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For 25 years, one of the Milky Way’s strangest stellar explosions hid its culprit behind its own debris. Now the dust has thinned, exposing two tightly bound stars — and clumps of gas hurtling away at nearly 9,000 kilometers per second.
The peculiar system, called V445 Puppis, appears to contain a white dwarf siphoning helium-rich gas from a rare stripped star. New observations also suggest that the pair has resumed transferring material since its eruption in 2000, potentially restarting the long buildup toward another blast.
Astronomers also found several compact knots of possibly oxygen-rich gas racing through the expanding debris at up to 20 million miles per hour. These high-speed clumps, nicknamed “bullets,” have never been seen in another nova, and researchers do not yet know what launched them.
The findings were presented Wednesday at the National Astronomy Meeting in Birmingham and described in a conference abstract. They offer the clearest look yet at the only confirmed helium nova in our galaxy, while reopening a debate over whether such systems can eventually produce Type Ia supernovae.
An Explosion Without Hydrogen
Most novae begin when a white dwarf — the compact remnant of a Sun-like star — pulls hydrogen-rich gas from a companion. The stolen material piles up until heat and pressure ignite runaway nuclear burning across the white dwarf’s surface. The star survives, but throws its accumulated outer layer into space.
V445 Puppis breaks the pattern, though. Its spectrum showed helium, carbon and other elements, but no hydrogen.
“V445 Puppis has long stood out amongst novae for its complete lack of hydrogen,” said John Mills, a doctoral researcher at the University of Warwick.
Mills combined infrared images from the Very Large Telescope, Hubble observations, years of spectra from the Southern African Large Telescope and brightness measurements from NASA’s TESS spacecraft. As the obscuring dust faded, the central source reappeared.
Strong ionized-helium emission indicates that gas is once again falling onto the white dwarf, while the light curve points to an orbital period of 3.7 days — twice the previously proposed value. Models favor a helium star swollen enough for the white dwarf to pull material directly from its outer layers.
A 2009 study of the expanding debris found a narrow, hourglass-shaped outflow moving at about 6,700 kilometers per second, with knots reaching roughly 8,450 kilometers per second. Mills’s analysis identifies discrete clumps at comparable speeds and suggests they may contain abundant oxygen.
“The origin of these ‘bullets’ is a mystery,” Mills said. He suspects they formed after the main eruption, although astronomers do not yet know what could have launched them or why similar structures have not appeared in other novae.
A Possible Supernova Path — or a Dead End
Astronomers are very interested in this obscure binary because a white dwarf that gains enough mass can become unstable. In some models, accumulated helium detonates first and triggers a second explosion inside the white dwarf, producing a Type Ia supernova. These explosions have helped astronomers measure cosmic distances and discover the universe’s accelerating expansion.
But V445 Puppis may be a natural laboratory for that pathway without being destined to follow it.
A 2023 analysis of its extraordinary dust production estimated that the eruption expelled 0.01 to 0.1 solar masses of gas — an enormous amount for a nova. A later study by Bradley Schaefer argued that the white dwarf ejected far more matter than it had accumulated, meaning it is losing mass over repeated eruptions rather than moving toward a supernova. That study also derived a 1.87-day orbit, the period Mills’s new analysis now challenges.
The disagreement is not a minor detail. The orbital period constrains the stars’ sizes and how they exchange matter; the ejected mass determines whether the white dwarf grows or erodes. Until the new observations undergo peer review and the competing measurements are reconciled, V445 Puppis cannot settle the Type Ia question.
It can, however, expose a kind of stellar violence that astronomers have almost nowhere else to study: a hydrogen-free explosion, an accreting binary emerging from its own dust and unexplained projectiles still racing outward decades later.
This story originally appeared on ZME Science. Want to get smarter every day? Subscribe to our newsletter and stay ahead with the latest science news.
