In looking at the ways in which stars die, there are two basic scenarios. Stars like our Sun eventually just sort of peter out as they run out of fuel in their core and stop creating energy via nuclear reactions. The light those reactions had been producing had been supporting the star against collapse under its own gravity, but in the absence of light, that core will collapse into a tiny, moon-sized, white dwarf star, while the remaining atmosphere, if any, drifts away. This is how the majority of stars will die. 

In a few instances, however, the collapsing core is so massive that instead of hunkering down into a smaller object, it also explodes. This can happen in a couple of different ways. If you pile a bunch of extra matter from a nearby star onto an already collapsed white dwarf, it will explode as a Type 1a supernova. If you have a truly giant star, greater than ten solar masses in size, it will collapse, and the core will become either a neutron star or black hole or nothing but energy as things around it explode in the collapse. 

Now, we are learning, a theory on a third way for stars to die may be true and may be able to explain the death of the star that formed the Crab Nebula. This third form of death is electron capture collapse. Elderly red super-asymptotic giant branch stars can reach a point where they stop having nuclear reactions, their core starts to collapse, and the ionized oxygen, neon, and magnesium grab onto all the free electrons and suddenly get tiny by forming atoms. This is the atomic difference between everyone taking their own car to work and taking rideshare. As soon as all those particles combine, they take up a lot less space, and infalling material around them, it ends up going supernova. 

While theories about this kind of explosion had existed for a long time, including theories that explained Chinese observations of the Crab Nebula’s birth in 1054, modern observations didn’t exist until now. In 2018, a weird supernova whose progenitor star was observed to be a super-asymptotic Giant Branch star in surveys was seen to explode. Careful observations of how that explosion evolved over time matched the electron capture supernova theories and taught us there are apparently more than two ways to explode a star. This work appears in Nature Astronomy and was led by Daichi Hiramatsu.

More Information

Las Cumbres Observatory press release

University of California, Davis press release

University of California, Santa Barbara press release

“The electron-capture origin of supernova 2018zd,” Daichi Hiramatsu et al., 2021 June 28, Nature Astronomy