A pair of very strange stars is behaving very ‘strangely’, say scientists

A newly discovered very rare binary star system seems to behave very strangely, according to the researchers who found it.

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

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

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

The twin star system is bright in X-rays and high in mass, say the astronomers who found it. But it’s particularly unusual because the two stars orbit each other in what they say is an “oddly circular” path.

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

Their strange orbit helped researchers discover that one of the two stars is a “depleted” supernova. That meant that when the star used up its fuel and its core collapsed, it had a relatively weak explosion.

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

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

The pair of stars is quite strange in itself. But scientists hope that finding systems like this might better help us with kilonovae, those dramatic explanations that are also believed to be the source of gold in the universe.

“For quite some time, astronomers have speculated about the exact conditions that could eventually lead to a kilonova,” said Dr. Chené. “These new results show that, at least in some cases, two sister 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 located about 11,400 light-years from Earth. It was first discovered 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 article, 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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