Far out cosmology: how stars manufacture elements (Introduction)

by David Turell , Friday, March 10, 2023, 19:20 (413 days ago) @ David Turell

Whole story is incomplete:

https://www.newscientist.com/article/2363058-how-are-the-atoms-that-form-us-forged-acro...

"The first description of how elements are formed came from Margaret Burbidge, Geoffrey Burbidge, William Fowler and Fred Hoyle (B2FH for short) in 1957. The quartet consisted of an astronomer, a modeller, a nuclear experimentalist and a theorist, reflecting the scientific diversity required to address such a fundamental problem. Together, they described how the combination of intense pressure and heat inside stars could fuse atomic nuclei together, so that heavier elements could be produced from lighter ones. With this the field of nuclear astrophysics was born.

"However, fusion reactions like this couldn’t explain how all elements are formed, only those with an atomic weight up to and including that of iron. Nuclei have a positive charge (because of their protons) and the heavier they get, the harder it is for them to overcome the repulsive electrostatic forces and fuse. Heavier elements do exist in the universe, of course, but the question of exactly how they get there has been open for decades.

"Fred Hoyle suggested that heavier elements could be formed by heavy nuclei capturing neutrons, as the particles’ lack of charge makes them easier to grab than positively charged protons. B2FH proposed two ways that these neutron captures could take place inside stars.

"The first involves stars with a relatively small number of neutrons (around 108 neutrons per cubic centimetre), where neutron-capture reactions take years to occur. This is the so-called s-process (s for slow). In this case, neutrons are captured only by stable isotopes – atoms of the same element with differing numbers of neutrons – and their closest neighbours. Stable isotopes have been studied by nuclear physicists for almost a century and so their properties and how they would work inside a star are well known.

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"We already had slow and rapid processes, but it seems that the missing piece of the puzzle was something in between. The i-process (short for intermediate) involves a middling amount of neutrons (between 1013 and 1015 per cubic centimetre) and neutron-capture reactions happen over the course of minutes. Adding this process into the mix means that the models fit the new observation data, suggesting it could explain what is going on in those mystery stars uncovered by more powerful telescopes.

***

"We are still far from a full understanding of how heavy elements are produced in the universe. The more we learn, the more we realise that our simple nucleosynthesis picture is incomplete. The introduction of the i-process brings us a step closer to the truth, but we still don’t know where it could take place and how much it contributes to the mix of elements in the solar system. And there’s still much to learn about the r-process too, like whether there are places in the universe outside of neutron star mergers where it could occur. The main challenge comes from the fact that the nuclei involved can’t be produced by our current particle accelerators. However, next-generation facilities, like the Facility for Rare Isotope Beams (FRIB) at Michigan State University, will be able to discover hundreds of new rare isotopes, never-before-produced on Earth."

Comment: still doesn't explain our giant universe, but locally in our galaxy we are learning more and more.