A strange "two-sided" star was discovered 1,300 light years away, one side is hydrogen and the other side is helium

An artist's impression of the newly discovered double-sided white dwarf "Janus"

Astronomers recently discovered a very strange white dwarf with completely different compositions on both sides: one side is made entirely of hydrogen, while the other side is made entirely of helium. This is the first time scientists have discovered such a "two-sided" white dwarf. Because of this, scientists named this unique white dwarf after the two-faced god "Janus" in Roman mythology .

The discovery of Janus was very accidental. While searching for the remains of a dead star at the Zwicky Transient Facility in California, the researchers accidentally discovered a strange signal. They then used several other telescopes to observe it several times and finally discovered Janus's amazing "double-sided" features. Dr. Ilaria Caiazzo, an astrophysicist at the California Institute of Technology who led the study, said: "When I showed these observations to others, they were stunned."

Observations show that Janus is located more than 1,300 light-years from Earth in the direction of the Cygnus constellation and rotates once every 15 minutes. If observed up close, the two sides of Janus will appear blue and have similar brightness, but the helium side will have a granular appearance like our sun, while the hydrogen side will appear smooth.

Schematic diagram of the magnetic field around the double-sided white dwarf Janus, whose surface magnetic field strength may be unevenly distributed.

How did Janus become "two-faced"?

The remnants of most stars after their death, white dwarfs are extremely dense, about the size of an Earth-like planet but with a mass comparable to that of our sun, making them among the densest objects in space, after neutron stars and black holes.

Under the influence of the enormous gravity at the white dwarf's surface, heavier elements sink toward the center, leaving the upper layers containing only the lightest elements, usually hydrogen or helium.

When a white dwarf is hot, helium occupies its surface. When the temperature of a white dwarf drops below 30,000 degrees Celsius, the helium sinks into the interior, while the hydrogen rises to the surface. When the white dwarf heats up again, the hydrogen sinks, while the helium reappears on the surface of the white dwarf. Janus may be somewhere in between these two transition stages, and scientists have never discovered a white dwarf in a similar transition stage.

As for the reason why Janus has two sides, researchers believe that it is still difficult to explain. One possible reason is that Janus is undergoing a rare change process.

The researchers speculate that it may also be related to the magnetic field inside Janus. The magnetic field can affect the distribution and movement of matter on the surface of a star. If there is a magnetic field inside Janus that is stronger on one side than on the other, it may cause hydrogen and helium to gather in different hemispheres, and a hydrogen "ocean" will form in the area with a stronger magnetic field. However, current observational data cannot prove this.

Could Janus be a binary star system consisting of a white dwarf whose spectrum is dominated by hydrogen and a white dwarf whose spectrum is dominated by helium? Current observations have ruled out this possibility.

In any case, Janus is a very special star that challenges astronomers' understanding of white dwarf evolution. In the future, researchers also hope to find more celestial bodies like Janus to uncover the secrets behind "double-sided" white dwarfs.

Compiled by: Youyou

Planning: Zhang Chao, Li Peiyuan, Yang Liu

Reviewed by: Li Xin, Research Librarian of Beijing Planetarium