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Giant Explosion in Space Illuminates Thousand-Year Mystery

Supernova 2018zd (the large white dot on the right) and the host starburst galaxy NGC 2146 (toward the left) credit NASA

Dr. Iair Arcavi, a researcher at Tel Aviv University’s Faculty of Exact Sciences, was part of a team that found a new type of stellar explosion called an electron-capture supernova. These supernovae result from the explosions of stars eight to nine times the mass of the sun. While they have been predicted for 40 years, empirical evidence has remained elusive. The discovery also throws new insight into the thousand-year-old enigma of the supernova seen by ancient astronomers in A.D. 1054 that evolved into the Crab Nebula we know today.

A supernova is the star’s explosion resulting from an abrupt imbalance between two opposing forces that fashioned the star over its lifetime. Gravity attempts to contract each star. For example, our sun counterbalances this force through nuclear fusion in its core, which generates pressure in the opposite direction of the gravitational pull. As long as sufficient atomic fusion occurs, gravity will be incapable of collapsing the star. However, nuclear fusion will eventually cease, much as gasoline does in a car, and the star will collapse. The collapsed core of a star, such as the sun, is referred to as a white dwarf. White dwarfs contain such dense mass that quantum forces between electrons prevent them from collapsing further.

However, for stars, ten times the mass of our sun, electron quantum forces are insufficient to overcome gravitational attraction, and the core continues to shrink until it collapses into a neutron star or a black hole, accompanied by a catastrophic explosion. Electrons are squeezed (or, more precisely, captured) onto atomic nuclei in the intermediate-mass range. This eliminates the quantum interactions between electrons, causing the star to collapse and finally explode.

Dr. Iair Arcavi

Historically, there have been two distinct types of supernova. The first is a thermonuclear supernova, which occurs when a white dwarf star accumulates mass in a binary star system. These white dwarfs are the dense cores of ash that remain when a low-mass star (one with a mass of up to around 8 times the sun) dies. Another major type of supernova is a core-collapse supernova, which occurs when a large star — one with a mass greater than around ten times that of the sun — runs out of nuclear fuel and collapses, forming a black hole or a neutron star. According to theoretical study, electron-capture supernovae would occur on the dividing line between these two types of supernovae.

That is the hypothesis proposed in the 1980’s by Ken’ichi Nomoto and others at the University of Tokyo. Theorists have developed predictions on what to look for in an electron-capture supernova for decades. Before exploding, the stars should shed a significant mass of a particular composition, and the supernova should be relatively faint, create little radioactive fallout, and produce neutron-rich materials.

The latest study, published in Nature Astronomy, focuses on the supernova SN2018zd, which was detected in 2018 by Koichi Itagaki, a Japanese amateur astronomer. Dr. Iair Arcavi of Tel Aviv University’s astrophysics department also participated in the investigation. The explosion in galaxy NGC 2146, has all the characteristics of an electron-capture supernova, which have not been observed in any previous supernova.
Additionally, because of the supernova’s proximity – only 31 million light-years -, the researchers could pinpoint the star in pre-explosion Hubble Space Telescope photos. Indeed, the star itself meets the predictions for the type of star that should explode as an electron-capture supernova, in contrast to other forms of supernovae observed.

While several previously discovered supernovae exhibited several of the indicators predicted for electron-capture supernovae, only SN2018zd exhibited all six – a progenitor star within the expected mass range, significant pre-supernova mass loss, an unusual chemical composition, a weak explosion with low radioactivity, and neutron-rich material. “We began by asking, ‘Who is this stranger?'” said Daichi Hiramatsu, the study’s lead author from the University of California, Santa Barbara and Las Cumbres Observatory. “Then we investigated every aspect of SN 2018zd and discovered that they can all be explained in terms of electron capture.”

The new results shed light on some riddles surrounding one of the most renowned supernovae in history. A supernova occurred in our own Milky Way Galaxy in A.D. 1054, and according to Chinese and Japanese accounts, it was bright enough to be seen during the day and cast shadows at night. The resulting relic, known as the Crab Nebula, has been thoroughly investigated and discovered to contain a unique composition. Previously, it was the best candidate for an electron-capture supernova, but this was uncertain due to the explosion’s roughly 1,000-year age. The new finding strengthens the case that the historic supernova 1054 was an electron-capture explosion.

“It’s fascinating how modern technologies can throw light on historical occurrences in the Universe,” Dr. Arcavi says. “Today, with the help of robotic telescopes that scan the sky with remarkable efficiency, we may uncover an increasing number of rare events that are crucial for comprehending the rules of nature, without having to wait a thousand years between events.”

Dr. Arcavi is a member of the Global Supernova Project and uses the Las Cumbres telescope network to examine extremely rare transient phenomena, including as supernovae, neutron star mergers, and stars ripped apart by black holes.

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