A white dwarf more than 1,300 light years away appears very different depending on which A side you’re looking at. Called Janus, this white dwarf has been baffling astronomers. On one side, observations indicate the dying star’s surface is made up of hydrogen. The other side appears to be made up of helium. It’s not an exact split right down the centre, but it has proven perplexing and compelling all the same.
As with many such discoveries, this one caught astronomers by surprise. It’s taken so long to spot Janus because the technology to do so has only become available relatively recently. In this case, researchers were making use of the Zwicky Transient Facility (ZTF) at Palomar Observatory, close to San Diego, California, which was commissioned in 2018. They were looking for rapidly rotating and highly magnetised white dwarfs in the hope of finding remnants of double white dwarf mergers. “ZTF takes photographs of a large portion of the northern sky each night so that we can look for objects whose brightness changes with time,” explains Jeremy Heyl of the University of British Columbia in Canada. “In the near future the Vera Rubin Observatory will be starting a similar but much deeper survey in
