Third type of supernova explosion confirmed
WHEN YOU THINK OF A SUPERNOVA, the type you most likely imagine is a type II, or corecollapse, supernova. This type of cosmic blast happens when a star at least 10 times the mass of our Sun runs out of fuel and collapses at the end of its life, leaving behind either a neutron star or a black hole. The other type of supernova, type I, occurs when the remnant of a Sun-like star, called a white dwarf, siphons material from a nearby companion. Once it tips over a certain mass limit, a runaway thermonuclear explosion rips the white dwarf apart.
However, calculations dating back to 1980 show that there should be a third type of supernova, called an electroncapture supernova, that occurs only in stars in a narrow mass range: 8 to 10 solar masses. Before such a star can run out of fuel completely, magnesium and neon atoms that have piled up in its core begin capturing free-floating electrons around them. As these electrons are absorbed, the outward pressure holding up the star decreases and the star’s inner regions collapse to a neutron star while the outer regions simultaneously blast outward as a supernova.
Now, for the first time, astronomers have confirmed one such supernova.
The culprit, SN 2018zd, was spotted in the galaxy NGC 2146 in March 2018 by Japanese amateur astronomer Koichi Itagaki. The results are detailed in a paper published June 28 in Nature Astronomy.
The key was finding the progenitor star, which astronomers were able to do by looking at Hubble and Spitzer space telescope images of its host galaxy before and after the supernova. Both the star and the explosion fit expectations perfectly, matching all six expected criteria for an electron-capture supernova.
Now that researchers have solid evidence for this new type of supernova, they hope to use SN 2018zd as a template to identify other such blasts. For example, astronomers have long thought the supernova of A.D. 1054, which created the Crab Nebula (M1), was an electroncapture supernova. The confirmation of SN 2018zd provides the strongest evidence yet that this is the case. And finding more of these supernovae will reveal more about the galaxy and universe we inhabit, including how the deaths of massive stars fling the building blocks of life across the cosmos.