Introducing Janus, the Fascinating 'Two-Faced' White Dwarf Star


Introducing Janus, the Fascinating 'Two-Faced' White Dwarf Star

WASHINGTON - Some individuals are said to be two-faced, metaphorically speaking, of course. In ancient Roman mythology, Janus, the god of transitions and duality, was quite literally depicted with two faces – one looking forward and the other backward. Now, astronomers have discovered a two-faced star, and it goes by the name Janus.

This unique star is a white dwarf, a hot stellar remnant known for its extreme density, residing within the vastness of our Milky Way galaxy, approximately 1,300 light-years away from Earth, in the direction of the Cygnus constellation. A light year, which measures the distance light travels in one year, spans a staggering 5.9 trillion miles (9.5 trillion km).


Janus, unlike typical white dwarfs, possesses a fascinating and peculiar characteristic: it consists of hydrogen on one side and helium on the other. Scientists have playfully given this star the name Janus, drawing inspiration from the Roman deity's dual nature.


Caltech postdoctoral fellow in astrophysics, Ilaria Caiazzo, who leads the study published in the journal Nature, explained the reasoning behind the name choice. "Janus is the Roman god with two faces, so we thought it was very appropriate. Moreover, Janus is the god of transition, and the white dwarf might be currently transitioning from having an atmosphere made of hydrogen to one made of helium," she said.


Janus, comparatively massive for a white dwarf, boasts a mass 20% larger than that of our sun, compressed into an object with a diameter half that of Earth's. What's even more astonishing is its remarkable rotational speed – Janus completes a full rotation on its axis every 15 minutes, whereas most white dwarfs usually take hours to days to achieve the same feat.


White dwarfs come into existence at the very end of a star's life cycle. In fact, approximately 97% of all stars, including our sun, are destined to become white dwarfs once they exhaust their nuclear fuel. Our sun is currently fusing hydrogen into helium in its core, but when the hydrogen supply is depleted, it will transition to burning helium into carbon and oxygen. Eventually, after further changes, it will form a white dwarf.


The discovery of Janus was made possible through the use of the Zwicky Transient Facility at Caltech's Palomar Observatory near San Diego, along with subsequent observations made by other ground-based telescopes.


As a white dwarf forms, its heavier elements are believed to sink to the core, while lighter elements, such as hydrogen followed by helium, remain at the surface. In some white dwarfs, a stage of strong mixing occurs, destroying this layered structure as hydrogen and helium blend together. Janus might be a white dwarf undergoing this transitional blending process, but what confounds researchers is the asymmetry it exhibits – one side composed predominantly of hydrogen and the other of helium.


The team of scientists speculates that Janus' magnetic field could be responsible for this peculiar asymmetry. If one side has a stronger magnetic field than the other, as is common in celestial objects, it could hinder the mixing of elements, resulting in one side being hydrogen-rich and the other helium-rich.


Caiazzo further remarked, "Many white dwarfs are expected to go through this transition, and we might have caught one in the act because of its magnetic field configuration."


Janus adds to the collection of exotic white dwarf stars known to astronomers. In 2021, Caiazzo and her research team reported on another peculiar white dwarf, which was incredibly small – with a diameter slightly larger than Earth's moon – yet possessed the greatest mass among all known white dwarfs.


Studying stars from different perspectives continues to be a source of surprises and revelations for astronomers. As Caiazzo noted, "Stellar phenomenology is extremely rich, and no two stars are the same if looked at closely enough." Janus, with its enigmatic duality, stands as a testament to the wonders of the cosmos and the mysteries that continue to captivate scientists exploring the universe.

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