Janus: White Dwarf Edition

by | August 4, 2023, 12:00 PM | Stars & Nebulae

The results of science are often weirder than anything humans can imagine. As researchers, we dedicate our lives to taking data and knowing that whatever we may want to believe, we have to accept the reality of what our data shows us. 

And sometimes the data is just plain weird. 

The press release this story draws from includes the quote “When I show the observations to people, they are blown away.” and in describing the results, the press officers explain “The team acknowledges they are baffled but have come up with some possible theories.”

While looking for stars with strong magnetic fields, researchers found a white dwarf that varies in brightness in a way that white dwarfs usually don’t. This wasn’t the high-speed pulsations that have been seen before. Rather, about every 15 minutes, they observed the star’s brightness vary. This duration isn’t consistent with anything previously seen, and in follow up observations, the team studied the star’s composition to see how else things may differ from your run of the mill white dwarf. Remarkably, the two sides of the star differed even more in composition, with one side being predominantly hydrogen while the other is helium. 

This is a new kind of weird.

We have known for decades that some white dwarfs have hydrogen atmospheres while others have helium atmospheres. We also know that below 30,000 Kelvin many helium dominated atmospheres become hydrogen dominated. Exactly why this happens… we’re still learning, but it’s thought the Helium syncs over time.

This entirely weird white dwarf, which the team has named Janus because of its two-faced nature, is currently 35,000 Kelvin. 

As best as anyone can figure, Janus has a small magnetic field that causes one side of the star to be a little bit warmer than the other. This temperature variation may in turn cause the cooler side to have already lost its helium to become hydrogen dominated while the warmer side has continued to support its helium atmosphere. 

This discovery is published in Nature and led by Ilaria Caiazzo. It was Caiazzo who pointed out these results blow people away. 

The universe is large, and the things we are most likely to see are the things that are common, and the things that list for a long period of time. While many stars become white dwarfs after nuclear reactions cease in their cores, only some of them start as Helium white dwarfs, and of those that do, only some transition to hydrogen white dwarfs. And the transition is likely brief… on the scales of the universe, and the white dwarf cools. 

It is truly remarkable that we caught a white dwarf with just the right combination of elements and with just the right alignment of a magnetic field, and at just the right time in its cooling.


Caiazzo, I., Burdge, K.B., Tremblay, PE. et al. A rotating white dwarf shows different compositions on its opposite faces. Nature 620, 61–66 (2023). https://doi.org/10.1038/s41586-023-06171-9