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Wednesday, December 6, 2023

Two-faced star Janus. Poles helped find it

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The star “with two faces” managed to be observed by an international team of scientists. The celestial body is covered with hydrogen on one side and helium on the other. Researchers suggest that this unusual feature may be related to the object’s magnetic field.

The research on the unusual star was published in the journal “Nature”. It is a white dwarf, half of which is covered with hydrogen and the other half with helium. The object was named Janus in honor of the Roman deity with two faces.

Changing and spinning

The star was discovered in data from the Zwicky Transient Facility (ZTF) sky survey, carried out at the Palomar Observatory in California. Doctor Przemysław Mróz from the Astronomical Observatory of the University of Warsaw was responsible, among other things, for measuring the brightness of over a billion stars observed by ZTF, thanks to which this unusual object was found, the University of Warsaw informed.

– The star was distinguished by large changes in brightness, repeated every 15 minutes. This is as long as it takes one revolution around its axis – explained the Polish researcher.

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Observations made with the Keck Telescope at the Mauna Kea Observatory in Hawaii revealed the unusual nature of the star. The scientists used an instrument called a spectrometer to split the star’s light, allowing them to study its chemical composition. Observations showed only hydrogen on one half of the star and only helium on the other.

The star Janus in the artist’s vision. Its brighter side is covered with hydrogen and the darker side with heliumK. Miller, Caltech/IPAC

What are white dwarfs?

White dwarfs are the remnants of sun-like stars. As Mróz said, when a star burns out the fuel inside it, it inflates to enormous proportions, becoming a so-called red giant. It then sheds its gaseous envelope, leaving only a dense, hot core. It is covered with a thin layer of helium or hydrogen, or, as Janus shows, both.

White dwarfs have masses similar to the mass of the Sun, but they are even a hundred times smaller and a million times denser. One teaspoon of white dwarf matter can weigh as much as a car.

– It is believed that the atmospheres of some white dwarfs change from hydrogen to helium. Due to its high density, and therefore strong gravity, heavy elements sink to the center. Hydrogen is the lightest of all elements and floats to the surface. However, it is believed that over time, as the white dwarf cools down, there should be mixing of layers of hydrogen and helium on the star’s surface. Janus is probably the first object in which this phenomenon was directly observed – explained Mróz.

Janus magnetic field – artist’s visionK. Miller, Caltech/IPAC

Why this inequality?

However, the reason for such significant differences in chemical composition on the surface of Janus is not known. Scientists speculate that this magnetic field could be the key to explaining them.

– The magnetic field of stars can be unbalanced or stronger on one side. This can prevent the mixing of elements, explained Ilaria Caiazzo of the California Institute of Technology (CalTech), the lead author of the paper, adding: ‘If the magnetic field is stronger on one side of the white dwarf, it prevents the mixing of elements and one side of the star is hydrogen.

Another hypothesis put forward by the authors of the paper is that the magnetic field may affect the pressure and density of matter in the star’s atmosphere.

“The field can reduce the pressure of the gas, creating an ‘ocean’ of hydrogen only where the magnetic field is strongest,” added James Fuller of CalTech, co-author of the study.

Testing which of these hypotheses is true requires finding and analyzing a larger group of similar objects. Scientists hope to provide it with future great surveys of the sky.

Main photo source: K. Miller, Caltech/IPAC

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