First Observed Nearly a Thousand Years Ago, Scientists Finally Confirm Elusive Third Type of Supernova

The stellar explosion may explain an odd event documented by Chinese astronomers in 1054 C.E.

A photo taken by the Hubble Space Telescope of the NGC 2146 galaxy and of the supernova SN 2018zd
The supernova SN2018zd (large white dot on the right) located 30 to 40 million light years away was first identified in 2018 and may be the first observed evidence of an electron-capture supernova. NASA/STScI/J. DePasquale and Las Cumbres Observatory

Within starry galaxies, astronomers have observed two types of supernovas. When a massive star reaches the end of its lifetime, it loses fuel and some of its mass starts to flow into its core. Eventually, it becomes so heavy that it collapses and explodes into an iron-core collapse supernova. Thermonuclear supernovas, on the other hand, occur when small white dwarf stars run out of matter and begin to siphon matter from another nearby white dwarf star, eventually accumulating so much mass that it explodes into a supernova.

But neither of these supernovae fully explain an event that occurred nearly a thousand years ago. In the summer of 1054, Chinese astronomers recorded a star that radiated so brightly it was visible in daylight and shined for 23 days. The explosion, now known as SN 1054, was a supernova, and its remnants formed the Crab Nebula. In the 1980s, University of Tokyo researchers first theorized that the blast was caused by a third type phenomena called an electron-capture supernova.

Now, a supernova event observed in 2018 may confirm the existence of electron-capture supernovae after all, reports Ashley Strickland for CNN. The study published this month in Nature Astronomy may provide new insights into how the Crab Nebula formed, how neutron stars are made, and how elements are created and scattered across the universe.

Electron-capture supernovae occur when stars within a slim range in size—eight to ten solar masses—explode. The enormous internal pressures force electrons to fuse with atomic nuclei as the star's core loses fuel. Normally, the electrons would repel each other. But when they combine, pressure inside the star drops, causing the star's core to collapse. The collapsed core sets off an explosion leaving behind a neutron star heftier than the sun, reports Charles Q. Choi for Space.com. Like iron-core collapse supernovas, electron-capture supernovas produce neutrons stars.

While records were kept from SN 1054 and astronomers developed predictions about what to look for in an electron-capture supernova and its progenitor star, researchers had yet to observe it occurring. Their chance came in 2018 when amateur astronomer Koichi Itagaki detected an exploding star in the starburst galaxy NGC 2146, which is located 30 to 40 million light-years away, reports Alison Klesman for Astronomy.

Two years after it was first seen, researchers at Las Cumbres Observatory and the University of California, Santa Barbara, gathered data on the 2018 supernova event dubbed SN 2018zd. Previous images captured by the Hubble and Spitzer telescopes showed SN 2018zd's host galaxy before and after the explosion, which allowed researchers to identify the exact star that caused the explosion.

Observations of SN 2018zd fit criteria for identifying an electron-capture supernova set by the 1980s researchers. Six key features are needed to place an electron-core supernova. The stars should have an enormous mass, such as red giants. The star needs to shed most of its mass before exploding, and the shed mass must mostly be made of helium, carbon, nitrogen, and little to no oxygen. When the star explodes, the explosion should be weak with no radioactive fallout, and the core should have neutron-rich elements, reports Jennifer Ouellette of Ars Technica.

The researchers hope to find more examples of the third type of supernova.

"This supernova is literally helping us decode thousand-year-old records from cultures all over the world," says study author Andrew Howell, an astronomer at the University of California Santa Barbara, in a statement. "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."