Astronomers have discovered an unusual blue-tinted white dwarf star with two distinct "faces": one side is hydrogen and the other side is helium, according to a new paper published in the journal Nature. Naturally, they nicknamed the star Janus, after the two-faced Roman god of duality and transition.

As previously reported, a white dwarf is essentially the burnt-out core of a dead star. One of the first white dwarf stars discovered, dubbed 40 Eridani B, had a density over 25,000 times that of the Sun, packed into a much smaller volume (roughly the size of Earth)—an observational deduction that astronomers initially deemed impossible. A second white dwarf, Sirius B (orbiting the star Sirius), was discovered soon after and appeared incredibly dense (about 200,000 times as dense as Earth).

That extreme density arises from the unusual mechanism behind the star's internal pressure to keep it from collapsing under the force of gravity. Regular stars rely on energy released via nuclear fusion, but fusion has stopped in white dwarfs. So gravity has compacted all the star's matter inward so tightly that its electrons are smashed together, forming "electron-degenerate matter." This happens because of quantum mechanics, notably the Pauli exclusion principle, which holds that only two electrons can be in the same energy level. Normal gases don't violate this principle because there's sufficient space between electrons to keep all the energy levels in the atoms from filling up. But in a degenerate gas, the electrons do fill up all the energy levels, and this results in an outward-pressing force to halt the star's collapse.

The more mass the white dwarf has, the smaller it becomes because it has to create sufficient internal pressure to support all that mass. And because the star's surface gravity is 100,000 times that of Earth, heavier atoms in its atmosphere sink, leaving lighter atoms at the surface. So white dwarf atmospheres are typically composed of pure hydrogen or pure helium.

That's why this latest discovery of a white dwarf is so intriguing. Astronomer Ilaria Caiazzo, a postdoc at Caltech, first spotted Janus (officially designated ZTF J203349.8+322901.1) while using the Zwicky Transient Facility (ZTF) to look for highly magnetized white dwarfs. The ZTF is a robotic camera attached to the 70-year-old Samuel Oschin telescope at the Palomar Observatory in San Diego County. ZTF performs robotic surveys of the night sky, looking for objects that erupt or vary in brightness: supernovas, stars being munched on by black holes, and asteroids and comets, for example. It scans the entire sky over three nights and the visible plane of the galaxy twice every night.

Follow-up observations with the CHIMERA instrument at Palomar and Spain's Gran Telescopio Canarias revealed that Janus spins on its axis about every 15 minutes. But it was the observational data gleaned with the W.M. Keck Observatory in Hawaii that revealed the star's unusual spectrum, i.e., its distinctive chemical fingerprint: one side hydrogen, the other helium. Caiazzo and her co-authors believe this may be a white dwarf caught in the midst of a rare transition from a hydrogen- to a helium-dominant surface.

That doesn't explain why one side is transitioning faster than the other, however. Astronomers currently have two hypotheses to explain this odd phenomenon, both related to magnetic fields. One posits that Janus' magnetic fields could be asymmetric. "Magnetic fields can prevent the mixing of materials," said Caiazzo. "So, if the magnetic field is stronger on one side, then that side would have less mixing and thus more hydrogen." Perhaps the helium side of Janus appears so bubbly because convection has removed the thin hydrogen layer on the surface, revealing the helium beneath.

The other is that the star's magnetic fields could be changing the pressure and density of the atmospheric gases. "The magnetic fields may lead to lower gas pressures in the atmosphere, and this may allow a hydrogen ‘ocean' to form where the magnetic fields are strongest," said co-author James Fuller, a theoretical astrophysicist at Caltech. "We don't know which of these theories are correct, but we can't think of any other way to explain the asymmetric sides without magnetic fields."

The next step is to locate more two-faced white dwarf stars, which should be easier to accomplish when the Vera C. Rubin Observatory comes online, along with the fifth Sloan Digital Sky Survey. Astronomers have already observed less extreme spectral variations in another white dwarf (GD 323). "Janus might therefore not be an isolated case but rather the most striking member of a class of double-faced white dwarfs," the authors concluded.

DOI: Nature, 2023. 10.1038/s41586-023-06171-9  (About DOIs).

Listing image by K. Miller/Caltech/IPAC