The Barbenheimer Star and Exploding White Dwarfs

When making a planetarium show that condenses cutting-edge science into a 25-minute story, it becomes necessary to gloss over some details. And the Academy’s latest planetarium production, Spark: The Universe in Us is no exception. The show features remarkable visualizations of stars at the ends of their lives, seeding the Universe with the elements for habitable planets—and life!

But at the 243rd American Astronomical Society meeting, currently taking place in New Orleans, Louisiana, the participants are all about the details! Including the ways in which stars end their lives…

On the third day of the conference, we heard from Alexander Ji, from the University of Chicago, about an object he and his team have nicknamed the Barbenheimer Star. Early in the history of the Universe, the Barbenheimer Star exploded, seeding space with the elements to form a star with the far less glamorous name J0931+0038.

What led Ji and his team of “stellar archaeologists” to develop this hypothesis was the highly unusual spectrum of J0931+0038. (I talked about spectra a little bit in the article on Monday, but basically, astronomers tease apart wavelengths of light to understand what stars, planets, and other objects are made of. This is called the object’s spectrum. If you’d like to know more, you can watch a video about how astronomers use spectra to search for life in the Universe.)

An initial spectrum came from the Sloan Digital Sky Survey (SDSS), the weirdness of which left Li and his team wanting more, so they used the Magellan telescopes at Las Campanas Observatory in Chile to acquire more detailed spectra. These revealed an unusual combination of elements in the star, unlike anything they’d seen before.

As the show Spark explains, when stars die explosively, they seed space with new elements, some of which can form a new generation of stars! To explain the unusual spectrum of J0931+0038, Li and his colleagues propose that an extraordinary star (nicknamed “Barbenheimer”) exploded in a supernova more than 13 billion years ago, seeding the early Universe with a strange mix of elements.

To create the strange mix, the star would have had to be 80 times the mass of the Sun. Those are the only stars that are that massive that make up enough iron, and when the researchers compared computer models to their observations, the 80 “solar mass” star (in astronomer parlance) came closest in terms of matching their observational data.

Closest… But still not a great match.

“Amazingly, no existing model of element formation can explain what we see,” says Sanjana Curtis of the University of California, Berkeley, co-lead of the published study.

Even more difficult to explain is that, based on our current theories of how stars die, a star 80 times the mass of the Sun shouldn’t go supernova at all! It should simply collapse into a black hole.

Ooops.

Astronomers often divide themselves into two camps: theorists and observationalists. Most astronomers dabble in both, but most pledge allegiance to one side or the other. When computer models don’t describe what we see, well, that’s a problem for the theorists.

As Li rephrased a question from one of the press conference attendees: “Why should we believe theorists ever?” But he was very welcoming of input from the astronomers developing the theoretical models. J0931+0038 “gives us a nice target system to understand some of these questions,” he said.

It might have started its life at 80 times the mass of the Sun, but there could be ways in which it loses mass… Maybe it was a binary and lost mass during interaction with it’s companion? Maybe it was rotating and developed a jet? Those are the kind of concepts that need to be refined into computational models and new predictions.“The theorists I hope are going to have a playground just explaining how you produce a system like this,” Li said.

But the conversation didn’t end there. Juna Kollmeier, from the University of Toronto and the director of the fifth phase of SDSS, then offered “a quick response in defense of theorists,” as she put it.

“The point is that you’re not supposed to believe. You’re supposed to understand.

“And the point of doing big surveys is to challenge our understanding by finding things that are outside of what our current calculations do. And so this is really a synergy between theory and observation to yield deeper understanding.

“And it’s exactly these types of falsifications that we’re seeking in big surveys and as theorists who actually want to understand deeper about the Universe.”

A more harmonious blending of observation and theory was described in the same press conference.

Estefania Padilla Gonzalez, from the University of California, Santa Barbara, described her work investigating supernova SN2022joj, another unusual object that requires astronomers to examine their ideas in greater detail.

Unlike J0931+0038, SN2022joj, was a transient event—a supernova that brightened then dimmed over time, similar to tens of thousands events that are observed every day.

The discovery of SN2022joj took place on May 8, 2022, at the Zwicky Transient Facility at Palomar Observatory. But to track how the brightness changed over the next few weeks, astronomers immediately turned to the Las Cumbres Observatory network of telescopes, where Gonzalez is a researcher.

This rapid response allowed the astronomers to observe details in the supernova that might often be missed. These details suggest that SN 2022joj is our best evidence yet for a potentially rare event—a supernova triggered by a process not too different from the one we showed in Spark. (Based on a computational model by Alexander Holas and Friedrich Röpke at the Heidelberg University & Heidelberg Institute for Theoretical Studies, with contributions from Rüdiger Pakmor at the Max Planck Institute for Astrophysics.)

Since evidence for this particular type of explosion disappears after hours or days, it could have been missed in previous events—and mistaken for other types of supernovae! That means these events could be much more common than previously thought.

While the rivalry between theorists and observationalists seems unlikely to settle down in the astronomical community, the two camps continue to collaborate to bring us remarkable new insights about the origin of elements in the Universe.