Avery brief and highly energetic explosion stemming from fused helium in the core of a star is called what?

The answer is Helium Flash.

A helium flash marks the looming end of a red giant. As a red giant expands in its outer layers, its core is actually contracting. The core would eventually form a white dwarf.

As the core reaches a certain temperature, helium will start to fuse to form carbon. As this occurs, the fusing of helium, the core heats up. The core is not able to expand to cool down, so the reaction increases the core until the core will ultimately explode. 

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