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Astronomers witness new type of star explosion, solving a long-standing mystery

Third, new type of supernova observed at a distance of about 31 million light-years from Earth.


Supernova 2018zd is the first example of a brand new type of supernova.

Joseph Depasquale, STScI

Just when you think we know a whole lot about the stars above, something comes along and illuminates them even more. Literally, in this case -- thanks to the observation of a brand new type of supernova. The discovery has interesting implications for a mysterious bright light that appeared in the sky in 1054. 

Published in Nature Astronomy on Monday, a report from an international team of scientists confirms a previously-unseen type of stellar explosion. Before the find, there were thought to be only two types of supernova: A core-collapse supernova (which occurs when a massive star runs out of fuel and its core collapses into a black hole or neutron star) and a thermonuclear supernova (which occurs when a white dwarf star explodes).

However, since the 1980s there was speculation another type may exist. Ken'ichi Nomoto of the University of Tokyo predicted a third type called an "electron-capture supernova" in 1980. This refers to supernovae arising from a dearth of fuel, which then had gravity force electrons into the core's atomic nuclei -- thus collapsing in on itself.

Evidence to suggest the existence of an electron-capture supernova involves huge stars losing much of their mass before exploding. The mass in question should be of an unusual chemical composition. Post-supernova there should be minimal radioactive fallout and the core should have neutron-rich elements.

Spectral analysis of a supernova, originally detected in March 2018, provides new evidence for the theory of an electron-capture supernova. Dubbed "Supernova 2018zd," there are several key factors that suggest its electron-capture nature: it displayed a high amount of its mass loss before exploding, has an unusual chemical composition, produced a weak explosion, shows little radioactivity and it leaves a neutron-rich core.

Understandably pleased to see his theory gain weight, Nomoto commented on the development and contributed to the paper.

"I am very pleased that the electron-capture supernova was finally discovered, which my colleagues and I predicted to exist and have a connection to the Crab Nebula 40 years ago. This is a wonderful case of the combination of observations and theory," he said.


This is a composite image of the Crab Nebula, which has long-since been hypothesized as the result of an electron-capture supernova.

NASA, ESA, NRAO/AUI/NSF and G. Dubner (University of Buenos Aires)

The Crab Nebula link

One of the brightest mysteries in supernova history, the origins of the Crab Nebula have long gone unexplained. It's believed that in A.D. 1054 a supernova occurred in the Milky Way. Historical accounts allege it was so bright that it could be seen in the daytime for 23 days, and at night for nearly two years. Nowadays its remains are known as the Crab Nebula.

Despite being studied extensively, determining whether the Nebula was likely the result of an electron-capture supernova was tricky -- largely because the explosion was almost a thousand years ago. 

With this new supernova discovery, however, it's believed we can more confidently declare the Crab Nebula to be the result of an electron-capture supernova.

Dr. Andrew Howell, leader of the Global Supernova Project and staff scientist at Las Cumbres Observatory, said "This supernova is literally helping us decode thousand-year-old records from cultures all over the world. And it is helping us associate one thing we don't fully understand, the Crab Nebula, with another thing we have incredible modern records of, this supernova."

"In the process it is teaching us about fundamental physics: How some neutron stars get made, how extreme stars live and die, and about how the elements we're made of get created and scattered around the universe."

With more still to learn about this new type of supernova, it's hard not to keep your eye more firmly on the night sky for any new and exciting flares of immense brightness.