Two-faced star seems to have one hydrogen side and one helium side

Artist’s impression of Janus, the two-faced star

K. Miller, Caltech/IPAC

Astronomers have found a two-faced star where each side has a different composition. This white dwarf star, nicknamed Janus after the Roman god of doorways and transitions, is the first of its kind ever spotted.

Jeremy Heyl at the University of British Columbia in Canada and his colleagues spotted Janus using the Zwicky Transient Facility in California, and then took more observations using several other telescopes. The observations indicated that one side of the star was made up completely of hydrogen, and the other side completely of helium. It is more than 1300 light years from Earth and rotates about once every 15 minutes.

We know that the entire surface of some white dwarfs can transition from helium to hydrogen and back to helium again, but we have never definitively caught one in the middle of this change before. The mechanism behind it remains murky.

“It seems likely to be connected with a magnetic field in the star that’s a bit stronger on one side than the other, but the data itself doesn’t show evidence for a very strong magnetic field,” says Heyl. “So we still don’t completely understand what’s going on.”

If there is such a magnetic field, it would suppress the star’s convection, or internal churning, only on the side where it is stronger. White dwarfs are made primarily of hydrogen and helium, so the lighter hydrogen would naturally float to the surface of the side with no convection, whereas on the other side the more abundant helium would bubble up to the top.

“If it was at the distance of the moon, you’d be able to clearly see these blotches from convection and it would look like one half is around 15 per cent brighter than the other,” says Heyl. “Its temperature is about five times the temperature of the sun so it would appear incredibly dazzling to the eye – it would be uncomfortable to look at.”

The fact that we have searched so much of the cosmos and only found one of these stars means that these objects are probably relatively rare. It also took an incredible coincidence for us to be able to identify it – the axis of rotation just happens to be perpendicular to the divide between the helium face and the hydrogen face, and we just happen to be positioned face-on to the star.

Now that they have had this stroke of luck, the researchers hope to take more detailed data on Janus to figure out how exactly its internal dynamics work. This star could be the key that unlocks the explanation of this strange phase of white dwarf evolution.


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