJ0528+2838 is different. Astronomers have found no evidence of an accretion disk, leaving them without the usual explanation for the powerful outflow and nebula.

“The surprise that a supposedly quiet diskless system could drive such a spectacular nebula was one of those rare ‘wow’ moments,” Scaringi says.

VLT observations confirm the structure’s origin

The unusual structure was first noticed in images taken with the Isaac Newton Telescope in Spain. Because of its strange appearance, researchers investigated it in greater detail with the MUSE instrument on ESO’s VLT.

“Observations with the ESO MUSE instrument allowed us to map the bow shock in detail and analyze its composition, which was crucial to confirm that its structure really originates from a binary star system and not from an unrelated nebula or interstellar cloud,” Ilkiewicz explains.

The size and shape of the bow shock suggest that RXJ0528+2838 has been producing strong outflows for at least 1,000 years.

That timescale creates another mystery. Scientists still do not know how a dead star without an accretion disk can sustain such long-term outflows. The system’s magnetic field may provide part of the answer.

Could the magnetic field power the outflow?

RXJ0528+2838 is known to have a strong magnetic field, which was also confirmed by MUSE observations.

Instead of allowing material from the companion star to settle into a disk, the magnetic field appears to direct the material straight onto the white dwarf.

“Our findings show that even in the absence of a disk, these systems can generate strong outflows, and reveal mechanisms that we do not yet understand. This discovery challenges the standard picture of how matter moves and interacts in these extreme binary systems,” Ilkiewicz explains.

Researchers believe the magnetic field may be connected to a hidden energy source—what Scaringi describes as a “mysterious engine.” However, this explanation remains incomplete.

Observations indicate that the white dwarf’s current magnetic field could withstand bow shocks for only a few hundred years. Because the structure appears to have existed for at least 1,000 years, magnetic fields alone cannot fully explain the phenomenon.

Astronomers search for the “mystery engine”

Astronomers will need to study many more binary star systems to determine how powerful outflows can form without an accretion disk.

ESO’s upcoming Extremely Large Telescope (ELT) could play an important role by allowing scientists to study both known systems and much fainter examples in greater detail.

Scaringi hopes the telescope will help researchers “map more of these systems, as well as fainter stars, detect similar systems in greater detail, and ultimately help us understand mysterious energy sources that remain unexplained.”