Supernova reveals rare pair of stars believed to be one of only about 10 like it in the Milky Way

The lackluster explosion, known as an "ultra-stripped" supernova, led researchers to discover the two stars 11,000 light-years away from Earth.

It's the first confirmed detection of a star system that will one day create a kilonova -- when neutron stars collide and explode, releasing gold and other heavy elements into space. The rare stellar pair is believed to be one of only about 10 like it in the Milky Way galaxy.

The discovery was a long time coming.

In 2016, NASA's Neil Gehrels Swift Observatory detected a large flash of X-ray light, which originated from the same region in the sky where a hot, bright Be-type star was located.

Astronomers were curious if the two could potentially be linked, so data was captured using the Cerro Tololo Inter-American Observatory's 1.5-meter telescope in northern Chile.

One of those interested in using this data to learn more about the star was Dr. Noel D. Richardson, now an assistant professor of physics and astronomy at Embry-Riddle Aeronautical University.

In 2019, Clarissa Pavao, an undergraduate student at the university, approached Richardson while taking his astronomy class to ask if he had any projects she could work on to gain experience with astronomy research. He shared the telescope data with her and throughout the pandemic, Pavao learned how to work with the data from the telescope in Chile and clean it up to reduce distortion.

"The telescope looks at a star and it takes in all the light so that you can see the elements that make up this star — but Be stars tend to have disks of matter around them," Pavao said. "It's hard to see directly through all that stuff."

She sent her initial results — which resembled something like a scatterplot — to Richardson, who recognized that she had pinned down an orbit for the double-star system. Follow-up observations helped them verify the orbit of the binary star system, named CPD-29 2176.

But that orbit wasn't what they were expecting. Typically, binary stars whirl around one another in an oval-shaped orbit. In CPD-29 2176, one star orbits the other in a circular pattern that repeats about every 60 days.

The two stars, a larger one and a smaller one, were whirling around one another in a very close orbit. Over time, the larger star had begun to shed its hydrogen, releasing material onto the smaller star, which grow from 8 or 9 times the mass of our sun to 18 or 19 times the mass of our sun, Richardson said. For comparison's sake, our sun's mass is 333,000 times that of Earth.

The main star became smaller and smaller while building up the secondary star — and by the time it had exhausted all of its fuel, there wasn't enough to create a massive, energetic supernova to release its remaining material into space.

Instead, the explosion was like lighting a dud firework.

"The star was so depleted that the explosion didn't even have enough energy to kick (its) orbit into the more typical elliptical shape seen in similar binaries," Richardson said.

What remained after the ultra-stripped supernova was a dense remnant known as a neutron star, which now orbits the rapidly rotating massive star. The stellar pair will remain in a stable configuration for about 5 to 7 million years. Because both mass and angular momentum were transferred to the Be star, it releases a disk of gas to maintain balance and make sure it doesn't rip itself apart.

Eventually, the secondary star will also burn through its fuel, expand and release material like the first one did. But that material can't be easily piled up on the neutron star, so instead, the star system will release the material through space. The secondary star will likely experience a similar lackluster supernova and turn into a neutron star.

Over time — that is, likely a couple billion years — the two neutron stars will merge and eventually explode in a kilonova, releasing heavy elements like gold into the universe.

"Those heavy elements allow us to live the way that we do. For example, most gold was created by stars similar to the supernova relic or neutron star in the binary system that we studied. Astronomy deepens our understanding of the world and our place in it," Richardson said.

"When we look at these objects, we're looking backward through time," Pavao said. "We get to know more about the origins of the universe, which will tell us where our solar system is headed. As humans, we started out with the same elements as these stars."

A study detailing their findings published Wednesday in the journal Nature.

Richardson and Pavao also worked with physicist Jan J. Eldridge at the University of Auckland in New Zealand, an expert on binary star systems and their evolution. Eldridge reviewed thousands of binary star models and estimated there are likely only 10 in the entirety of the Milky Way galaxy similar to the one in their study.

Next, the researchers want to work on learning more about the Be star itself, and hope to conduct follow-up observations using the Hubble Space Telescope. Pavao is also setting her sights on graduating — and continuing to work on space physics research using the new skills she has acquired.

"I never thought I would be working on the evolutionary history of binary star systems and supernovas," Pavao said. "It's been an amazing project."

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