lithium-rich-stars-puzzle-solved

Context: A Study led by Indian astrophysicists challenges the present understanding of nucleosynthesis in stars.

Understanding the lithium content in the stars and contradictions thereafter:

  • Stars, as per the known evolution mechanism destroy lithium as they evolve into red giants. Planets have more lithium than their stars. The Sun, for instance, has about a factor of 100 lower amount of lithium than the Earth.
  • The puzzle and contradiction: About 40 years ago, a few large stars were spotted that were lithium-rich. That posed a puzzle — if stars do not produce lithium, how do some stars develop to become lithium rich?
  • Possible answers to the puzzle, the planet engulfment theory: As per the theory, Earth-like planets may increase the star’s lithium content when they plunge into their star’s atmosphere when the latter become Red Giants. 
    • But this theory was found to be unsatisfactory.

Nucleosynthesis is the process of creating new atomic nuclei from preexisting nucleons (protons and neutrons). The primordial preexisting nucleons were formed from the plasma of the Big Bang as it cooled below ten million degrees.

 

About presence of Lithium in universe

  • Lithium, a light element commonly used today in communication device technology, was first produced in the Big Bang, around 13.7 billion years ago when the universe was born, along with other elements. 
  • How it is measured: There is a method of measuring lithium content using low-resolution spectra in a large number of stars, with facilities provided at the Indian Institute of Astrophysics.
  • Lithium is destroyed in the Universe: While the other elements grew a lot in abundance, the present abundance of lithium in the universe is only four times the original value that was produced in the Big Bang because it is actually destroyed in the stars. 

Findings of the new study by Indian astrophysicists

  • This is the first study to demonstrate that lithium abundance enhancement among low mass giant stars is common. Until now, it was believed that only about 1% of giants are lithium rich.  
  • They also set a lower limit for helium abundance which will classify the star as “lithium-rich” which is about 250 times lower than the previous limit.
  • The scientists group studied over 200,000 stars using the Galactic Archaeology survey of the Anglo-Australian Telescope, Australia which is a dedicated facility for obtaining high-resolution spectra for a large number of stars.
  • The study has been published in the journal Nature Astronomy.
  • Puzzle solved: The study has found that when stars grow beyond their Red Giant stage into what is known as the Red Clump stage, they produce lithium in what is known as a Helium Flash. This enriches them with lithium. 

Stellar evolution: It is a description of the way that stars change with time.

The life of a star: 

  • Stars are born out of the gravitational collapse of cool, dense molecular clouds.
  • Death of star: Stars spend most of their lives repetitively compressing two hydrogen atoms into a single helium atom – plus a lot of energy, which is released as light and heat.
  • Once the hydrogen in the core has all been burned to helium, energy generation stops and the core begins to contract.
  • This causes the star to expand enormously and increase in luminosity – the star becomes a red giant.
  • Once the helium has all been converted, the inert carbon core begins to contract and increase in temperature.

A helium flash is a very brief thermal runaway nuclear fusion of large quantities of helium into carbon through the triple-alpha process in the core of low mass stars during their red giant phase.

Source: http://abyss.uoregon.edu/~js/ast122/lectures/lec16.html