Astronomers Discover Pair of Super-Rare ‘Two-Faced’ Stars – ryan

Scientists have discovered two rare white dwarfs—which are among the dimmest stars in the universe—that feature changes in their surface composition caused by magnetic fields. This brings the number of known “two-faced” white dwarfs to just seven.

White dwarfs are dense, core remnants of stars that have shed their outer layers and cool over billions of years as they steadily burn off their thermal energy.

The surface atmosphere of white dwarfs were originally thought to be composed mostly of hydrogen. However, in 2023, astronomers discovered an unusual white dwarf that had a surface made of hydrogen on one side and helium on the other.

Since then, a handful of other white dwarfs with differing surface compositions have been found, forming a growing class of “double-faced” white dwarfs.

“Given the several patchy-atmosphere objects known at this time, we are able to define the class of double-faced objects for the first time,” the researchers wrote in their study.

These unique white dwarfs exhibit different compositional “faces” as they spin, resembling one type of white dwarf in one moment and another type in another.

Four of the seven known double-faced white dwarfs are confirmed to be “strongly magnetic,” the study noted.

While most white dwarfs maintain a homogeneous surface atmosphere throughout their evolution, double-faced white dwarfs form as “a result of the magnetic field influencing the motion and mixing of either H (hydrogen) or He (helium), creating an atmosphere with varying surface abundances,” the astronomers explained.

“Since magnetism is quite rare in He-dominated white dwarfs, it follows that these inhomogeneous atmospheres are quite rare among the DBA (white dwarfs with a mixed surface composition of hydrogen and helium) population,” the researchers said.

Adam Moss from the University of Oklahoma, one of the lead authors of the study, and his team believe that magnetic fields can impact the convection of a white dwarf—process that brings helium-rich material to the surface of the star—and this alters its the fundamental composition.

So, two-faced stars may result from stronger magnetic fields at the star’s magnetic poles inhibiting convection, while weaker fields near the equator allow convection to take place.

“While the exact nature of the atmosphere geometry cannot be directly confirmed due to the complexities in how magnetism affects convection, we are confident that magnetism is the source of the inhomogeneities in this class of objects,” the astronomers concluded.

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Reference

Moss, A., Kilic, M., Bergeron, P., Jewett, G., & Brown, W. R. (2025). The Emerging Class of Double-faced White Dwarfs. The Astrophysical Journal, 983(1), 14.