Close-up images show how stars explode

ATLANTA — Astronomers have captured images of two stellar explosions — known as novae — within days of their eruption and in unprecedented detail. The breakthrough provides direct evidence that these explosions are more complex than previously thought, with multiple outflows of material and, in some cases, dramatic delays in the ejection process.

The international study, published in the journal Nature Astronomy, used a cutting-edge technique called interferometry at the Center for High Angular Resolution Astronomy (CHARA Array) in California. This approach allowed scientists to combine the light from multiple telescopes, achieving the sharp resolution needed to directly image the rapidly evolving explosions.

“The images give us a close-up view of how material is ejected away from the star during the explosion,” said Georgia State’s Gail Schaefer, director of the CHARA Array. “Catching these transient events requires flexibility to adapt our night-time schedule as new targets of opportunity are discovered.”

Novae occur when a dense stellar remnant called a white dwarf undergoes a runaway nuclear reaction after stealing material from a companion star. Until recently, astronomers could only infer the early stages of these eruptions indirectly, because the expanding material appeared as a single, unresolved point of light.

Revealing how the ejecta are expelled and interact is crucial to understanding how shock waves form in novae, which were first discovered by NASA’s Fermi Large Area Telescope (LAT). In its first 15 years, Fermi-LAT detected GeV emission from more than 20 novae, establishing these explosions as galactic gamma-ray emitters and highlighting their potential as multi-messenger sources.

A Tale of Two Novae

The team imaged two very different novae that erupted in 2021. One, Nova V1674 Herculis, was among the fastest on record, brightening and fading in just days. Images revealed two distinct perpendicular outflows of gas — evidence that the explosion was powered by multiple interacting ejections. Remarkably, these newly emerging flows appeared in the images while NASA’s Fermi Gamma-ray Space Telescope also detected high-energy gamma rays, directly tying the shock-powered emission to the colliding outflows.

The second, Nova V1405 Cassiopeiae, evolved much slower. Surprisingly, it held onto its outer layers for more than 50 days before finally ejecting them, providing the first clear evidence of a delayed expulsion. When the material was finally expelled, new shocks were triggered — again producing gamma rays seen by NASA’s Fermi.

“These observations allow us to watch a stellar explosion in real time, something that is very complicated and has long been thought to be extremely challenging,” said Elias Aydi, lead author of the study and a professor of physics and astronomy at Texas Tech University. “Instead of seeing just a simple flash of light, we’re now uncovering the true complexity of how these explosions unfold. It’s like going from a grainy black-and-white photo to high-definition video.”

Revealing Hidden Structures

The ability to resolve such fine detail comes from the use of interferometry, the same technique that made it possible to image the black hole at the center of our galaxy. These sharp images were further complemented by spectra from major observatories such as Gemini, which tracked the evolving fingerprints of the ejected gas. As new features appeared in the spectra, they lined up with the structures revealed in the interferometric images, providing a powerful one-to-one confirmation of how the flows were shaping and colliding.

“This is an extraordinary leap forward,” said John Monnier, a professor of astronomy at the University of Michigan, a co-author of the study and an expert in interferometric imaging. “The fact that we can now watch stars explode and immediately see the structure of the material being blasted into space is remarkable. It opens a new window into some of the most dramatic events in the universe.”

Implications for Stellar Physics

The results not only reveal unexpected complexity in novae but also help explain their powerful shock waves, which are known to produce high-energy radiation such as gamma rays. NASA’s Fermi telescope has been the key instrument in discovering this connection, establishing novae as natural laboratories for studying shock physics and particle acceleration.

“Novae are more than fireworks in our galaxy — they are laboratories for extreme physics,” said Professor Laura Chomiuk, a co-author from Michigan State University and an expert on stellar explosions. “By seeing how and when the material is ejected, we can finally connect the dots between the nuclear reactions on the star’s surface, the geometry of the ejected material and the high-energy radiation we detect from space.”

The findings challenge the long-held view that nova eruptions are single, impulsive events. Instead, they point to a variety of ejection pathways, including multiple outflows and delayed envelope release, reshaping our understanding of these cosmic blasts.

“This is just the beginning,” Aydi said. “With more observations like these, we can finally start answering big questions about how stars live, die and affect their surroundings. Novae, once seen as simple explosions, are turning out to be much richer and more fascinating than we imagined.”For more information about Georgia State University research and its impact, visit research.gsu.edu. For more information about the CHARA Array, visit the CHARA Array website.

The observations of the two novae were obtained as part of the CHARA Array open-access program funded by the National Science Foundation under Grants No. AST-2034336 and AST-2407956. Institutional support for the CHARA Array is provided by Georgia State’s College of Arts & Sciences, Office of the Provost and Office of the Vice President for Research and Economic Development.