Lopsided ordinary supernovas can account for halo stars once credited to hypernovas, UCL models find
UCL modellers say uneven, non-spherical ejection from ordinary supernovas can explain halo stars once attributed to explosions 10 times more powerful. The work tests a small set of stars, so it weakens the case for hypernovas without ruling them out.
The Scientist · Science desk
Share chart: of the four stars suspected to have been forged from hypernova material, three were judged by the UCL team more likely to have been created after supernovas.
- Likelier made after supernovas, claim 6 3 of 4 stars 75% computed
Likelier made after supernovas The rest of the 4
| Share | Part | Whole | Percent | Claim |
|---|---|---|---|---|
| Likelier made after supernovas | 3 stars | 4 stars | 75% (computed) | 6 |
What happened
- The model asks how much material from each layer or region of the pre-explosion star would be needed to reproduce the chemistry of an observed star.
- Of four stars suspected to have formed from hypernova debris, the team concluded that three were more likely made after ordinary supernovas.
- The team traced a halo red giant with an odd chemical mix to a neutron star merger combined with an ordinary supernova.
- The work appeared as two papers in Monthly Notices of the Royal Astronomical Society.
Why it matters
- contradiction Space.com's headline says the explosions may never have happened, but the team's claim covers only whether stellar chemistry requires them.
- decision The authors say models of galactic chemical evolution and of the interstellar medium now have to build in incomplete mixing of supernova material.
- exposure We think any halo star labelled a hypernova product on its abundance pattern alone now faces the same lopsided-ejecta alternative.
Ralph Schoenrich of Mullard Space Science Laboratory at UCL described the model's picture of a star on the eve of collapse. "Just before a star goes supernova, it has layers like an onion, with heavier elements at the center and lighter ones further out," he said. [14]
Anmol Aggarwal, the team leader, said in a statement why that matters for the stars in question. "If we look at a supernova today, the star's materials are not spread evenly. Oxygen might predominantly fly in one direction, sulphur in another. New stars might form from a mix of pieces of the supernova, not its overall mix of elements," Aggarwal said. [4]
Set that against the count: three of four suspect stars, or 75 percent [15]. Aggarwal's summary is wider. His models, he said, "found in all cases that stars with a very unusual mix of ingredients were most likely formed from ordinary supernovas," and "All the evidence we see for hypernovae is suddenly gone." [5][16] Even in the first quote the wording is probabilistic. The report does not say how the team classified the fourth star.
The red giant is the more intricate case. It has an overabundance of heavy elements such as silver and uranium. Elements that heavy can only be made in the cores of the very largest stars, the sort that end as hypernovas. [7] A neutron star merger can also make such elements, but this star lacks certain other heavy elements that a merger would leave behind. [8]
The team used a Milky Way model rapidly enriched, by the deaths of the first generation of stars, with elements heavier than hydrogen and helium. [9] They took the star's rapid spin and its place on the galaxy's outskirts as signs that it escaped a metal-poor dwarf galaxy and was later dragged into the Milky Way. [10] In that account the supernova half does not have to be a hypernova. An asymmetrical supernova is enough. [11]
Underneath sits a quantity the authors call poorly known. "We still don't know how well the supernova material gets mixed before it forms new stars. That is despite decades of work in this area. Our research suggests some mixing goes on, but incomplete mixing," Schoenrich said. [12]
The thing this doesn't tell you is which explosion actually made any one of these stars. A fit shows what an ordinary supernova can produce, and we'd expect it to carry that mixing uncertainty with it.
What to watch
- Whether the two Monthly Notices papers show how the fourth suspect star was classified, and whether the red giant is a separate object.
- Whether other groups refit the same four stars with their own ejecta models and reach the same three-in-four split.
- Whether chemical-evolution models that adopt incomplete mixing change their predicted halo abundances.
Clarity's read
What the record supports and how the coverage leans. The claims behind it follow.
Reality
- Evidence50
- Adoption
- Insufficient
- Hype gap+40
- Incentives
- Insufficient
- Confidence40
Claim ledger
Ranked by verification strength, evidence, and original report placement.
- [1]
Hypernovas were first proposed to explain unusual stars in the halo of the Milky Way and in a neighboring dwarf galaxy.
- [2]
It was suggested that the unusual halo stars could only form from the debris of an explosion 10 times more powerful than the average supernova.
- [3]
The UCL team suggests ordinary supernovas could account for these stars when the uneven or non-spherical way they fling out matter is considered.
- [4]
"If we look at a supernova today, the star's materials are not spread evenly. Oxygen might predominantly fly in one direction, sulphur in another. New stars might form from a mix of pieces of the supernova, not its overall mix of elements," said team leader Anmol Aggarwal of University College London in a statement.
- [5]
"We developed mathematical models that took account of this and found in all cases that stars with a very unusual mix of ingredients were most likely formed from ordinary supernovas."
- [6]
The team began by observing four stars suspected to have been forged from material ejected in a hypernova explosion, and concluded that three of those stars were more likely to have been created after supernovas.
- [7]
The team studied a single red giant in the Milky Way halo with an odd chemical composition, including an overabundance of heavy elements like silver and uranium, which are too heavy to be forged in the cores of all but the most massive stars, the kind that go hypernova.
- [8]
Such heavy elements could also be created when two neutron stars collide and merge, but the red giant does not have certain other heavy elements that would be present after a neutron star collision.
- [9]
The team considered the red giant's origins using a model of the Milky Way rapidly enriched with elements heavier than hydrogen and helium (metals) through the death of the first generation of stars.
- [10]
The star's rapid spin and its position on the outskirts of the Milky Way led the team to conclude it escaped a metal-poor dwarf galaxy and was eventually dragged into the Milky Way; they concluded the event that created it was the combination of a neutron star merger and an ordinary supernova.
- [11]
No hypernova was required for the red giant; an asymmetrical supernova would be enough, the scientists found.
- [12]
"We still don't know how well the supernova material gets mixed before it forms new stars. That is despite decades of work in this area. Our research suggests some mixing goes on, but incomplete mixing."
- [13]
"We need to take this into account in our models of how chemicals evolve in galaxies and of how the interstellar medium works."
- [14]
"Just before a star goes supernova, it has layers like an onion, with heavier elements at the center and lighter ones further out. Our model looks at how much material from each layer or each region would be needed to fit the observed star."
- [15]
Three of the four suspect stars is 75 percent of the sample.
- [16]
"All the evidence we see for hypernovae is suddenly gone."
- [17]
The team's research was published as two papers in the journal Monthly Notices of the Royal Astronomical Society.
- [18]
Space.com's headline says that some of the universe's biggest explosions "may have never happened at all".
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Sources
1 independent publisher whose own reporting we read for this story.
- space.comScientists discover that some of the universe's biggest explosions may have never happened at all
1 article · October 14, 2026
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Topics
- SupernovaeFollow
- HypernovaeFollow
- Galactic chemical evolutionFollow
Entities
- Anmol AggarwalFollow
- Monthly Notices of the Royal Astronomical SocietyFollow
- Space.comFollow
- Ralph SchoenrichFollow
- University College LondonFollow