Janus: a two-faced white dwarf with a mysterious composition

White dwarfs are stars that have passed through a burned-out shell stage and left behind a compact core. They are usually composed mostly of hydrogen or helium, with levels of the basic element about 1000 times higher than others. However, astronomers recently discovered a white dwarf that doesn’t fit this description. This star, named Janus after the two-faced Roman god, has two different faces – one half is made up mostly of hydrogen and the other half of helium. This is the first time this phenomenon has been observed for an object of this type and raises many questions.

The researchers discovered Janus using the Zwicky Transient Facility (ZTF), an observatory designed to observe objects that change their brightness in the night sky. They noticed that this white dwarf changes its brightness as it rotates. Further observations with a spectrometer showed that the star’s composition varies greatly and correlates with the changes in brightness.

There are several ideas about how Janus became so two-faced. One suggests that some white dwarfs go from hydrogen-dominated to helium-dominated during their lifetime, and Janus may be the first to be caught in this act. However, it is still a mystery why he changes his face at a time.

Researchers believe that the answer may be related to Janus’ magnetic field. If the magnetic field is stronger on one side than the other, it could prevent hydrogen and helium from mixing on that side, leaving a stronger hydrogen signal. A magnetic field can also change the pressure and density of gases. This explanation is supported by the fact that there are no other ways to explain the asymmetry of sides without magnetic fields.

However, to solve this mystery completely, we need to find other white dwarfs that are in a similar state of separation. The researchers hope that further observations and studies will help solve the mystery of Janus and shed light on the processes occurring in these mysterious stars.

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