For the first time in the case of white dwarfs, astronomers have discovered that at least one member of this cosmic family, the burnt-out cores of dead stars, has two faces. One side of a white dwarf is made of hydrogen and the other is made of helium.

“The white dwarf’s surface changes completely from one side to the other,” says Caltech postdoctoral researcher Ilaria Giazzo, who led the new study on the journal’s findings. Nature. “When I show the observations to people, they are impressed.”

White dwarfs are the burning remains of stars that were once like our Sun. As stars age, they become red giants; Eventually, their fluffy outer material sloughs off and their cores shrink into fiery, dense white dwarfs. Our Sun will become a white dwarf in about 5 billion years.

The newly discovered white dwarf, nicknamed Janus after the two-faced Roman god of transition, was initially discovered by the Zwicky Transient Facility (ZTF), an instrument that scans the sky every night from Caltech’s Palomar Observatory near San Diego. ZTF was looking for highly magnetized white dwarfs, such as an object known as J1901+1458. One candidate object stood out for its rapid changes in brightness, so Caiazzo decided to investigate further. Chimera Tool in Palomar, also hypercom in Gran Telescopio Canarias in the Canary Islands, Spain. Those data confirmed that Janus rotates on its axis every 15 minutes.

Follow-up observations with the WM Keck Observatory on Mauna Kea, Hawaii, revealed the spectacular two-sided nature of the white dwarf. The team used an instrument called a spectrometer to diffuse light from the white dwarf across a rainbow of wavelengths that carry chemical signatures. The data revealed the presence of hydrogen when one side of the object was in view (no sign of helium), and only helium when the other side was in view.

Why does a white dwarf floating alone in space have so many different faces? The team admits they’re confused, but they’ve come up with a few possible theories. One idea is that we are seeing Janus go through a rare white dwarf phase of evolution.

“Not all, but some white dwarfs have surfaces dominated by hydrogen instead of helium,” explains Giazzo. “We may have caught a white dwarf in action.”

After white dwarfs form, their heavier elements sink to their cores and their lighter elements (hydrogen being the lightest of all) float to the top. But over time, as white dwarfs cool, the materials are thought to mix. In some cases, hydrogen mixes inside and helium is diluted to become more diffuse. Janus may embody this transition phase, but a pressing question is: Why does the transition occur in such an ironic fashion, with one side evolving before the other?

According to the scientific team, the answer may lie in magnetic fields.

“The magnetic fields around cosmic bodies are asymmetrical, or strong on one side,” explains Giazzo. “Magnetic fields can prevent materials from mixing. So if the magnetic field is stronger on one side, that side will mix less and have more hydrogen.

Another theory proposed by the team to explain the two faces depends on magnetic fields. But in this scenario, the fields are thought to change the pressure and density of atmospheric gases.

“Magnetic fields lead to lower gas pressures in the atmosphere, and this may allow a ‘sea’ of hydrogen to form where magnetic fields are stronger,” says James Fuller, professor of theoretical astrophysics at Caltech. “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.”

Reference: “A rotating white dwarf shows different structures on its opposite faces” Ilaria Giazzo, Kevin P. Burdge, Pierre-Emmanuel Tremblay, James Fuller, Lilia Ferrario, Boris D. Consicke, J.J. Hermes, Jeremy Hale, Adela Gavka, Thomas R. Kulkarni, Thomas R. Kulkarni. Richer, Antonio C. Rodríguez, John Van Rostel, Zachary B. Vanderbosch, Stephen Wennes, Dayal Wickramasinghe, Vikram S. Dillon, Stuart B. Littlefair, James Munday, Ingrid Bellisoli, Daniel Perley, Ridin Perley, Eric C. Breitken, Elmex Pruitt, Margaret J. Dyer, Mateo J. GrahamMatthew J. Verde, Russ R. Lahr, Paul Kerry, Steven G. ParsonsReid L. Riddle, Ben Rusholm, and Dave I. Sahman, Jul 19, 2023, Nature.
DOI: 10.1038/s41586-023-06171-9