Lopsided supernova debris can reproduce the halo-star chemistry once credited to hypernovas

The case for hypernovas in these stars was made by elimination. Astronomers read a star's ancestry from its element ratios, because different kinds of explosion leave different chemical patterns in the gas that later forms new stars ^[5]. For a group of old Milky Way halo stars low in heavy elements, the ratios looked impossible to produce with ordinary events alone, so hypernovas, a hyper-energetic kind of supernova, were proposed as the source ^[11][1].

That kind of argument holds only if the ordinary alternatives were modelled fairly. The new work goes after one step in that modelling: how ejecta mix. Models have often treated a supernova's expelled material as if it blends evenly into the gas around it ^[6]. Observations and simulations show explosions can spread elements asymmetrically, with a clump of oxygen going one way and nickel another ^[7]. Schönrich said those overabundances are unlikely to average out, particularly in the earliest stellar generations, which have fewer heavy elements ^[8]. Two stars forming on opposite sides of the same explosion could then end up with different compositions ^[8].

With that unevenness built in, the model matched the halo stars' element ratios as well as or better than the hypernova model, according to the paper in the September Monthly Notices of the Royal Astronomical Society ^[12][2]. Schönrich said the evidence for hypernovas is now very seriously challenged ^[3].

Anna Frebel, an astronomer at the Massachusetts Institute of Technology, is not fully convinced and still counts both supernovas and hypernovas as plausible parents ^[4]. "A good fit tells us that a particular enrichment scenario is possible, but it does not necessarily tell us that it is the only scenario," she said ^[13]. In her view, the new models do not rule hypernovas out ^[14].

Both readings can hold at once. The halo stars counted as evidence because nothing ordinary seemed able to make them. A supernova model that fits at least as well removes that reason without disproving the hypernova explanation, so on this evidence the stars' chemistry no longer singles out a parent ^[1].

How much the asymmetric model's edge means is harder to judge. The published account does not give the number of stars fitted or the statistics behind "as well as or better." Those details matter here. An explosion that seeds neighbouring stars with different compositions can reach a wider spread of ratios than one whose debris mixes evenly ^[2], and a fair comparison would penalise that extra freedom.

Adam Burrows of Princeton University, who studies supernovas and was not involved in the research, said irregular mixing is well known among supernova researchers but has not been well shared with astronomers who study galaxy evolution and generations of stars ^[9]. "They're taking seriously what we have seen for a long time," he said ^[10].