Why do young stars contain so much lithium? That might seem an odd question, but it’s a question that has nagged astronomers for quite some time.

The story of lithium in the universe begins with the big bang. In the early moments of the big bang, the first elements were created through a process known as nucleosynthesis. Most of the matter created through nucleosynthesis was hydrogen and helium (numbers 1 and 2 on the periodic table), but the big bang also produced a bit of lithium (the number 3 element). According to our model of the big bang, for every 10 billion hydrogen atoms produced, only one lithium atom would form. This might seem like an extraordinarily small amount, but it’s actually more lithium than we actually observe in the universe. It would seem our prediction didn’t quite match reality.

But we also know that lithium can be consumed in the heart of a star. Stars produce light and heat through a complex process of nuclear reactions. As a result, lighter elements such as hydrogen can be fused into heavier elements like carbon, oxygen and iron. But some elements are easier to fuse than others, and it turns out that lithium readily fuses into other elements, a process known as lithium burning. As a result, stars can reduce the amount of lithium in the universe. Perhaps that would explain why there is less lithium than our initial model predicted. It’s a good idea, but there’s just one problem. If lithium burning were the solution, then we would expect older stars to have more lithium and younger stars less, since over time there would be less lithium available. What we actually observe is that older stars actually have less lithium than younger stars. What started with one mystery then became two.

But back in the 1970s it was proposed that the higher levels of lithium in younger stars might be due to nova explosions of older stars. Unlike a supernova, where a star is completely destroyed (with the exception of a remnant neutron star or black hole) a nova is due to a runaway nuclear reaction on the surface of a white dwarf. This nuclear reaction could produce lithium, and thus create the abundance of lithium in younger stars. New observations have actually confirmed this effect.

The results come from a nova known as V1369 Cen, which was a bright nova that appeared in the constellation Centaurus in 2013. Because of its brightness, the team was able to observe its spectrum in detail, and they found clear evidence of lithium in the nova. This lithium was cast out into interstellar space, making it available for later stars that happen to form. The measured amount of lithium was small, but combined with the estimated billions of novae that have occurred throughout the history of our galaxy, it is enough to account for the observed rise in lithium levels.

So it seems that the lithium enigma isn’t such an enigma after all.

Paper: Luca Izzo et al. Early optical spectra of nova V1369 Cen show presence of Lithium. ApJ 808 L14 (2015)

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One of the big successes of the big bang model is its prediction of elemental abundances. The first elements were produced in the early moments of the universe through a process known as baryogenesis. This process is very complex, and it is highly dependent upon the temperature and density of the universe at that time. Change the temperature a bit one way or the other, and the initial ratio of primordial elements would be different. Knowing the temperature of the early universe, we can predict the amount of hydrogen vs. helium produced by the big bang, and this agrees fairly well with observation.

But the model also predicts that trace amounts of lithium should also be produced. According to the theory, for every 10 billion hydrogen atoms the big bang produced, it should also have produced a lithium atom. That may seem like a tiny amount, but it is much higher than we observe in the early universe. In other words, the model predicts much more lithium in the early universe than we observe, and this is known as the “lithium problem.”

There have been a couple of proposed solutions to this problem. One is that we simply aren’t seeing the amount of lithium that’s actually there. Lithium can be difficult to detect, and it is actually consumed in the fusion process of stars, so it is possible that the big bang prediction is right, and we simply need to determine where it went. Another idea is that somehow our model of baryogenesis is wrong. If, for example, there exists some complex nuclear interactions we don’t yet understand (perhaps due to physics beyond the standard model) then we could be over-predicting the amount of lithium in the early universe.

But now a new paper in Physical Review Letters shows the latter idea doesn’t work. The work was done at the Laboratory for Underground Nuclear Physics (LUNA) where they study nuclear fusion reactions. This new work presents the results of lithium-6 production at temperatures equal to the early universe. They found the rate of lithium production agrees with that predicted by big bang cosmology. So it seems the big bang model works yet again, and this means we still have a lithium problem.

Paper: M. Anders et al.  First Direct Measurement of the 2H(α,γ)6Li Cross Section at Big Bang Energies and the Primordial Lithium Problem. Physical Review Letters 113, 042501 (2014).

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