‘Medium’ Nova 
The dead stellar cores known as white dwarfs are usually pretty placid. They no longer produce significant nuclear reactions, so they shine only by radiating the heat they stored up during their long lifetimes. But their surface gravity is strong, which sets up some “second acts” that can be quite theatrical.
A white dwarf’s gravity can pull gas from the surface of a companion star. The gas piles atop the white dwarf, where it can trigger an explosion — one that blows up the extra gas, the entire white dwarf, or perhaps only part of the white dwarf.
When hydrogen gas from the companion builds up on the white dwarf’s surface, the gas can get so hot that it triggers a nuclear explosion, known as a nova — a stellar H-bomb. The white dwarf survives, so it can stage more of these outbursts.
But if the white dwarf is heavy, then the extra gas can push it beyond its ability to support itself, and the star is blasted to bits as a supernova, which can outshine an entire galaxy.
Astronomers recently identified a couple of dozen stellar explosions that don’t seem to fit either category. They’re far brighter than a nova, but much fainter than a white-dwarf supernova.
Such a blast may be triggered not by hydrogen, but by helium. As the helium heats up it causes a more powerful blast than hydrogen does, so it destroys part of the white dwarf but may leave the rest intact — perhaps to begin the process all over again.
More about exploding stars tomorrow.
Script by Damond Benningfield, Copyright 2013