September 29, 2023

Science – Janus has different hemispheres – knowledge

Pasadena (dpa) – A white dwarf star 1,300 light-years away shows astronomers two very different faces: the surface of one hemisphere is made of helium, and the surface of the other half is made of hydrogen. This is evidenced by observations made by an international research team using several large telescopes. It’s the first time sky researchers have observed such a phenomenon in a white dwarf. Perhaps the reason for this strange phenomenon is due to magnetic fields, the scientists wrote in the journal “Nature”.

“White dwarfs are the extremely dense remnants of ordinary stars like our Sun,” explain Ilaria Kayazo 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 same name

The star ZTF J203349.8+322901.1, named Janus — named after the two-faced Roman god — first came to researchers’ attention during observations made with the observatory’s Zwicky Transit Facility 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 colleagues 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. 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 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.

A white dwarf at the moment of transition

The result of the observations was surprising: Janus’ surface consists almost exclusively of helium on one side and only hydrogen on the other. Until now, astronomers only knew of white dwarfs whose surface is composed 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,” the researchers explain. However, when a dwarf star cools below 30,000 degrees, the upper layers are mixed, and thus helium is dominant in white dwarfs with a cooler surface.

“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.

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