Language
Country/Area
No result found
The mystery of the helium flash: Why does it generate such incredible energy in the cores of stars?
A helium flash is a stunning event in the life of a star, especially during the vigorous red giant phase of a low-mass star. The helium flash is said to be caused by the rapid nuclear fusion of large amounts of helium in the core, mainly through the triple alpha process.
In about a billion years, the Sun is predicted to undergo a helium flash, which will be the last thing on Earth that will occur after it leaves the main sequence.
This process occurs primarily in stars with masses between 0.8 solar masses (M☉) and 2.0 M☉. In these low-mass stars, as the hydrogen in their cores is rapidly consumed, although nuclear fusion continues in the outer hydrogen shells, a helium-rich substance is formed in the core. As the hydrogen becomes depleted, the remaining helium is compressed into degenerate matter, whose resistance to gravitational collapse comes from the principles of quantum mechanics rather than traditional thermal pressure.
When the temperature of the core rises to about 100 million degrees, the nuclear fusion process of helium begins. The reason why this process is amazing is that the core at this time is composed of degenerate matter. Therefore, in such a material environment, the increase in temperature does not lead to a significant increase in pressure. This phenomenon causes a rapid reaction with a temperature surge, which is extremely rare and destructive in the evolution of a star.
The rate of helium nuclear fusion increases dramatically, quickly reaching 10 billion times the original energy released, and this lasts only a few seconds.
As energy from the nuclear fusion of helium is released, the degenerate state of the core is altered, allowing the core to thermally expand, with the remaining energy being absorbed into the star's superstructure. This means that although the instantaneous energy release of the helium flash is astonishing, most of it cannot be observed. Because of this, astronomers rely primarily on theoretical models to understand this phenomenon.
Over time, the star's surface will rapidly cool and shrink at a rate of about 100,000 years, eventually reducing its radius and brightness to about 2% of its original value. It is worth mentioning that in this process, about 40% of the star's mass will be converted into carbon, which is crucial to the future evolution of the star.
After the helium flash, the pulsation instability of the secondary flash will drive the star, and this process often lasts for hours to years.
The helium flash is then followed by a series of secondary flashes, which are usually relatively weak pulsation instabilities and are not necessarily destructive. Compared to helium flashes, they are inherently more peaceful, but they play an important role in the final stages of stellar evolution.
In addition, in some extremely low-mass stars, the degenerate helium core may never reach a high enough temperature to initiate helium fusion, and will eventually evolve into a helium white dwarf. This shows a strong connection between the mass of a star and its evolutionary consequences.
Although a similar process is followed in white dwarfs, when hydrogen gas from a binary star system accumulates on the surface of a white dwarf, the fusion of hydrogen sources can also lead to an unstable helium flash. However, the occurrence of these events is rarely observed directly because their dynamics are generally hidden deep in the core.
The nuclear fusion process of a star is a long and unpredictable journey, with changes at each stage making the star's fate different.
It is worth pondering how the helium flash triggers such a violent release of energy in the life of a star, and how many undiscovered cosmic mysteries are hidden behind it?
