A very rare pair of stars is behaving extremely ‘weirdly’, scientists say

Pair of stars is a ‘one-in-ten-billion system’, scientists say

Andrew Griffin
Friday 03 February 2023 23:21 GMT
Related video: Take a Look At This Glorious Moment Two Neutron Stars Colliding

A newly discovered, very rare binary star system appears to be behaving very weirdly, according to the researchers who found it.

The system is so unusual that there are thought to be only be about 10 of them throughout our vast Milky Way galaxy.

It has all the conditions to go on and trigger a kilonova, or the explosion that happens when neutron stars collide, triggering an ultra-powerful blast that can be detected across the universe.

 “We know that the Milky Way contains at least 100 billion stars and likely hundreds of billions more. This remarkable binary system is essentially a one-in-ten-billion system,” said NOIRLab astronomer and co-author on the new study, André-Nicolas Chené. “Prior to our study, the estimate was that only one or two such systems should exist in a spiral galaxy like the Milky Way.” 

The twin star system is bright in X-rays and high in mass, the astronomers who found it say. But it is particularly unusual because the two stars orbit around each other in what they say is a “weirdly circular” route.

It appears to have been formed when an exploding star or supernova fizzled out, rather than exploding with the usual dramatic bang.

Its weird orbit helped researchers find that one of the two stars is a “depleted” supernova. That meant that when the star consumed its fuel, and its core collapse, it had a relatively weak explosion.

Usually, that explosion kicks the stars into a long, elliptical orbit. But there wasn’t even enough energy left in the star to create such a blast, so the two stars instead stayed closely aligned in a round orbit.

Over time, they will merge, sending powerful gravitational waves through the cosmos and leaving behind heavy elements such as silver and gold.

The pair of stars is odd enough in itself. But scientists hope that finding systems like this could help us better under kilonovae, those dramatic explanations that are also thought to be the source of gold in the universe.

“For quite some time, astronomers speculated about the exact conditions that could eventually lead to a kilonova,” said Dr Chené. “These new results demonstrate that, in at least some cases, two sibling neutron stars can merge when one of them was created without a classical supernova explosion.” 

The system is known as CPD-29 2176, and is situated about 11,400 light years from Earth. It was first spotted by Nasa’s Neil Gehrels Swift Observatory and it was later observations using the SMARTS 1.5-meter Telescope in Chile that confirmed its unusual nature.

The findings are described in a new paper, published in the journal Nature today, under the title ‘A high-mass X-ray binary descended from an ultra-stripped supernova’.

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