White dwarfs are remnants of stars that once looked like our Sun. As stars age, they swell to become red giants, then their outer material is shed and their cores shrink into dense, incandescent dwarfs. For example, the Sun will turn into a white dwarf after about 5 billion years.

Two-faced Janus

The new white dwarf, named after the two-faced Roman god Janus, was discovered by the Palomar Observatory at the California Institute of Technology. Astronomers noticed an object whose brightness changes very rapidly and discovered that the star rotates around its axis 15 times per minute.

Further observations revealed the unusual nature of the white dwarf. The researchers determined the wavelength of the starlight using a spectrometer. The data showed the presence of only hydrogen when one side of the object was visible, and only helium when the other side was visible.. In this way Janus evenly distributed his matter in different directions, and this is the first known example of our science in history.

What could have caused a white dwarf to become this way? The exact cause is unknown, but researchers have several convincing theories.

According to one of the versions, perhaps we are observing a rare stage of the evolution of the star. Astronomers believe that some white dwarfs have switched from hydrogen to helium composition on their surface.

Not all, but some white dwarfs are transitioning from hydrogen dominance to helium on their surface. We may have caught one of these white dwarfs in the process,
— says the study’s lead author, Ilaria Caiazzo, of the California Institute of Technology.

Normally, the gravity of an average white dwarf pulls the heavier helium into its core, while the lighter hydrogen drifts toward the surface, forming separate layers. But as the star cools, this hierarchy breaks down as the elements mix and change their appearance and composition.

This should occur more or less evenly across the star, not just on one half. Caiazzo suggests that asymmetrical magnetic fields may be responsible.

“Magnetic fields can prevent materials from mixing. So if the magnetic field is stronger on one side, there will be less mixing and therefore more hydrogen on that side.”– terminates the researcher.