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Meet the Electron Capture Supernova
In looking at the ways in which stars die, there are two basic scenarios. Stars either just kind of stop and shrink in on themselves, or they explode in an element generating burst of nuclear reactions that we call a supernova. Up until today, there were two generally observed kinds of supernovae: exploding white dwarf stars, and exploding massive stars that leave behind either a compact object or nothing. Now, thanks to one weird object that exploded in 2018, a theorized third-form of supernovae — the Electron Capture Supernova — has been identified. With these single-star explosions, the core collapses for the most boring of reasons: Ionized atoms are pressured into taking back their electrons, and this collapses the core, making it possible for the outer layers to bounce out in a spectacular supernova. This work is published in Nature Astronomy and led by Daichi Hiramatsu.
The Death of a Normal Star, and the Detonation of its Ember
Stars like our Sun pass gently into the night. As they age, they loose the ability to burn the fuel in their cores as they run out of light weight materials like easy-to-fuse hydrogen. They are simply too small to produce the pressures and temperatures necessary to fuse heavier elements. As they peter out, their outer atmosphere (if there is any) drifts away, and the remaining core will (except for the smallest stars) collapse down into a white dwarf star — a star made of normal protons and electrons that are packed so densely that it is the pressure of the electrons repelling one another that supports the star. A star like our Sun can leave behind something as small as the moon.
That white dwarf is not the most stable object in the universe. If you pile mass onto this stellar ember’s surface, bad things can happen. This extra mass generally comes from the white dwarf gravitationally pulling material off a companion star. It is pretty common to see novae flaring up as material on or near a white dwarf gets dense enough to explode, temporarily, with nuclear reactions. Pile too much material on a white dwarf… well, electrons can’t support infinite gravitational force, and under the weight of too much added stuff, the protons and electrons in a white dwarf will merge into neutrons, releasing a vast amount of energy in the process, and blasting out a Type 1a supernova.
