Astronomy: An observable star with two faces.

International scientists write that the cause of this strange phenomenon is most likely magnetic fields in the journal “Nature”.

says the research team led by an astrophysicist Ilaria Kiazo From the California Institute of Technology: “They pack a mass the size of our Sun into an object about the size of Earth.” In about five billion years, when our sun has exhausted its supply of nuclear fuel, it will first expand into a red giant star and then collapse into a white dwarf star that will slowly cool over billions of years.

Named after the Roman god

The star ZTF J203349.8+322901.1, named Janus — named after the two-faced Roman god — first caught researchers’ attention during observations with Zwicky Transit Facility At the observatory on Mount Palomar in California. This telescope looks for significant and transient changes in the brightness of stars. Janus exhibits very strong fluctuations in its brightness every 15 minutes, which is unusual behavior for white dwarfs. Reason enough for Kazuo and her team to take a closer look at the object using various telescopes.

Among other things, the Gran Telescopio Canarias on the Canary Island of La Palma and one of the Keck telescopes in Hawaii were used. The researchers used a spectrophotometer to break down the light from Janus into its spectral colours. In this way, astronomers can find out the chemical elements that make up celestial bodies. Because each element emits radiation at very specific wavelengths. These spectral lines act as a kind of fingerprint to identify the elements of interest.

Magnetic fields at work?

The result of the observations was surprising: Janus’ surface consists almost exclusively of helium on one side and only hydrogen on the other. So far, only white dwarfs have been known whose surface consists of either hydrogen or helium. “With the strong gravity of white dwarfs, all heavy elements sink to the depths, and at first only hydrogen remains at the top,” says the research team. But when the dwarf star cools below 30,000 degrees, the upper layers mix, and so helium dominates over the cooler-surfaced white dwarfs.

“Apparently, with Janus, we just found a white dwarf at the moment of this transition,” says Kiazo. The question remains why this transition is so uneven in the two stellar hemispheres. Scientists suspect that magnetic fields are at work here. This is because the magnetic fields of stars are often asymmetric, that is, stronger on one side than the other. When the magnetic field is stronger, it can hinder the transition. The team now hopes to use the Zwicky Transit Facility to find more two-faced white dwarfs, and thus trace the origin of the phenomenon.