A sweep of 26,000 stars has found one with a composition like nothing else yet seen. Its strange mix of elements matches what would be expected if the star SMSS J200322.54-114203.3 formed from a type of explosion that astronomers have theorized but never seen, known as a magneto-rotational hypernova.
As the name suggests, hypernovas are mightier even than supernovas, releasing around 10 times as much energy. They're very rare, but a few have been witnessed in quite distant galaxies. There are hopes Eta Carinae will one day give us a ringside view.
Dr Chiaki Kobayashi of the University of Hertfordshire has modeled a specific type of hypernova. Now in Nature, Kobayashi and a team at the Australian National University have announced the discovery of a star that appears to have risen phoenix-like from the remnants of such an event in the halo of the Milky Way.
SMSS J200322.54-114203.3 was spotted as being unusual as part of a SkyMapper survey. It's chemical composition has four unusual features. Known processes can explain each of them, but putting them all together requires something new. On the other hand, all four aspects fit well with the expected product of a rapidly spinning star that formed in the earliest beginnings of the Milky Way and then went hypernova.
The timing is indicated by the exceptionally low concentration of iron, relative to hydrogen, within J200322.54-114203.3, 3,000 times lower than in the Sun. Since iron is produced by fusing carbon in high-mass stars, and then distributed through the galaxy when the star dies, only a star that formed in its galaxy's earliest days could be so anemic.
Meanwhile J200322.54-114203.3 is rich in heavy elements such as uranium and zinc. Only two processes are thought to produce these in any great quantities, supernova explosions and neutron star mergers. Since the first observation of a neutron star merger in 2017, these events have become the favored explanation among many astronomers for most of the universe's heavy metals, but the new paper's authors think the spectral fingerprint seen here is much closer to what would be expected from a truly enormous supernova. “The high zinc abundance is definite marker of a hypernova,” said senior author, Nobel Prize winner Professor Brian Schmidt.
Spin can be inferred from the concentration of nitrogen. First author David Yong told IFLScience that rapid spin drives a mixing process within the star that brings hydrogen and carbon – separated in most stars – together. These then fuse to produce nitrogen. J200322.54-114203.3's rotation rate is insufficient for the purpose, so the mixing must have been done in the predecessor.
Together these create a picture of a star at least 25 times as massive as the Sun, highly magnetized and spinning staggeringly fast before exploding with a force less than one in a thousand supernova can match. A black hole was probably left behind, but billions of years later its location is impossible to guess.
When IFLScience asked Yong whether the authors had any evidence for the ancient presence of a magneto-rotational hypernova besides the chemical composition of the star thought to have formed from its remains he replied honestly “None whatsoever,” but J200322.54-114203.3's composition is hard to explain otherwise.
Other stars like J200322.54-114203.3 may turn up, but Yong says many will have lived and died long before humans evolved to see them. Hypernova debris will have often mixed with the remains of more ordinary supernovas and galactic gas, leaving a signal too faint for us to detect.
The discovery could be important for one of astronomy's biggest question, whether the heavy metals that hold such importance to a technological society come primarily from hypernovas, or from collisions between neutron stars.