New images reveal what really happens when stars explode
New high-resolution images show that novae are anything but simple stellar fireworks. One exploded with multiple gas streams colliding almost immediately, while another shockingly delayed its eruption for more than 50 days before unleashing a powerful blast. These complex outflows create shock waves that produce intense gamma rays, confirming long-standing theories with direct visual evidence. The findings reveal novae as evolving, multi-stage events rather than single, instant explosions.
Astronomers have obtained remarkably detailed images of two stellar explosions -- called novae -- just days after they began. The new observations offer clear proof that these outbursts are not as simple as once believed. Instead of a single blast, the explosions can send out more than one stream of material and may even delay some of the ejection in dramatic ways.
The international research team reported the work in Nature Astronomy. They used interferometry at the Center for High Angular Resolution Astronomy (CHARA Array) in California, a method that combines light from multiple telescopes to create extremely sharp views. That added resolution made it possible to directly image these fast-changing events as they evolved.
"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."
What a Nova Is and Why Shock Waves Matter
A nova happens in a close binary system when a white dwarf, the dense leftover core of a star, pulls gas from a nearby companion. As the stolen material builds up, it can ignite in a runaway nuclear reaction, triggering a sudden brightening in the sky. Until recently, astronomers mostly had to piece together the earliest stages indirectly because the expanding debris looked like a single pinpoint of light.
Seeing exactly how the ejecta blast outward and interact is key to explaining how shock waves form in novae. Those shocks were first linked to novae by NASA's Fermi Large Area Telescope (LAT). During its first 15 years, Fermi-LAT detected GeV emission from more than 20 novae, showing that these eruptions can produce gamma rays in our galaxy and pointing to their promise as multi-messenger sources.
Two 2021 Novae With Very Different Behavior
The team focused on two novae that erupted in 2021 and found that they behaved in strikingly different ways. Nova V1674 Herculis was one of the fastest ever recorded, rising and fading within days. The images revealed two separate gas flows moving in perpendicular directions -- a sign that the event involved multiple ejections interacting with each other. The timing was especially telling: the new outflows appeared in the images while NASA's Fermi Gamma-ray Space Telescope was also detecting high-energy gamma rays, directly connecting the shock-powered radiation to those colliding streams.
Nova V1405 Cassiopeiae unfolded much more slowly. It unexpectedly held onto its outer layers for more than 50 days before releasing them, offering the clearest evidence yet for a delayed expulsion in a nova. When that material finally broke free, it set off fresh shocks, and NASA's Fermi again observed gamma rays tied to the renewed violence.