On the case of the "missing" helium
Oct 27, 2006
Astrophysicists may have solved the embarrassing problem of why there is so much less helium-3 in the universe than predicted by standard cosmology and star-evolution theories. Peter Eggleton of the Lawrence Livermore National Laboratory in the US and colleagues at Monash University in Australia have calculated that when aging low-mass stars swell to become "red giants", the large amount of helium-3 they have produced is pushed down into the stars’ hot interiors, where it is then burnt up. The result suggests that our understanding of the Big Bang is correct after all (Sciencexpress 1133065).
The helium-3 isotope -- together with hydrogen and lithium -- is one of the very few elements to have been synthesised in the Big Bang. Further quantities of helium-3 are also produced by low-mass stars (about one to two times as heavy as our Sun) when they burn up the hydrogen in their cores.
But once a low-mass star has spent all its hydrogen, it expands and cools to become a red giant, during which the outer layers of the star become turbulent. Scientists believe that any helium-3 inside the star becomes mixed up into these layers by convection. The helium is then carried away from these surface layers into space by winds.
The flaw with this model, however, is that it predicts that there should be a lot of helium-3 in the universe, whereas astronomers have only detected about a tenth of that value, which is just the amount that was produced in the Big Bang.
Eggleton and co-workers may now have solved this problem by modelling a red giant star in 3D. The simulations show that turbulence at the base of the star’s convection layer causes deep "hydrodynamic" mixing that destroys the helium-3 so none of it can be released into space. The helium-3 is converted into another helium isotope, helium-4, and hydrogen.
"The apparent problem with the Big Bang has been solved," says team member John Lattanzio of Monash University. "The helium-3 in the universe comes from the Big Bang, and low mass stars -- although they produce helium-3 -- do not release any into the universe because they destroy it."
About the author
Belle Dumé is a science writer based in France