It is one of the strangest stars ever discovered.
Strange star with one side made of hydrogen, the other made of helium
In a novel study published in the journal Nature, astronomers have discovered a strange star, a white dwarf, with one side made of hydrogen, and the other made of helium. “The star’s surface is entirely different from one side to another,” says Ilaria Caiazzo, a postdoctoral fellow at the California Institute of Technology (Caltech) leading the groundbreaking research.
White dwarfs, remnants of sun-like stars, stir great intrigue among astronomers. Once these stars exhaust their lifetimes, they swell into red giants, shedding their outer material while their cores contract and ignite into dense, fiery white dwarfs. Our Sun is expected to experience this transformation in about 5 billion years.
Unveiling the Janus Star: The God of Transition
The white dwarf, affectionately named Janus after the Roman god of transition, was initially spotted by the Zwicky Transient Facility (ZTF), which daily scans the sky from Caltech’s Palomar Observatory near San Diego.
Caiazzo was primarily focused on identifying highly magnetized white dwarfs, like the ZTF J1901 1458, discovered earlier with her team. One potential object’s swift brightness changes piqued her interest, leading to further investigation using the CHIMERA instrument at Palomar and HiPERCAM on the Gran Telescopio Canarias in the Spanish Canary Islands. The data confirmed Janus’s swift rotation, completing an axis spin every 15 minutes.
The Dual-Faced Phenomenon of the Janus Star
Further scrutiny with the W. M. Keck Observatory atop Maunakea, Hawaii, revealed Janus’s stunning dual-sided nature. By spreading the light from the white dwarf into a spectrum of wavelengths, chemical signatures emerged. This showed hydrogen dominating one side (no sign of helium), and exclusively helium on the other side. This distinctive feature left the team pondering several theories.
Janus – A White Dwarf in Transition?
“We might be witnessing Janus in a rare phase of white dwarf evolution,” suggests Caiazzo. When white dwarfs form, their heavier elements descend towards the core while the lighter ones—hydrogen, the lightest—rise to the surface. As white dwarfs age and cool, their materials intermix. In some instances, hydrogen blends in such a way that helium becomes predominant. Janus may be representing this phase, although why this transition appears uneven is still a question.
According to the researchers, the answer could lie within magnetic fields. “Magnetic fields around celestial bodies tend to be asymmetric, or stronger on one side,” Caiazzo mentions. A stronger magnetic field on one side could inhibit material mixing, resulting in more hydrogen. Another theory implies the atmospheric gas pressure and density alteration through magnetic fields.
The Mystery Continues
James Fuller, study co-author and professor of theoretical astrophysics at Caltech, suggests, “Magnetic fields could decrease gas pressure in the atmosphere, permitting a ‘hydrogen ocean’ where magnetic fields are strongest.” The team is still unsure about which theory is correct, but asymmetric sides without magnetic fields seem improbable.
To solve this puzzle, researchers are hopeful about discovering more Janus-like white dwarfs with ZTF’s sky survey. “ZTF excels in locating peculiar objects,” Caiazzo remarks. Future studies, particularly by the Vera C. Rubin Observatory in Chile, should ease the process of finding these fascinating white dwarf variables.
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