Astronomers witness the rare break up of a star couple

Astronomers have witnessed a rare and important life event in the evolution of binary star couplings for the first time. 

The team discovered a tight binary star surrounded by an expanding shell of material. This shell is matter is leftover from a stage in the stars' evolution called the common envelope phase. 

This phase occurs when material from one star swells out and engulfs the other in a cosmic 'embrace.' This results in a mass transfer from the swelled star to its companion that can run out of control. The aftermath of this phase is something astronomers had not glimpsed until now.

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"The common envelope phase is a missing link in the very long and complex chain of events making up the life of stars," Australian National University (ANU) associate professor Christian Wolf and part of the team that made the observations, said in a statement. "Now we are starting to fix that link."

Half of all stars in the universe come in binary pairs and though the initial stages of partnerships can be uneventful, when one star runs out of hydrogen for nuclear fusion things get interesting for the pairing. 

The initial step in these events is the collapse of the hydrogen-exhausted core of the star while its outer layers 'puff out'  —  a process that the sun will experience in around 5 billion years  —  creating a red giant star. But, this proceeds differently for stars in binary pairs than it will for our lonely star.

"When one of the stars grows into a red giant, it does not just claim more empty space the way a single star will do," Wolf said. "Instead, it 'embraces' or engulfs its companion, and they appear as one star under an opaque envelope. That's when things get really exciting."

Wolf explains that friction created in the envelope caused by the motion of the stars within it has profound effects on the next step in the evolution of binary stars. "It not only causes heat but slows down the stars, so they spiral into an ever-tighter orbit; the envelope finally overheats and gets blown away," he said.

As a result of this, the stars can end up over 100 times closer together at the end of the common envelope phase than they were at its beginning after heat from the process causes the surrounding matter to be expelled in a violent 'blow-out.'

The blow-out for the binary stars observed by Wolf and colleagues occurred around 10,000 years ago. The researchers predict that the binary stars they observed, now a white dwarf and a hot subdwarf which will eventually evolve into a white dwarf itself, will continue to spiral together eventually merging.

The team's findings and the first glimpse of the aftermath of the common envelope phase of stellar evolution could help other researchers spot more binary stars in the critical stage of their lives. 

"It may be easier to recognize them now we have a clearer idea of what to look for. There may be others that have been under our nose the whole time," Wolf said, adding that the findings could also have ramifications for other cosmic unions. "It could even help us better reconstruct gravitational wave events, such as black hole mergers."

The team's research was published in the journal Monthly Notices of the Royal Astronomical Society.

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