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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