Sky & Telescope - April 2021 - 17

Branch 1

Branch 2

Branch 3

Proton
Neutron

Positron

Ȗ
Ȟ

Gamma ray
Neutrino

Hydrogen-1
Beryllium-7

Beryllium-7

Ȗ
3

Helium-3

Lithium-7
Ȟ

H
He

Hydrogen-2

Helium-4

Beryllium-8

Ȟ
Boron-8

PP CH AINS: LE A H TISCIONE / S&T, SOURCE: N ASA / JPLCA LTECH; OV ERSHOOT SIM UL ATION: T. ROG ERS E T A L. /
ASTR OPHYSICAL JOUR NAL 20 0 6

S RARE REACTIONS Most solar fusion occurs through the proton-proton chain, which fuses hydrogen into helium. About 70% of proton-proton
fusion occurs through the string of reactions in Branch 1, with the rest following Branches 2 and 3 (30.9% and 0.1%, respectively). By detecting neutrinos coming from the latter two sets of reactions, the Borexino experiment has provided tentative evidence that helioseismologists are right and that
the new solar metal abundances are too low. Detections of neutrinos from another, even rarer set of reactions involving carbon, nitrogen, and oxygen
would provide more definitive evidence.

helium-poor. " We've already seen evidence of this happening
with other young stars, " Zhang says.
Combined, the three processes create a complex interplay
that explains observations from both sides of the standoff.
" Our model gives a perfect match to the results from
helioseismology, " Zhang says. The catch is that these processes have to happen in just the right proportions to make
all the pieces ﬁt. " The range of values needed to make it work
is small, " he says, " but we think it is reasonable. " It's a rare
suggestion that would see both the spectroscopists and the
helioseismologists vindicated.
Anton Sokolov (Institute for Nuclear Research of the
Russian Academy of Sciences) believes he has another solution, albeit one that relies on an idea far from mainstream
scientiﬁc thinking. He looked for ways to change the opacity
of solar material without tweaking the abundance of metals.
One option is to make the radiative zone a little cooler and
denser, in turn making it opaquer. To achieve this, Sokolov
looked beyond the Standard Model of particle physics - scientists' cookbook for the way particles
and forces interact with one another. Physicists have long suspected there are particles
beyond what the Standard Model prescribes, not least because they need them
to explain dark matter, the invisible glue
X DEFINING BOUNDARIES A 2D slice from a
computer simulation shows the convection and
radiative zones in the Sun. Some of the convective
plumes spill past the boundary between the zones,
carrying material deeper into the radiative zone. This
phenomenon is known as overshoot, and taking it into
account could help resolve the solar abundance problem.

thought to bind galaxies like our Milky Way together.
According to Sokolov's calculations, conditions at the
top of the radiative zone are ideal for turning some of the
ordinary photons the Sun creates into particles called dark
photons. Crucially, dark photons don't interact with ordinary
solar material; instead, they stream out freely in all directions, robbing the radiative zone of energy. However, this
energy loss needs to be compensated for elsewhere in the Sun
to tally with observations.
For that, Sokolov turned to another so-far-hypothetical
entity: millicharged particles. They are the product of dark
matter that has settled in the solar core after being hoovered
up by the Sun as it journeys through the Milky Way. According to Sokolov's theory, these particles produce energy that
warms the core and balances the heat lost farther up in the
radiative zone. " Such a process would solve the solar abundance problem, " he says.
Sokolov's solution is an extreme one. After all, as Carl
Sagan famously espoused, extraordinary claims require
extraordinary evidence. Yet the solar abundance/
modelling problem has been a thorn in astronomers' collective sides for nearly two decades.
Although the recent Borexino CNO neutrino haul has provided a real shot in the
arm, it's clear there is much work left to
do to resolve one of the trickiest issues in
modern astronomy. Nothing short of our
understanding of stars depends on it.

COLIN STUART (@skyponderer) is an astronomy author and speaker. Get a free ebook
at colinstuart.net/newsletter.
sk yandtelescope.org * APRIL 2 021

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Sky & Telescope - April 2021

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