A vibrant display of glowing gases around a dead star is fueled by a mysterious source that scientists can’t explain, in a phenomenon without precedent.
As stars move through space, they push away material and create a so-called bow shock ahead of them akin to a curved ridge of water that forms in front of a moving boat. Using the European Southern Observatory’s Very Large Telescope, a team of astronomers found a shock wave that shouldn’t be there.
The star RXJ0528+2838 is a white dwarf, the remains of a dead star that no longer generates energy through nuclear fusion. And yet, the white dwarf is surrounded by a bright shock wave that glows in red, green, and blue—and its origin is unknown. The discovery, published Monday in Nature Astronomy, can’t be explained by any known mechanism of the universe.
Cosmic mystery
The white dwarf star is located approximately 730 light-years away from Earth, which is quite close in cosmological terms. The leftover core of the low-mass star has a stellar companion, a Sun-like star orbiting around it. In these types of binary systems, the material of the companion star often gets transferred to its dead neighbor, forming a disc around the white dwarf. The disc fuels the white dwarf, while some of the material is ejected into space to form an outflow.
The recently discovered star, however, shows no sign of a disc. “We found something never seen before and, more importantly, entirely unexpected,” Simone Scaringi, associate professor at Durham University, England, and co-lead author of the study, said in a statement. “The surprise that a supposedly quiet, discless system could drive such a spectacular nebula was one of those rare ‘wow’ moments.”
As the white dwarf orbits the center of the Milky Way, it interacts with surrounding gas to form a vibrant bow shock ahead of it. These bow shocks are usually created by material outflowing from the star, but the white dwarf shows no such evidence of generating any type of material.
“Our observations reveal a powerful outflow that, according to our current understanding, shouldn’t be there,” Krystian Ilkiewicz, a postdoctoral researcher at the Nicolaus Copernicus Astronomical Center in Warsaw, Poland, and study co-lead, said in a statement.
Origin unknown
The scientists behind the discovery mapped the bow shock in detail and analyzed its composition, confirming that it is originating from the binary star system. They found that the shape and size of the bow shock suggests that the white dwarf has been expelling this material for at least 1,000 years, furthering the mystery of how a lifeless star with no disc produces such a long-lasting outflow.
Although this is a first-of-its-kind cosmic mystery, the team of scientists does have some clues as to how it may have come to be. The results point to a possible hidden energy source, likely a strong magnetic field. The white dwarf’s magnetic field could be channeling material stolen from its companion star directly onto its dead stellar remains without forming a disc around it.
The theory, however, still needs some work. A magnetic field would only be strong enough to power a bow shock for a few hundred years, only solving part of the cosmic mystery.
“This discovery challenges the standard picture of how matter moves and interacts in these extreme binary systems,” Ilkiewicz said. “Our finding shows that even without a disc, these systems can drive powerful outflows, revealing a mechanism we do not yet understand.”
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