We know that a hot soup of plasma as it cools (as space expands) can produce light elements if the baryon-photon ratio is just right and the space expands quickly enough so that the neutrons and protons fall out of equilibrium - a theory advocated by Ralph Alpher. We also know that Deuterium and Helium can be produced by fusion of hydrogen atoms in stars, a theory advocated by Fred Hoyle. The internet is full of articles that tell us that BBN is one of the pillars of Cosmology, but I can't find a single one that says why it's favored over Stellar Nucleosynthesis.

Please explain the reason/evidence for preferring BBN over Stellar nucleosynthesis for the production of light elements.

  • $\begingroup$ isn't it just basically "the ratio of heavy elements to helium is too low for stellar nucleosynthesis"? $\endgroup$ Mar 29, 2021 at 20:48
  • $\begingroup$ @JerrySchirmer - It could be, if the statement is followed by "because..." and then an explanation of why it's too low. This is the part that seems to be missing from the public documents available. $\endgroup$
    – Gluon Soup
    Mar 29, 2021 at 21:04
  • $\begingroup$ Wait, how does this paper have only 305 citations!? I mean, it's a great paper, and the author list just kills it! ui.adsabs.harvard.edu/abs/1948PhRv...73..803A/abstract $\endgroup$
    – Kyle Oman
    Mar 29, 2021 at 21:09
  • $\begingroup$ @KyleOman: you'd expect that with Hans Bethe being the second author on that! $\endgroup$ Mar 29, 2021 at 21:41

1 Answer 1


Deuterium and lithium are generally destroyed by nuclear reactions inside stars. They fuse at lower temperatures (about $10^6$ K and $3\times 10^6$ K for D and 7Li respectively) than the $>10^7$ K it takes to convert hydrogen into helium.

Therefore, if the conditions are right to produce helium from hydrogen, then this should destroy all the deuterium and lithium.

NB. 7Li must be created somewhere because the cosmic abundance is higher than the primordial abundance. The most likely candidates are AGB stars or novae. But then 7Li would be accompanied by lots of other heavy elements and wouldn't explain why we see lithium in extremely old stars, even when they have almost no metals.

It is also worth mentioning that the exquisite concordance of the measured primordial He and D measurements at a single value of baryon/photon ratio that itself matches the value estimated from the cosmic microwave background (for example see this "Schramm plot") is a coincidence beyond any variant of the steady state theory.

  • $\begingroup$ OK. That's very useful. Putting aside the production of Deuterium and lithium, are there any studies on how much Helium has been produced by stars in 13 billion years?That is, I'd like to know, of the 24% of helium observed, what fraction of that was produced in stars? $\endgroup$
    – Gluon Soup
    Mar 29, 2021 at 23:52
  • $\begingroup$ @GluonSouo By looking at the He abundance in the most metal-poor galaxies and comparing it with the He abundance of gas in our galaxy. The difference is a few percent. $\endgroup$
    – ProfRob
    Mar 29, 2021 at 23:54
  • 1
    $\begingroup$ Present day He abundance in the galaxy is about 27%, primordial He is about 24%. $\endgroup$
    – ProfRob
    Mar 30, 2021 at 0:01
  • $\begingroup$ Gluon Soup, please have a look at Weinberg's book The First Three Minutes, it will answer your questions in a straightforward and easily comprehended way. Highly recommended. $\endgroup$ Mar 30, 2021 at 4:52

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