December 11, 2023

The ingredients in stars are usually well mixed, but now astronomers have discovered a strange star with two different faces, each made of different elements, which cannot be explained.

White dwarfs are the burnt-out outer shells of stars like our sun that have shed their outer layers, leaving only a dense core. Their atmospheres are usually dominated by hydrogen or helium, with the main element being about 1,000 times more abundant than other elements.

But now, astronomers have discovered a white dwarf that doesn’t fit that description—and how exactly it formed remains a mystery. Half of the white dwarf is composed almost entirely of hydrogen, while the other half is helium, marking the first time such a sharp split has occurred for such an object. As such, it was nicknamed Janus, named after the two-faced Roman god.

“The surface of the white dwarf completely changes from one side to the other,” said Ilaria Caiazzo, lead author of the study. “When I showed people the observations, they were taken aback.”

Janus was discovered through the Zwicky Transient Facility (ZTF), an observatory designed to observe objects in the night sky that change in brightness over time. The team noticed that the white dwarf changed its brightness as it rotated. Follow-up observations using a spectrometer, which can analyze the light to reveal the elements that make up the light source, revealed clear differences in the composition associated with changes in brightness.

Researchers have some ideas about how this strange star became so two-faced. Some white dwarfs are thought to transition between hydrogen dominance and helium dominance during their lifetimes, and Janus may be the first white dwarf to fall into this behavior. But even so, it’s still unknown why it changes one face at a time.

The answer to this question, the team says, may be due to the object’s strong magnetic field. If one side is stronger than the other, it prevents the hydrogen and helium from mixing on that side, leaving a stronger hydrogen signal. A similar explanation might be that the magnetic field changes the pressure and density of the gas.

“Magnetic fields could cause lower atmospheric pressure, which could create a hydrogen ‘ocean’ where the magnetic field is strongest,” said study co-author James Fuller. “We don’t know which of these theories are correct, but We couldn’t think of any other way to explain the asymmetric plane without the magnetic field.”

More clues to this mystery could be revealed by looking for other white dwarfs in a similarly separated state.

The study was published in the journal nature. The team describes Janus in the video below.

Janus, the strange two-faced star

source: Caltech