Missing Helium Mystery Solved: Big Stars Ate It
Deep in the interior of a red giant star, hydrogen rich clouds (red) are seen to float above the hydrogen burning shell (blue).
Credit: Lawrence Livermore National Laboratory

For years, astrophysicists have tried to reconcile a cosmic discrepancy: the universe held much less helium 3 gas than was predicted by models of stellar evolution. But by using new 3-dimensional models, scientists think they've discovered where all the helium 3 went - it was destroyed by the very stars that were thought to eject it into space, according to a new study.

Just after the Big Bang, the gases that made up the universe were predominately hydrogen, with 10 percent helium 4 and just .001 percent helium 3.

But astrophysicists thought that "several times that much [helium 3] ought to have come by later [stellar] evolution," said Peter Eggleton, an astrophysicist at Lawrence Livermore National Laboratory and lead author of the study, which appeared in the Oct. 26 issue of Science Express.

According to previous models of stellar evolution, low mass stars (about 1 to 2 times the size of our Sun) should have produced large amounts of helium 3 and increased its percentage in the universe to .01.

But observations showed that the amount of helium 3 in the universe still at .001 percent.

"It was odd that the helium 3 didn't seem to pile up," Eggleton said.

Astrophysicists had tried to come up with what Eggleton called "flimsy ideas" to explain the discrepancy.

In what he described as a case of serendipity, he and his colleagues found the answer of the missing helium 3 while they were modeling a "near explosion" called a helium flash, which occurs when a star switches from burning hydrogen to burning helium.

Stars like the Sun burn hydrogen at their cores for nearly 10 billion years. As the star ages, it exhausts all the hydrogen at its core to become a red giant and begins to burn helium. The star also loses much of its mass through stellar winds. The expelled material was thought to be rich in helium 3. (Heavier elements like carbon, nitrogen, and oxygen have accumulated in the universe through this same mechanism.)

While modeling the helium flash, Eggleton and his colleagues found an unexpected instability elsewhere in the star that "seemed to explain two phenomena that had been a bother for several years," Eggleton said.

The instability mixed helium 3 in the outer layers of the star into deeper layers where it was hot enough for it to be burned up ─ solving the problem of where all the helium 3 went to.

The instability also explained why older stars were observed to have increasing abundances of carbon 13 and nitrogen 14 when they weren't expected to. Like the helium 3, carbon 12 and nitrogen 13 located nearer the surface were also mixed deeper into the star where they were converted into carbon 13 and nitrogen 14 respectively.

"Now it seems that a perfectly respectable mechanism is" behind the discrepancies astrophysicists have been trying to explain for decades, Eggleton said.

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