A white dwarf star 1,300 light-years away shows astronomers two very different faces: the surface of one hemisphere consists of helium, the surface of the other consists of hydrogen. This is shown by observations by an international research team using several large telescopes. It is the first time that sky researchers have observed such a phenomenon in a white dwarf. The reason for the strange phenomenon are probably magnetic fields, write the scientists in the journal “Nature”.
“White dwarfs are extremely dense remnants of ordinary stars like our Sun,” explain Ilaria Caiazzo of the California Institute of Technology and her colleagues. “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 such a white dwarf, which will slowly cool over billions of years.
The Roman god is the namesake
The star ZTF J203349.8 322901.1, christened Janus – named after the two-faced Roman god – first caught the attention of researchers during observations with the Zwicky Transient Facility at the Observatory on Mount Palomar in California. This telescope looks for prominent, transient changes in the brightness of stars. Janus shows very strong fluctuations in its brightness every 15 minutes, an unusual behavior for white dwarfs. Reason enough for Caiazzo and her colleagues to take a closer look at the object with various telescopes.
Among other things, the Gran Telescopio Canarias on the Canary Island of La Palma and one of the Keck telescopes on Hawaii were used. With the help of special devices – spectrometers – the researchers separated the light from Janus into its spectral colors. In this way, astronomers can find out what chemical elements a celestial object is made of. Because every element emits radiation at very specific wavelengths. These spectral lines serve as a kind of fingerprint to identify the respective elements.
A white dwarf in the moment of transition
The result of the observations was surprising: the surface of Janus consists almost exclusively of helium on one side and only hydrogen on the other side. So far, astronomers have only known white dwarfs whose surface consists of either hydrogen or helium. “With the strong gravity of the white dwarfs, all heavy elements sink to the depths, initially only hydrogen remains at the top,” the researchers explain. But when a dwarf star cools below 30,000 degrees, the upper layers mix, and helium therefore dominates in cooler surface white dwarfs.
“Apparently, with Janus, we just caught a white dwarf at the moment of this transition,” says Caiazzo. The question remains why this transition is so uneven in the two hemispheres of the star. The scientists suspect that magnetic fields are at work here. This is because the magnetic fields of stars are often asymmetrical, i.e. stronger on one side than on the other. Where the magnetic field is stronger, it could impede the transition. The team now hopes to use the Zwicky Transient Facility to find more double-faced white dwarfs and thus trace the origin of the phenomenon.
