An international team of astronomers has announced the discovery of an unusual white dwarf star that is made of hydrogen on one side and helium on the other. They reported their findings in the latest issue of Nature. Dr Przemysław Mróz from the Astronomical Observatory is a co-author of the study.

White dwarfs are the scalding remains of stars that were once like our sun. As the stars age, they puff up into red giants, but eventually their outer fluffy material is blown away and their cores contract into dense, fiery-hot white dwarfs. Our sun will evolve into a white dwarf in about 5 billion years.

In a first for white dwarf, astronomers have discovered that at least one member of this cosmic family is two faced. One side of the white dwarf is composed of hydrogen, while the other is made up of helium. The newfound 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 skies every night from Palomar Observatory in California. The object stood out for its rapid changes in brightness that repeat every 15 minutes.

Subsequent observations made with the W. M. Keck Observatory atop Maunakea in Hawaiʻi revealed the dramatic double-faced nature of the white dwarf. The team used an instrument called a spectrometer to spread the light of the white dwarf into a rainbow of wavelengths that contain chemical fingerprints. The data revealed the presence of hydrogen when one side of the object was in view (with no signs of helium), and only helium when the other side swung into view.

What would cause a white dwarf floating alone in space to have such drastically different faces? The team acknowledges they are baffled but have come up with some possible theories. One idea is that we may be witnessing Janus undergoing a rare phase of white dwarf evolution.

After white dwarfs are formed, 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 the white dwarfs cool, the materials are thought to mix together. In some cases, the hydrogen is mixed into the interior and diluted such that helium becomes more prevalent. Janus may embody this transition phase, but one pressing question is: why is the transition happening in such a disjointed way, with one side evolving before the other? 

The answer, according to the science team, may lie in magnetic fields. 

“Magnetic fields around cosmic bodies tend to be asymmetric, or stronger on one side,” says Ilaria Caiazzo, a postdoctoral scholar at the California Institute of Technology who leads a new study. “Magnetic fields can prevent the mixing of materials. So, if the magnetic field is stronger on one side, then that side would have less mixing and thus more hydrogen.”

Another theory proposed by the team to explain the two faces also depends on magnetic fields. But in this scenario, the fields are thought to change 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,” says co-author James Fuller, professor of theoretical astrophysics at Caltech.

To help solve the mystery, the team hopes to find more Janus-like white dwarfs in the data from current and future large sky surveys.

The study – A rotating white dwarf shows different compositions on its opposite faces – was published in Nature on July 19, 2023.