Scientists Stumbled upon a Two-Faced White Dwarf (And They Can’t Really Explain It)

Astronomers are puzzled. Because on one side the newly discovered star is made of hydrogen, while the other is made of helium.

Meanwhile, many white dwarf stars have already been discovered in the universe. This means that researchers already know a lot about these “dead stars”. However, it turns out that they still have surprises in store for us. Because for the first time, scientists have encountered a white dwarf that remarkably has two different faces.

white dwarfs
As you may already know, white dwarfs are the boiling remnants of stars that once looked like our own sun. As stars age, hydrogen fusion in the star’s core stops and it swells. A red giant then forms. When the star then blows off its outer layers of gas and dust, the core contracts, leaving a white dwarf. The star has reached the end of her life. Our sun will also eventually become a compact white dwarf in five billion years.

Ash
In a new study, researchers took a closer look at a recently discovered white dwarf. The star used the Zwicky Transitional Facility (ZTF), an instrument that scans the sky every night. Follow-up research has revealed that the star rotates around its own axis every 15 minutes.

two sides
So far nothing special. But later observations using the WM Keck Observatory in Hawaii revealed something striking. The team used a spectrometer to analyze the light from the white dwarf. And the data revealed the presence of hydrogen when one side of the star was in view (with no sign of helium), and only helium when the other side came into view. In short, the star looks like it has two faces, with one side in hydrogen and the other in helium. “The surface of the star is therefore fundamentally different on the one hand than on the other,” said researcher Ilaria Caiazzo.

janus
Due to the dual nature of the white dwarf, the star is named Janus, after the Roman god of beginnings and endings, who is also depicted as having two faces. Researchers still don’t know what the exact cause is. They did, however, offer a few possible theories for the puzzling phenomenon.

hydrogen to helium
“Some white dwarfs first have a hydrogen-dominated surface and then get a helium-dominated surface,” Caiazzo explains. “Maybe we caught a white dwarf red-handed.”

That’s right
When a white dwarf sees daylight, the heaviest elements “flow” towards the core. The lightest elements – hydrogen being the lightest of all – then float to the surface. Over time, as the white dwarf continues to cool, all the elements mix together. In some cases, however, the hydrogen inside becomes so dilute that the helium takes over. And Janus could well embody this transition phase. Although in this case, the question arises as to why the transition occurs so inconsistently, with one side moving earlier than the other.

Magnetic field
The researchers suspect it might have something to do with the white dwarf’s magnetic field. “They are usually asymmetrical or stronger on one side,” Caiazzo explains. “They can even prevent materials from mixing. So if the magnetic field is stronger on one side, less elements are mixed here, resulting in more hydrogen. Another option is that the magnetic field changes the pressure and density of atmospheric gases. “Magnetic fields can cause the pressure of gas in the atmosphere to drop,” says co-author James Fuller. “Where they are strongest, an ocean of hydrogen can then form.”

More white dwarfs
At the moment, however, that remains speculation. “We don’t know which of these theories is correct, but we can’t think of any other way to explain asymmetric sides without magnetic fields,” Fuller says. To help solve the mystery, the team hopes to find more Janus-like white dwarfs using the ZTF. And they’re actually pretty confident about it. “ZTF is very good at detecting foreign objects,” says Caiazzo. “And with the help of the Vera C. Rubin Observatory in Chile, which is still under construction, it will soon be even easier to reveal variable white dwarfs.”

And that creates hope. Because maybe in a few years we’ll learn more about the dual nature of Janus, who is still the cosmic weirdo right now.