A unique system of two interacting white dwarf stars stripped down to their helium layers has been exposed thanks to a line-of-sight eclipse of the stars as seen from Earth.

The system, known as CSS 41177, lies 1,140 light years away in the constellation Leo, and was studied using the robotic two-metre Liverpool Telescope in the Canary Islands and the eight-metre Gemini Telescope on Hawaii.

"There are around 30 systems thought to be double helium white dwarf binaries," lead author of the study Steven Parsons of the University of Warwick tells Astronomy Now. "CSS 41177 is unique as it's the only known eclipsing double helium white dwarf binary."

Both of the original stars began life at the same time, but one was more massive than the other, therefore it burned its fuel faster and reached the end of its life quicker, swelling into a red giant before collapsing into a white dwarf. This is the fate of the majority of stars in the Universe – and will be the fate of own Sun – but most white dwarfs tend to have largely inert cores of carbon and oxygen, left behind once their hydrogen and helium content has expired.

"To form a helium white dwarf there must have been some kind of process to remove mass from the evolved star that formed it before the helium core ignited, which would lead to the creation of the common carbon-oxygen white dwarf," explains Parsons. "The best way to remove this mass is with another nearby star and therefore it's no surprise that most helium white dwarfs are found in binary systems."

In this scenario the most massive star would begin to expand first, with the companion ripping off its outer hydrogen envelope and preventing it from fusing helium, thus leaving the helium layer exposed. Then, when the companion star started expanding, the first star repaid the favour by ripping off its hydrogen layer too. But since the first star was already reduced to a helium white dwarf it could not use that new material, instead leaving it to waste away into the star system, leaving behind two helium white dwarfs.

The two stars will spiral inwards and eventually merge in about one billion year's time, at which point their helium will finally ignite and the stars will become what is known as a hot subdwarf – of which around 2,400 are known – and will exist for a further 100 million years.

The results will be published in the Astrophysical Journal.

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