When stars explode

Why an electron-capture supernova is different

1 Mass range of electron-capture supernovae

Howell says astronomers aren’t sure of the exact mass range for electron-capture supernovae, but it’s larger than Type Ia and smaller than Type II supernovae: “It could be anything in between eight and ten solar masses.”

2 Oxygen and neon

An electron-capture supernova needs a core made of oxygen, neon and magnesium, and that’s why it has to be between eight and ten solar masses. “Heavier stars make iron cores, and lighter ones make carbon-oxygen cores,” Howell explains.

3 Fusion stops

Ongoing fusion prevents a star from collapsing. If fusion has ceased, then the star won’t collapse if the atoms cannot be any more tightly packed. In electroncapture supernovae, fusion stops but some electrons in the core smash into their atomic nuclei.

4 Removal of electrons

The collisions cause electrons to be removed by the neon and magnesium, and the core of the star collapses under its own weight. This causes a stellar explosion. “The star can’t have too much mass loss before it explodes,” says Howell.

5 Shining bright

SN 2018zd – just like SN 1054 – shone brightly, and that has been explained by the likelihood that matter ejected from the explosion collided with circumstellar material being cast off by the progenitor star.