Space

Two stars are showing ‘peculiar outburst activity’ that scientists can’t explain

A star system located some 16,000 light-years away from Earth is flaring in an unexpected manner, and scientists are working to explain what’s going on with it.

The star in question is called AG Draconis: a well-known binary star that’s been observed by astronomers since the late 19th century.

But while AG Draconis may have been watched for a very long time, researchers aren’t sure what’s behind its latest bout of strange stellar activity.

Like the name suggests, a binary star is actually a system made up of two separate stars that orbit around one another – a pairing that may seem surprising, but is actually the gravitational arrangement that the vast majority of stars are thought to have.

Depending on the proximity of the stars tied together in a binary system, and their relative stages of stellar evolution and activity, binary stars can exhibit some bizarre and breathtaking interactions.

In the case of AG Draconis, it’s pretty weird, too.

This binary pair, which consists of a cool giant star and a hotter white dwarf, is known for manifesting symbiotic flaring activity that alternates between a two-step sequence of quiescent and active stages.

Every nine to 15 years or so, AG Draconis characteristically demonstrates a stellar outburst: an intense phase of brightening, which repeats on an almost annual basis for a number of years at a time, before the binary goes quiet again for several flare-free years (until the cycle starts to repeat).

In a new study led by astrophysicist Rudolf Gális from the Pavol Jozef Šafárik University in Slovakia, the researchers identified 36 of these outburst events since 1932, across at least six separate stages of outburst activity.

That might sound like a peculiar habit to keep up, but the latest intel on AG Draconis is stranger still.

According to Gális’s team, the way the binary system flares during these outburst events seems to have changed recently – as if the two stars had decided to suddenly discard a consistent phenomenon that’s been the same across decades of observations.

Historically, ultraviolet and X-ray observations had showed that AG Draconis manifests two different kinds of outbursts: cool ones, which usually show up at the beginning of active stages (and in major outbursts); and hot ones, which occur on a smaller scale later in the cycle.

We don’t know for sure why these periodical outbursts actually happen, but one hypothesis is that thermonuclear reactions are ignited when the accretion rate of material pulled from the giant star into the dwarf star “exceeds some critical value, and luminosity of the hot component increases significantly,” the researchers explain.

In any case, something about this phenomenon appears to be different now.

For some reason, in 2015 AG Draconis marked the commencement of its latest active stage with an outburst that resembled the minor, hot type of event, not the cool outburst scientists expected to see – and the binary star then followed this with successive hot outbursts in 2016, 2017, and 2018.

In other words, for reasons nobody entirely understands, AG Draconis has shifted the long-established patterns of its active outburst stages.

“Such behaviour is considerably peculiar in [an] almost 130-year history of observing of this object, because the major outbursts at the beginning of active stages are typically cool ones,” the authors write in their paper, which has not yet been peer-reviewed but is available for the astronomy community on the pre-print server arXiv.org.

“The future evolution of AG [Draconis] is an open question. Can we expect (finally) a major, cool or (again) minor, hot outburst?”

That’s the question on astronomers’ minds, and we shouldn’t have to wait too long to find out what the binary’s fiery fate spells.

Per the researchers’ estimation, AG Draconis’ next event should have already occurred by the time you read this, although scientists haven’t yet had a chance to write up any new observations.

When they do, we’ll get to hear the latest strange news from another star – ‘the latest’, in this instance of course, meaning faraway weirdness that occurred across space 16,000 years ago.

The findings are available on arXiv.org.

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