Hypernovae Under Fire: Ordinary Supernovae May Explain the Universe’s Strangest Stars

For decades, astronomers have pointed to a handful of chemically bizarre stars as the fingerprints of the most violent explosions the Universe can produce. Hypernovae, theorised to be at least ten times more energetic than an ordinary supernova, were invoked to explain why certain ancient stars carry such unusual mixes of elements. Now, two new studies from University College London argue that this dramatic explanation may be unnecessary, and that ordinary supernovae, once modelled more realistically, can account for all of the evidence previously attributed to hypernovae.

The research, published in the Monthly Notices of the Royal Astronomical Society, was led by Anmol Aggarwal, a PhD student at UCL’s Mullard Space Science Laboratory, together with Dr Ralph Schoenrich, also based at the same laboratory. Their central insight is deceptively simple: when a star explodes, its material is not flung evenly into space. Different elements are ejected in different directions, with oxygen potentially streaming one way and sulphur another. New stars that condense from this debris may therefore inherit a lopsided sample of the supernova’s ingredients rather than its overall chemical blend.

Previous models, the researchers argue, assumed that the gas ejected by a supernova was thoroughly mixed before it formed new stars. Under that assumption, the peculiar chemical signatures observed in certain stars could not be produced by an ordinary supernova, and something far more energetic was required. Aggarwal and Schoenrich built mathematical models that instead accounted for the uneven, asymmetric way material is expelled, fitting how much material from each layer or region of an exploding star would be needed to match the composition of the observed stars.

Across the two studies, the team examined four stars that had previously been judged to have formed from material expelled by a hypernova. In the first study, they found that for three of these stars, formation from an ordinary supernova was statistically favoured over a hypernova. The unusual abundance patterns that had seemed to demand an ultra-powerful explosion could instead arise naturally if the stars formed from incomplete, patchy samples of supernova debris. As Aggarwal put it, stars with a very unusual mix of ingredients were most likely formed from ordinary supernovae, and all the evidence for hypernovae suddenly disappears.

To understand why this matters, it helps to consider how a supernova actually works. When a massive star exhausts its nuclear fuel, its core collapses under gravity, triggering an immense explosion that blasts the star’s outer layers into space. Just before this happens, the star is structured like an onion, with the heaviest elements concentrated at the centre and progressively lighter elements in the outer layers. A hypernova is theorised to occur when the same collapse happens in an even more massive, rapidly spinning star, producing an explosion of vastly greater energy.

These explosions seed nearby star-forming clouds with heavy elements, and the process gives astronomers a powerful investigative tool. By analysing the composition of a star, researchers can infer its family history, effectively reading the chemical record of the stellar explosions that contributed to the gas from which it formed. But this forensic method depends on assumptions about how well ejected material mixes with the surrounding interstellar gas before new stars are born. Schoenrich emphasised that, despite decades of work in this area, scientists still do not know how thoroughly supernova material gets mixed before forming new stars. The new research suggests that some mixing occurs, but that it is incomplete, a conclusion with consequences far beyond the handful of stars studied.

The second study focused on a single red giant in the Milky Way’s halo, the vast cloud of stars surrounding the galaxy’s disc. This star has an exceptionally peculiar chemical make-up, including an abundance of certain heavy elements such as silver and uranium. Elements this heavy can only be produced by neutron star mergers, the collisions of ultra-dense stellar remnants, or, in theory, by the hypernova of a highly magnetised massive star capable of forging both these heavy elements and the standard supernova elements no heavier than iron and nickel.

Earlier work had ruled out a neutron star merger as the origin of this star’s material, on the grounds that the star is extremely metal-poor, meaning it contains a very low proportion of heavier elements created in stellar explosions. Such metal-poor stars are understood to have formed from gas barely enriched by earlier generations of stars. The previous analysis relied on a model of the Milky Way in which the galaxy’s gas was enriched with heavy elements very quickly, implying that by the time the star was born, insufficient time had passed for neutron stars to form, spiral together, and collide. That reasoning left the hypernova as the only remaining explanation, and the star became one of the flagship cases for these ultra-powerful explosions.

Aggarwal and Schoenrich presented a different picture. They noted the star’s exceptional speed and the fact that it orbits the Milky Way in the opposite direction to most other stars, characteristics typical of stars captured from smaller galaxies. They argued that the star most likely originated in a small dwarf galaxy, where metal-poor stars can form much later, many millions of years after the Big Bang, before being pulled into the Milky Way. In tiny galaxies, star formation proceeds more slowly, and when stars do explode, most of the ejected material is blasted out into intergalactic space, keeping the galaxy’s gas metal-poor for far longer than in a large galaxy like our own. Using their mathematical model of asymmetric explosions, the researchers concluded that this star most likely formed from a neutron star merger combined with a single ordinary supernova, removing the need to invoke a hypernova at all.

The implications of the two studies extend well into the future of galactic chemistry modelling. If supernova ejecta are not well mixed, then models of how chemicals evolve in galaxies, and of how the interstellar medium behaves, need to incorporate this incompleteness explicitly. Chemical evolution models that treat each supernova as delivering a uniform, averaged blend of elements may misinterpret unusual abundance patterns as signs of exotic events, when they are in fact the natural consequence of patchy, directional ejecta. The UCL team’s work suggests that astronomers should be cautious before attributing strange stellar chemistries to the most extreme explosions imaginable, and that the humble supernova, viewed through a more realistic lens, may be capable of far more chemical variety than previously assumed. Whether genuine hypernovae exist at all remains an open question, but according to this new analysis, the evidence once marshalled in their favour has suddenly gone.

Subject of Research: The chemical signatures of hypernovae and supernovae in metal-poor stars

Article Title: Ultra-powerful star explosions might not have occurred after all

Article References: Ultra-powerful star explosions might not have occurred after all. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: hypernova, supernova, stellar nucleosynthesis, metal-poor stars, neutron star merger, Milky Way halo, dwarf galaxy, chemical evolution, interstellar medium, UCL, Monthly Notices of the Royal Astronomical Society, stellar explosions