Since its existence was predicted four decades ago by first predicted by University of Tokyo astronomer Ken’ichi Nomoto, astrophysicists have had a theory about electron-capture supernovae. 

A supernova is a powerful and luminous stellar explosion. of a star following a sudden imbalance between two opposing forces that shaped the star throughout its life. Gravity tries to contract every star. 

For stars like the sun, the collapsed core is called a “white dwarf” whose material is so dense that quantum forces between electrons prevent further collapse. Our sun, for example, counter balances this force through nuclear fusion in its core, which produces pressure that opposes the gravitational pull. As long as there is enough nuclear fusion, gravity will not be able to collapse the star. But eventually, nuclear fusion will stop, just like gasoline runs out in a car, and the star will collapse.

Electron capture is a process in which the proton-rich nucleus of an electrically neutral atom absorbs an inner atomic electron, usually from the K or L electron shells. This process thereby changes a nuclear proton to a neutron and simultaneously causes the emission of an electron neutrino. Real-world examples have been elusive. Such supernovas arise from the explosions of stars eight or nine times the mass of the sun.

For 40 years after Nomoto’s initial proposal, uncertainties in his theoretical predictions of the signatures of such supernovae meant that no clear evidence for electron-capture supernovae was ever confirmed. But three years ago, a promising candidate emerged in the galaxy NGC2146, roughly 31 million light–years away. Over the next two years, the subsequent rise and fall in brightness of the explosion was observed by a global network of telescopes. From archival images taken by both the Hubble and Spitzer Space Telescopes, Daichi Hiramatsuteam at the University of California at Santa Barbara and the Las Cumbres Observatory who led the study also identified the most likely progenitor for the supernova.

Dr. Iair Arcavi, a researcher at the Sackler Faculty of Exact Sciences at Tel Aviv University (TAU), participated in a study that discovered this new type of stellar explosion – an electron-capture supernova. The discovery also shed new light on the thousand-year mystery of the supernova from 1054 CE that was seen by ancient astronomers, before it eventually became the Crab Nebula, that we know today. 

Arcavy is a member of the Global Supernova Project and makes use of the Las Cumbres telescope network to study rare transient phenomena like supernovae, neutron star mergers and stars torn apart by black holes. 

For stars 10 times more massive than our sun, however, electron quantum forces are not enough to stop the gravitational pull, and the core continues to collapse until it becomes a neutron star or a black hole, accompanied by a giant explosion. In the intermediate mass range, the electrons are squeezed (or more accurately, captured) onto atomic nuclei. This removes the electron quantum forces and causes the star to collapse and then explode. 

Historically, there have been two main supernova types. One is a thermonuclear supernova — the explosion of a white-dwarf star after it gains matter in a binary star system. These white dwarfs are the dense cores of ash that remain after a low-mass star (one up to about 8 times the mass of the sun) reaches the end of its life. Another main supernova type is a core-collapse supernova where a massive star — one more than about 10 times the mass of the sun — runs out of nuclear fuel and has its core collapsed, creating a black hole or a neutron star. Theoretical work suggested that electron-capture supernovae would occur on the borderline between these two types of supernovae. 

Over the decades, theorists have formulated predictions of what to look for in an electron-capture supernova. The stars should lose a lot of mass of particular composition before exploding, they suggested, and the supernova itself should be relatively weak, have little radioactive fallout and produce neutron-rich elements.  

The new study, published as a letter in prestigious Nature Astronomy under the title “he electron-capture origin of supernova 2018zd,” focuses on this supernova, which was discovered in 2018 by Japanese amateur astronomer Koihchi Itagaki.This supernova, located in the galaxy NGC 2146, has all of the properties expected from an electron-capture supernova and were were not seen in any other supernova. 

In addition, because the supernova is relatively nearby – only 31 million light years away – the researchers were able to identify the star in pre-explosion archival images taken by the Hubble Space Telescope. Indeed, the star itself also fits the predictions of the type of star that should explode as an electron-capture supernovae, and is unlike stars that were seen to explode as the other types of supernovae.

While some supernovae discovered in the past had a few of the indicators predicted for electron-capture supernovae, only SN2018zd had all six – a progenitor star that fits within the expected mass range, strong pre-supernova mass loss, an unusual chemical composition, a weak explosion, little radioactivity and neutron-rich material. 

“We started by asking ‘what’s this weirdo?’” said Hiramatsu. “Then we examined every aspect of SN 2018zd and realized that all of them can be explained in the electron-capture scenario.” 

The new discoveries also illuminate some mysteries of one of the most famous supernovae of the past. In 1054 CE, a supernova happened in our own Milky Way Galaxy, and according to Chinese and Japanese records, it was so bright that it could be seen in the daytime and cast shadows at night. The resulting remnant, the Crab Nebula, has been studied in great detail, and was found to have an unusual composition. It was previously the best candidate for an electron-capture supernova, but this was uncertain partly because the explosion happened nearly a thousand years ago. The new result increases the confidence that the historic 1054 supernova was an electron-capture supernova. 

“It’s amazing that we can shed light on historical events in the Universe with modern instruments,” concluded Arcavi. “Today, with robotic telescopes that scan the sky in unprecedented efficiency, we can discover more and more rare events which are critical for understanding the laws of nature, without having to wait 1,000 years between one event and the next.”