Moderate supernova reveals a rare pair of stars in the Milky Way

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An unusual galaxy caused more fizz and less pop when it exploded in a supernova.

Known as an “ultra-stripped” supernova, the matte explosion led researchers to discover the two stars 11,000 light-years away from Earth.

It’s the first confirmed detection of a galaxy that will one day create a kilonova — when neutron stars collide and explode, ejecting 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 coming from the same region of the sky that contained a hot, bright Be-type star.

Astronomers were curious to see if the two could possibly be linked, so the data was captured using the 1.5-meter telescope at the Cerro Tololo Inter-American Observatory 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 in astronomical research. He shared the telescope data with her, and during the pandemic, Pavao learned how to work with the data from Chile’s telescope and clean it up to reduce distortion.

“The telescope looks at a star and picks up all the light so you can see the elements that make up that star — but Be stars usually have disks of matter around them,” Pavao said. “It’s hard to see right through all that stuff.”

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

But that job was not what they expected. Typically, binary stars swirl around each other in an oval-shaped orbit. In CPD-29 2176, one star revolves around another in a circular pattern that repeats approximately every 60 days.

The two stars, one larger and one smaller, revolved around each other in a very close orbit. Over time, the larger star began to shed its hydrogen, releasing material onto the smaller star, which grows from 8 or 9 times the mass of our sun to 18 or 19 times the mass of our sun, Richardson said. For comparison, the mass of our sun is 333,000 times that of the Earth.

The main star got smaller and smaller as the secondary star built up — and by the time it used up all of its fuel, there wasn’t enough to create a massive, energetic supernova to release the remaining material into space.

Instead, the explosion was like setting off a dud firecracker.

“The star was so exhausted that the explosion didn’t even have enough energy to kick (its) orbit into the more typical elliptical shape seen in similar binary stars,” Richardson said.

What was left after the ultra-stripped supernova was a dense remnant known as a neutron star, now orbiting 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 equilibrium and keep it from tearing itself apart.

Eventually the secondary star will also burn up its fuel, expand and release material as the first did. But that material can’t be easily piled onto the neutron star, so instead the star system will release the material through space. The secondary star is likely to experience a similar matte supernova and turn into a neutron star.

Over time – that is, probably a few billion years – the two neutron stars will merge and eventually explode into a kilonovareleasing heavy elements such as gold into the universe.

“Those heavy elements enable us to live the way we do. For example, most of the gold is made by stars similar to the supernova relic or neutron star in the binary system we studied. Astronomy deepens our understanding of the world and our place in it,” Richardson said.

“When we look at these objects, we look back in time,” Pavao said. “We are learning more about the origin of the universe, which will tell us where our solar system is headed. As humans, we started with the same elements as these stars.”

A study describing their findings was published Wednesday in the journal Nature.

Richardson and Pavao also collaborated with physicist Jan J. Eldridge at the University of Auckland in New Zealand, an expert on binary star systems and their evolution. Eldridge assessed thousands of binary star models and estimated that there are probably only 10 in the entire Milky Way galaxy, similar to those 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 also sets her sights on graduating – and continuing to work on space physics research using the new skills she’s acquired.

“I never thought I would be working on the evolutionary history of binary star systems and supernovae,” Pavao said. “It’s been a great project.”