Helium flash

A helium flash is a sudden, intense burst of nuclear fusion. It occurs deep within the cores of low-mass stars. These stars have a mass between 0.8 and 2.0 times that of our Sun.

This event happens during the red giant phase of a star's life. During the flash, the star uses the triple-alpha process to turn helium into carbon. This process is a runaway reaction that happens very quickly. For a few brief minutes, the energy produced can rival the output of the entire Milky Way galaxy. It is a critical moment in the evolution of many stars.

To understand the mechanism, we must look at how a star's core changes. As a star exhausts its hydrogen, a helium-rich core is left behind. In low-mass stars, there is not enough gravitational pressure to start normal helium fusion. This causes the core to contract and become extremely dense. The matter becomes degenerate matter. This state is supported by quantum mechanical pressure rather than thermal pressure.

While the core is degenerate, hydrogen fusion continues in a shell surrounding it. This shell fusion adds more mass to the core. This extra mass increases the core temperature until it reaches about 100 million kelvins.

Once the temperature hits this threshold, helium fusion begins. In a normal star, rising heat causes the core to expand and cool. This expansion regulates the temperature and keeps the star stable. However, degenerate matter behaves differently. In this state, an increase in temperature does not produce a significant increase in pressure. Because the pressure does not rise to push the core outward, the heat cannot escape through expansion. The helium fusion increases the temperature, which in turn increases the fusion rate. This creates a runaway reaction that rapidly spreads through the entire core.

This runaway reaction can climb to 100 billion times the star's normal energy production for a few seconds. Eventually, the temperature becomes so high that thermal pressure finally exceeds the degeneracy pressure. At this point, the degeneracy is eliminated. The core can then expand and cool down. This expansion consumes most of the energy released during the flash. Any remaining energy is absorbed into the star's upper layers. Because the energy is released so deep inside, the flash is mostly undetectable by direct observation. Scientists must rely on astrophysical models to understand it.

After the flash, the star undergoes significant changes. The core expands and becomes non-degenerate. The star's surface rapidly cools and contracts. This process can take as little as 10,000 years. After this period, the star's radius and luminosity may drop to roughly 2% of their former values.

It is estimated that the electron-degenerate helium core makes up about 40% of the star's total mass. During the process, about 6% of that core is converted into carbon.

Different types of stars experience different paths. A star with a mass greater than 2.25 solar masses will burn helium before its core becomes degenerate. These stars do not experience a helium flash. Conversely, very low-mass stars under 0.5 solar masses never reach the heat needed for ignition. Their cores keep contracting until they become helium white dwarfs.

Our own Sun is expected to undergo a helium flash. This will occur approximately 1.2 billion years after it leaves the main sequence phase.

There are also other related phenomena in space. Some stars experience subflashes, which are pulsational instabilities. These occur in stars that lack strong convective or radiative boundaries. Subflashes can last from hours to days and may occur for many years. Additionally, helium shell flashes can occur in asymptotic giant branch stars. These are less violent because they happen without degenerate matter. In binary systems, a white dwarf might accrete hydrogen from a companion. This can lead to a nova or an unstable helium flash on the star's surface.