A 40-Year-old problem in Astrophysics is Finally Solved!!!!
After 40 years, astrophysicists have finally solved the puzzle regarding the production of Lithium in Stars.
As many of us know that Stars — our Sun included — contain mostly Hydrogen and Helium. And Stars creates energy by nuclear fusion — Hydrogen atoms fuse together at high temperature and pressure — while another product is also Helium.
Two different isotopes of Hydrogen, Deuterium — Hydrogen with one proton and one neutron — and Tritium — Hydrogen with one proton and two neutrons — undergo nuclear fusion at high heat — 15.7 million kelvin — and pressure — 25.33 trillion KiloPascals — and release out enormous amounts of energy —17.58 MeV — with a Helium atom and a neutron.
If one ton of deuterium were to be consumed through the fusion reaction with tritium, the energy released would be 8.4 × 10²⁰ joules. This can be compared with the energy content of one ton of coal — namely, 2.9 × 10¹⁰ joules. In other words, one ton of deuterium has the energy equivalent of approximately 29 billion tons of coal.
Variety of other nuclear fusion reaction also happen such has Hydrogen-Hydrogen, and also Deuterium-Deuterium fusion reactions also occur. Now we wonder would Helium and Hydrogen undergo nuclear fusion to produce Lithium in the Star?
Fifty years ago Ann Merchant Boesgaard found one of the first Lithium rich star. The star had more Lithium in it compared to other stars and meteorites.
Stars, as per the known mechanism of evolution, actually destroy Lithium as they evolve into a Red Giant. They destroy the Lithium via low temperature Nuclear Burning.
There were numerous theories which theorized ways of why stars were Lithium rich. One popular theory was Planet Engulfment theory. For example, a Earth-like planet may increase the star’s lithium content when their plunge into the star’s atmosphere when the star becomes a Red Giant.
Planets are known to have more Lithium than their stars, for an example take our Sun and Earth. Earth has more Lithium than the Sun.
But evidence was found to contradict this. Some stars were found to be Lithium-rich. Here are a few papers on the observation of such stars:
When such stars were observed it was found that stars in the Red Clump stage of the evolution cycle of a star — Red Clump is a later on process which happens in the Red Giant stage of the star cycle— had much higher levels of Lithium, with an increase of a factor of 40 over the end of the Red Giant phase of the star cycle.
Red Clump: A cluster of Red Giants at around 5000 K temperature and are slightly hotter than stars of the same luminosity.
This new information shows that stars of low mass undergo a Lithium production phase between the Red Giant and Red Clump phase. Lithium enrichment in such stars happens when a process called Helium Flash occurs.
It was first theorized in 1971 in this paper.
In summary: The mechanism of Lithium production happens in the stage of Helium shell flashes. Due to the thermal instability of the Helium shell resulting in a flash causes an appearance of a convection zone starting from the middle of the burning shell outwards as a well to the non-burning helium layers. The convection zone enriches the Helium shell with Carbon. After nine relaxation cycles, the convection zone touched the Hydrogen burning shell. Maximum penetration will occur after the 12th cycle, and diminishes in late cycles.
The Hydrogen envelope is mixed down to a temperature of 10⁸ K, the mixing of particle at the base of the convection zone happens for a few hours. The reaction of ³He(α, γ) ⁷Be has a lifetime of a few hours near the temperature of 8 x 10⁷ K. It is then evident that the ³He envelope would be converted to ⁷Be.
Most of the ⁷Be will be mixed to a much cooler region before the formation of ⁷Li before electron capture takes place. ⁷Be(e⁻ν) ⁷Li .
Most of the ⁷Li will be preserved on the surface once the outer envelope relaxes to its former state. ^7Li will be preserved if it is in a region of temperature of less than 3 x 10⁸ K thus the ⁷Li will not undergo thermonuclear reaction.
This new work done by Bharat Kumar a Post-doctoral fellow at the NAO of China, Beijing and an international team of colleagues have solved an important puzzle in star evolution and nucleosynthesis. Here is a link to his paper:
But what does this mean?
This is a key puzzle in understanding the star cycle and how heavy metals are exactly formed because as we know heavy metals are a key to life and compose a higher percentage of mass of planets, asteroids, and other celestial bodies.
