Janus, ‘two-faced’ hydrogen / helium white dwarf star

Scientists have observed a white dwarf star – a hot stellar remnant that is among the densest objects in the cosmos – that they have nicknamed Janus owing to the fact it has the peculiar distinction of being composed of hydrogen on one side and helium on the other.


The star is located in our Milky Way galaxy about 1,300 light years from Earth in the direction of the Cygnus constellation. A light year is the distance light travels in a year, 5.9 trillion miles (9.5 trillion km).

Janus is fairly massive for a white dwarf, with a mass 20% larger than that of our sun compressed into an object with a diameter half that of Earth. It rotates on its axis every 15 minutes – very fast considering these stars usually rotate every few hours to a few days.

“White dwarfs form at the very end of a star’s life. About 97% of all stars are destined to become white dwarfs when they die,” Caiazzo said.


After a white dwarf forms, its heavier elements are thought to sink to the star’s core while its lighter elements – hydrogen being the lightest, followed by helium – float to the top. This layered structure is believed to be destroyed at a certain stage in the evolution of some white dwarfs when a strong mixing blends the hydrogen and helium together.

Janus may represent a white dwarf in the midst of this transitional blending process, but with the puzzling development of one side being hydrogen while the other side is helium.

The researchers suspect that its magnetic field may be responsible for this asymmetry. If the magnetic field is stronger on one side than the other, as is often the case with celestial objects, one side could have less mixing of elements, becoming hydrogen heavy or helium heavy.

“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,” Caiazzo said.


Source: Introducing Janus, the exotic ‘two-faced’ white dwarf star | Reuters

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