Final answer:
A helium flash is the explosive ignition of helium in the dense core of a red giant star's triple-alpha process, causing rapid heating and leading to a stable period in the star's life cycle.
Step-by-step explanation:
What is a Helium Flash?
The 'helium flash' refers to a nearly explosive ignition of helium in the triple-alpha process in the dense core of a red giant star. This event occurs when a star has used up the hydrogen in its core and starts to burn helium. The temperature and pressure at the core reach a critical point where helium nuclei begin to rapidly fuse into carbon, releasing a tremendous amount of energy in a very short time, leading to a flash. Since the star's core is degenerate—meaning electron pressure supports it against gravity—this energy does not expand the core but rather increases the temperature until the helium fusion becomes non-degenerate and the star stabilizes.
When the triple-alpha process begins in low-mass stars, the entire core is ignited almost instantaneously during the helium flash. The energy released is quickly transferred throughout the core, causing rapid heating, accelerating the nuclear reactions, and resulting in an energy runaway. This process is crucial for the star's evolution and leads to a period of stability in its life cycle. Helium, while well-known for its use in balloons, plays a critical celestial role as the second-most abundant element in the universe, participating in nuclear fusion processes that light up the stars.
The helium flash is not directly observable because it happens in the core of the star, but its effects can be seen as the star changes size and brightness, transitioning into a different stage of its lifespan as a giant once again.