Sky & Telescope - April 2021 - 16

Solar Trouble

The Devil's in the Details
In the meantime, others have been searching for alternative
solutions - some of them highly speculative. Qian-Sheng
Zhang (Chinese Academy of Sciences) thinks that we can
keep spectroscopists' lower abundances and match helioseismic measurements if we revise the way we model the Sun.
This requires resolving the entire abundance problem, which
isn't solely about a discrepancy in the amount of metals - the
amount of helium is also in question.
Standard solar models using spectroscopists' lower metal
abundances usually result in a lower amount of helium in
the convection zone; otherwise the Sun would be the wrong
brightness. But less helium is at odds with helioseismic measurements. In August 2019, Zhang's team presented a revised
model of the Sun's interior that employs lower metal abundances while also increasing the amount of helium in the
outer convection zone.
Currently, our picture of the convection zone is that material at its base heats up, rises to the surface, then cools and

Helium was discovered by spectroscopists on the Sun
before chemists identiﬁed it on Earth. That's why it's
called helium: Helios was the Greek god of the Sun.
sinks back down. As the plasma boils, some helium settles out
due to gravity, dropping into the radiative zone. But Zhang's
team suggests that if some boiling material overshoots the
lower boundary, it will drag the base of the convection zone
deeper inside the Sun, preventing helium from settling as
much and keeping the convection zone richer in helium.
Accounting for this process allows the Sun to have lower
metallicity while still maintaining the deep convection zone
that helioseismologists measure.
Another factor to consider is the solar wind, the stream
of material blown outward by the Sun. Solar models don't
typically include it, but it also affects helium abundance. Evidence from missions such as Ulysses suggests the solar wind
is helium-poor compared to the normal solar surface composition. If models incorporate this effect, they see a boost in
helium's abundance in the convection zone.
Finally, Zhang and colleagues found they also needed
to reduce the amount of helium in the radiative zone, deep
inside the Sun - a fundamental change likely made early on:
The material falling onto the infant Sun would have had to be

S LITTLE MESSENGERS The Borexino experiment is housed underground in the Gran Sasso National Laboratory in Italy. Photomultiplier tubes line
the walls of the chamber pictured here. The flashes of light they detect within a central sphere of liquid signal neutrinos coming from the Sun.

APRIL 2 021 * SK Y & TELESCOPE

VOLK ER STEG ER / SCIENCE SOURCE

" Carbon-nitrogen-oxygen neutrino ﬂuxes will be the
cleanest test, " Basu says. But she cautions against a quick
answer. Although the ﬁrst tentative results don't look good
for the spectroscopists, she says " it may take a few decades to
get the error bars down. It's a waiting game. "

Sky & Telescope - April 2021

Table of Contents for the Digital Edition of Sky & Telescope - April 2021