White dwarfs are called stellar remnants because they are what remains of main sequence stars after they have lost most of their mass and stopped merging. But even though they are merely remnants for which fusion is only a distant memory, they can still be sites of enormously powerful thermonuclear explosions called novae.
White dwarfs are extremely dense and hot cores of former stars. They only glow with residual heat and can take billions of years to cool completely. A nova explosion can only occur in a binary star system in which one of the stars is a white dwarf. These dense objects extract hydrogen from their companion stars, which accumulates on the white dwarf’s surface. The white dwarf heats this material until it triggers an out-of-control fusion explosion. Sometimes the white dwarf can be completely destroyed, and these cases are called type 1a supernova. But most novae are not destructive.
Instead, they eject their accumulated shells into space in nova explosions. Astronomers have captured detailed images of two different novae, illustrating the complexity of these events. Their findings are published in a new research article in Nature Astronomy. It is titled “Multiple outflows and delayed ejections revealed by early imaging of novae” and the lead author is Elias Aydi, a professor of physics and astronomy at Texas Tech University.
“These observations allow us to observe a stellar explosion in real time.” Elias Aydi, Texas Tech University
“Novae are thermonuclear eruptions on proliferating white dwarfs in interacting binary star systems,” the authors write. “Although most of the accumulated envelope is ejected, the mechanism – impulsive ejection, multiple outflows or sustained winds, or interaction between the shared envelope – remains uncertain.” The authors say that astrophysicists have detected gigaelectronvolt gamma-ray emissions from more than 20 novae and that novae are like nearby laboratories where astrophysicists can study shock physics and particle acceleration and how novae eject their shells.
“The mechanisms behind the energetic shocks that lead to the emission of GeV-γ radiation from novae are still poorly understood,” explain the authors. Recent work suggests that the tremors occur inside the ejecta, at the intersection of at least two ejecta, perhaps more. The interacting flows create the collisions that are responsible for accelerating the particles and producing high-energy gamma-ray emissions.
To understand how they eject their shells, the authors report on two novae known for their gamma-ray emissions.
One is V1674 Her, a fast nova from 2021. The other is the slow nova V1405 Cas, also from 2021. V1674 Her is called a fast nova because images taken just two to three days after the explosion show material being ejected in two vertical outflows, evidence of multiple interacting ejections. It is one of the fastest known novae, flaring brightly and then disappearing within a few days.
*These numbers from the study illustrate how V 1674 Her experienced multiple explosions in quick succession. (a) shows how a slow explosion first ejected material, then a second, faster explosion ejected more material, which slammed into the existing material, producing shocks and gamma ray radiation. (b) are CHARA images showing the nova explosion 2.2 and 3.2 days after its discovery in 1674. (c) shows Hβ (H-beta) spectral line profiles for hydrogen atoms. Photo credit: Aydi et al. 2025. NatAstr*
V1405 Cas is called a slow nova because images show that most of the ejected material was not visible until 50 days after the explosion. It is the first evidence of delayed ejection from a nova. When V1405 Cas finally ejected the material, it triggered new shocks that produced even more gamma rays.
“These observations allow us to observe a stellar explosion in real time, which is very complicated and has long been considered extremely challenging,” said lead author Elias Aydi. “Instead of just seeing a simple flash of light, we are now discovering the true complexity of how these explosions occur. It’s like going from a grainy black and white photo to a high-resolution video.”
“The fact that we can now see stars explode and immediately see the structure of the material being thrown into space is remarkable.” John Monnier, University of Michigan.
To study the novae, the researchers used two types of observations: interferometry and spectrometry. For interferometry, they turned to the CHARA (Center for High-Angular Resolution Astronomy) array at Georgia State University. For spectrometry, they used data from other observatories such as Gemini. Using interferometry, astronomers were able to reveal fine details in the explosions, and using spectrometry they were able to identify new chemical fingerprints in the developing ejecta.
The crucial part, however, is that the spectra match the structures revealed by interferometry. This is important confirmation of how the material flows collided.
“This is an extraordinary leap forward,” said co-author John Monnier, a professor of astronomy at the University of Michigan and an expert in interferometric imaging. “The fact that we can now see stars exploding and immediately see the structure of the material being thrown into space is remarkable. It opens a new window into some of the most dramatic events in the universe.”
*Early images from V1405 Cas show that material ejection was delayed for more than 50 days after the eruption. CHARA pictured it at 53, 55 and 67 after the explosion. H-alpha emissions were measured 53, 55 and 65 days after the explosion. It generated shock waves that also triggered gamma ray emissions. Image source: Aydi et al. 2025. NatAstr*
Extreme astrophysical environments like nova explosions are important because they define some of nature’s boundaries. Without understanding these boundaries, we cannot truly understand nature. Because of their shock waves and high-energy gamma radiation emissions, they are natural laboratories for extreme events in the cosmos.
“Novae are more than fireworks in our galaxy – they are laboratories for extreme physics,” said co-author Professor Laura Chomiuk from Michigan State University and an expert in stellar explosions. “By seeing how and when the material is ejected, we can finally make the connections between the nuclear reactions on the star’s surface, the geometry of the ejected material, and the high-energy radiation we observe from space.”
Everything in nature seems to be due to increasing complexity. And this complexity only becomes apparent as we improve our telescopes and observatories. While scientists once thought nova explosions were single explosion events, these results show the opposite. There are numerous drains and delayed ejections, and who knows what else there is to discover. These are not simple phenomena.
“This is just the beginning,” said Aydi. “With more observations like these, we can finally begin to answer big questions about how stars live, die, and influence their surroundings. Novae, once thought to be simple explosions, are turning out to be much richer and more fascinating than we imagined.”
Are these two novae outliers? Are there any that are much easier? The next step in understanding novae is to collect more data.
“By increasing the number of novae observed with CHARA and other optical and NIR interferometers in the future, we can confirm whether this delayed ejection is common to other novae, which would make novae ideal laboratories in our galactic backyard for constraining the physics of interaction between shared envelopes,” the researchers conclude.
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