It is generating energy through the thermonuclear fusion of helium at its core.
Eventually the internal pressure and temperature rise sufficiently for thermonuclear fusion to begin.
That turned out to be a mistake, Zeta's neutrons were not from thermonuclear fusion.
The central mass became increasingly hot and dense, eventually initiating thermonuclear fusion in its core.
This proved the lack of thermonuclear fusion in its core.
The key problem in achieving thermonuclear fusion is how to confine the hot plasma.
I have a final comment on thermonuclear fusion, although I know that many here take a different view from mine.
In either case, it is generating energy through the thermonuclear fusion of hydrogen at its core.
This is thermonuclear fusion, the energy of the H-bomb.
After his retirement Tuck became a prominent public supporter of research into thermonuclear fusion for power generation.