Sun-like stars and stars smaller than the Sun end their lives as white dwarfs. Without a steady source of energy from hydrogen fusion, these stars eventually collapse under their own weight. Without the pressure of the electrons, they would continue to collapse. As long as the star’s residual mass is less than about 1.4 Suns, electron pressure and gravitational force will balance each other to form a white dwarf.

White dwarfs are about the same size as Earth, but have the mass of a star, so their surface gravity is very strong. As a result, elements inside the white dwarf tend to settle so that the heaviest elements sink and the lightest elements are pushed to the surface. This means that most white dwarfs have a surface of pure hydrogen or pure helium, but some have an “open” carbon or oxygen surface. But recently, astronomers discovered an unusual white dwarf with two faces. It has a hydrogen surface on one side and a helium surface on the other.

It is known as ZTF 203349.8+322901.1 and was first detected by the team while searching for strongly magnetized white dwarfs with the Zwicky Transient Facility (ZTF) at Palomar Observatory. It fluctuated in brightness for a period of about 15 minutes. Strongly magnetized white dwarfs often have fluctuations in brightness, but not at normal proportions. Other observations confirmed that the star rotates with one side brighter than the other. The team then performed spectral observations of ZTF 203349.8+322901.1 and found that the bright side is almost pure helium and the dark side is almost pure hydrogen.

Of course, that doesn’t explain why this white dwarf has a double surface. It is not part of a binary system, so there is no obvious mechanism to cause such a dramatic difference. Even the team isn’t entirely sure, but one idea might be that the star has an asymmetrical magnetic field. Strong magnetic fields can hinder the movement of elements inside the star, so hydrogen will prefer the strong magnetic side. Another idea is that the asymmetric magnetic field interacts with the thin atmosphere of the white dwarf, creating a sea of ​​hydrogen alongside the strong magnetic field.

Whatever the mechanism, the discovery is exciting because the white dwarf could be in a transition state between a hydrogen-surfaced star and a helium-surfaced star. As white dwarfs cool, it’s thought that mixing of near-surface elements could sink hydrogen deeper into the star, leaving helium on the surface. This may explain why some white dwarfs have a hydrogen surface and some have a helium surface, but astronomers have never seen a transiting white dwarf until now. Source