Astronomers discover Electron Capture Supernova - A New, Third Type of Supernova

An international team of astronomers from the Los Cumbres Observatory has confirmed that they have gathered the first convincing evidence for a new type of supernova or stellar explosion called an electron capture supernova (aka supernova 2018zd).

Before this new discovery, there were thought to be only two types of supernovae – first is a core-collapse supernova, which occurs when a massive star more than 10 times the mass of our sun runs out of fuel and its core collapses into a black hole or a neutron star, now the second is a thermonuclear supernova, which occurs when a low-mass star remnant called a white dwarf (usually less than eight times the mass of our sun) explodes.

Electron Capture Supernova is nearly four decades old

However, in 1980, astrophysicist Ken’ichi Nomoto at the University of Tokyo predicted that a third type of supernova called an “electron-capture supernova” may exist. As theorized by Nomoto this electron capture supernova, would be on “the border between the other two types of supernovas, which happens when the star’s core loses fuel, gravitational forces push the core’s electrons and fuse them with atomic nuclei.” This sudden drop in electron pressure results in “star collapsing in on itself leaving a dense neutron star with a little more mass than our sun.”

Supernova 2018zd first example of an electron capture supernova
Supernova 2018zd, the first example of a new, third type of supernova predicted 40 years ago. Credit: Joseph Depasquale, STScI

And since two years, a team led by Daichi Hiramatsu, a graduate student at the University of California, Santa Barbara, and Las Cumbres Observatory, collected data on the supernova 2018zd, first spotted in March 2018, “just hours after it exploded 31 million light-years away in the galaxy NGC 2146.” The deeper they collected the information about it, the more they realized that supernova 2018zd is the first example of a new third type of supernova – an electron capture supernova.

The team found that SN 2018zd displayed six key indicators predicted by Nomoto’s theory, including unusual stellar chemical composition, a strong loss of mass prior to the supernova, a weak explosion, small radioactivity, a core rich in elements like oxygen, neon, and magnesium, and a Super-Asymptotic Giant Branch (SAGB) type star. (These SAGB stars, which are rare, are bloated old red giant stars) The team gets to know about SAGB while seeing 2018zd by Hubble Space Telescope before the explosion.

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

After seeing his theory gaining weight, Ken’ichi Nomoto said,

“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.”

A study based on this new research is published in the journal Nature Astronomy.

Electron Capture Supernova link with The Crab Nebula

Crab Nebula
This composite image of the Crab Nebula was assembled by combining data from five telescopes spanning nearly the entire breadth of the electromagnetic spectrum. Credit: NASA, ESA, NRAO/AUI/NSF and G. Dubner (University of Buenos Aires)

The confirmation of this third type of supernova – an electron capture supernova – may also shed some light on a 1000-year-old mystery, the origins of the Crab Nebula.

It’s believed that in A.D. 1054 a supernova occurred in our Milky Way that was so bright, that it could be seen in the daytime for 23 days, and at night for almost two years – even today, we can see it as the Crab Nebula. However, with this discovery of third type of supernova it’s believed that the Crab Nebula was created by an electron capture supernova.

“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,” said Dr. Andrew Howell, leader of the Global Supernova Project and staff scientist at Las Cumbres Observatory.

“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 getting created and scattered around the universe,” he added.

This new discovery has taught us that we still have more to learn about this new type of supernova and the mysteries of the cosmos.

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