Why Are Two Stars in Our Galaxy Suddenly Acting Very Strange?

Pedro_P

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This illustration shows what a binary star system with a red giant feeding material into a white dwarf might look like.
Credit: European Southern Observatory
▼ There's a binary star system out there in the Milky Way, and it's acting very weird.
"AG Draconis," as astronomers call it, is made up of two stars: a relatively cool giantand a relatively hot white dwarf— the stellar corpse of a low- to medium-size star. They're 16,000 l ight-years awayfrom Earth. (A light-year is the distance light travels in a year, meaning everything we see happening on these stars happened 16,000 years ago). And that distance makes them difficult to observe in detail. But we do know some things about them.
The two stars are probably interacting, with material flowing off the surface of the big, cool star and onto the surface of the small, hot star. And every once in a while, about once every nine to 15 years dating back to the 1890s, they become active — going through a period of several years where, once a year, they get much brighter in certain wavelengths that Earth's telescopes can detect. They're in an active period now, with flashes (or "outbursts" of energy) detected in April 2016, May 2017 and April 2018. (The 2016 outburst was a bit weird itself, having two peaks two weeks apart.) Researchers expect another outburst in April or May of this year, though it's too soon for any reports to have been published.
But there's something weird about this period of activity, as researchers reported in a paper uploaded May 10 to the preprint server arXiv, which has not yet been through peer review.
In the past, AG Draconis' active periods almost always followed a simple pattern: The first couple of outbursts are "cool," with the temperature of the white dwarf appearing to drop during each of its outbursts. Then,  (▪ ▪ ▪)

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