The study captured detailed images of two stellar explosions known as novae within days of their eruption, showing that these events involve multiple outflows of material and, in some cases, significant delays before mass is ejected. Aydi, who teaches in Texas Techs Department of Physics and Astronomy, served as lead author on the work.
"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," Aydi said. "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."
Novae occur when a dense stellar remnant called a white dwarf experiences a runaway nuclear reaction after accumulating material from a companion star. Before this campaign, astronomers could only study the early stages of such eruptions indirectly, because the expanding gas appeared as a single unresolved point of light.
Resolving how the ejecta are launched and interact is central to understanding how shock waves form in novae, which were first identified as gamma-ray sources by NASAs Fermi Large Area Telescope (LAT). During its first 15 years of operations, Fermi-LAT detected giga-electron volt gamma-ray emission from more than 20 novae, establishing these systems as Galactic gamma-ray emitters and highlighting their role as potential multi-messenger targets.
The team observed two very different novae in 2021. Nova V1674 Herculis was one of the fastest on record, brightening and fading within only a few days, whereas Nova V1405 Cassiopeiae evolved much more slowly over an extended period.
CHARA images of V1674 Herculis showed two distinct, nearly perpendicular flows of gas, indicating that the eruption involved at least two major outflows that interacted as they expanded. These structures developed at the same time that NASAs Fermi Gamma-ray Space Telescope detected high-energy gamma rays from the nova, directly linking the shock-powered emission to collisions between the different ejecta components.
V1405 Cassiopeiae presented a contrasting behavior. The system retained its outer layers for more than 50 days before finally ejecting them, providing direct evidence of a delayed expulsion phase in which material remains bound for weeks before being driven off.
When V1405 Cassiopeiae ultimately expelled its envelope, new shocks formed in the outflow and again produced gamma rays observed by Fermi, connecting late-time high-energy emission to the delayed release of material. Taken together, the two novae show that shocks in these systems can arise both from early colliding flows and from later episodes when previously confined gas is released.
Spectra from large observatories, including Gemini, complemented the interferometric images by tracing the evolving signatures of the ejected gas. Features that appeared in the spectra matched structures revealed in the CHARA images, giving a one-to-one confirmation of how different flows were shaped and how they collided.
The project was funded by NASA, with the Fermi mission and the CHARA Array playing key roles in providing the high-energy and high-angular-resolution data needed to connect shocks to specific ejecta geometries. The results indicate that nova eruptions often proceed through multiple ejection episodes and complex outflow patterns rather than a single, impulsive event, and they point to a variety of pathways that include both multiple outflows and delayed envelope release.
"This is just the beginning," added Aydi. "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."
Research Report:Multiple outflows and delayed ejections revealed by early imaging of novae
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Stellar Chemistry, The Universe And All Within It
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