18 July 2020 | New Scientist | 21Views
Columnist
Chanda Prescod-Weinstein
is an assistant professor of
physics and astronomy, and
a core faculty member in
women’s studies at the
University of New Hampshire.
Her research in theoretical
physics focuses on cosmology,
neutron stars and particles
beyond the standard model
Y
OU should never stare
directly at the sun. This
isn’t a euphemism of
some kind – you really shouldn’t
because it will damage your eyes.
Of course, some among us may
have stared at it anyway, perhaps
in what we irresponsibly perceived
to be a show of daring. But maybe
you have been good and have
never stared at it or only used
special glasses to look, if you have
ever witnessed a solar eclipse.
Either way, you have probably
never noticed that the sun
seemed different. Sure, clouds,
fog or even the moon might
get in the way, but, in general,
it is just this extremely bright
yellowish circle in the sky that
we are either longing for, annoyed
with or contented by. It is, by all
other measures, timeless.
But the sun is more fantastical
than it appears to be to us in
everyday life. It is easy to forget
that, like many of the pinpoints
we see in the sky at night, it is
just an average so-called main
sequence star – and that means
it is changing all the time.
To start with, the sun hasn’t
always existed. In fact, it isn’t a
first-generation star. If it were,
we wouldn’t be here on Earth.
The first generation of stars,
counter-intuitively called
population III stars, were made
entirely of primordial hydrogen
and helium, the by-products
of big bang processes. These
stars formed when clumps of
gas collapsed under their ownThis column appears
monthly. Up next week:
Graham Lawton
“ We are carbon-based
life forms, requiring
heavy elements that
didn’t exist in the era
of the first stars”What I’m reading
Stuart Hall’s essays
about cultural studies
are challenging to read
but very enriching.
What I’m watching
Probably a few too many
horror films!
What I’m working on
I’m about to roll out a
new academic paper on
the timescales involved
in the condensation of
axion dark matter into
a unique state called a
Bose-Einstein condensate.
Chanda’s week
Our ever-changing star
The sun may seem timeless, but it is constantly evolving and is already
halfway through its life, says Chanda Prescod-WeinsteinLetters
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ignition of nuclear fusion.
Fusion in stars is the universe’s
primary source of elements that
are more massive than hydrogen
and lighter than iron. The process
changes with time. At the start of
its life, a star has a lot of hydrogen.
By the end, there will be less
because some will have fused into
helium and other more massive
elements like oxygen and carbon.
One quirk of the astronomycommunity is that everything
heavier than hydrogen or helium
is called a “metal”. So, in our lingua
franca, you are mainly metal.
At the end of their lives,
population III stars either
underwent supernova explosions
or slowly blew off their outer
layers. Both scenarios left behind
a lot of hydrogen, helium and
some metal-rich gases. In the
case of supernovae, elements
heavier than iron might have
fused during the explosion.
The evaporated remains,
mixed with other hydrogen and
helium gas, eventually repeated
the gravitational collapse and
nuclear ignition process, birthing
second generation population II
stars. These stars have a different
composition to their ancestorsbecause metals were available
in their nursery.
It is from the mix of population
III and population II remains that
stars like our sun were born. These
generational stages are vital for our
existence: those metals are what
make up the planets orbiting the
sun, including our own, and we are
carbon-based life forms, requiring
heavy elements that didn’t exist in
the population III era.
Our sun, like its forebears, has
a finite lifetime. Our calculations,
and dating of elements on Earth,
suggest it is about 4.5 billion years
old, halfway through its life. With
each passing day, the sun has less
hydrogen and more helium.
It also has an outer magnetic
field whose behaviour and effects
on Earth remain somewhat
unpredictable. Several of my
colleagues at the University
of New Hampshire’s Space
Science Center work in the area
of heliophysics, trying to figure
out the exact dynamics of the
magnetosphere and other parts
of the sun that are constantly
in flux. This work, sometimes
known as space weather, points
to how dynamical and constantly
evolving our sun actually is.
It might look the same to
you from the park, but the sun
is changing every day in ways
that are noticeable with missions
like NASA’s Parker Solar Probe.
In about 4.5 billion years, when
it begins its transition into a
planetary nebula, the sun will look
different by anyone’s standards. ❚