We Don’t Know What the Sun Is Made Of
Summary
TLDRThe composition of the Sun has puzzled astronomers for decades. Using spectrography, Cecilia Payne determined in the 1920s that the Sun is made mostly of hydrogen and helium. The remaining elements are lumped together as 'metals.' Knowing the exact metallicity of stars allows inferences about planet formation. In 2009, a new high-precision model suggested the Sun has far fewer metals than previously thought - only 1.34%, versus 1.9%. However, this conflicts with data on the Sun's interior structure from helioseismology. Since then, multiple new analyses have tried to resolve the discrepancy by re-evaluating spectral measurements and opacity of metals like iron under solar conditions. Yet over a decade later, the question of the Sun's precise metallicity remains unsettled.
Takeaways
- 😯 Astronomers still don't know the exact composition of the Sun, even though it's close by
- 🔬 Cecilia Payne used spectroscopy to show the Sun is mostly hydrogen and helium
- 👩🔬 Astronomers call all elements besides hydrogen and helium "metals"
- 🌟 A star's metals come from previous generations of stars and are key for planet formation
- 😕 Conflicting measurements make it hard to pin down exactly how much of the Sun is metals
- 📊 A 2009 study using new models found the Sun has far fewer metals than thought before
- 🚨 This significantly reduced the expected metals across the universe
- ⏺ But helioseismology shows the Sun's interior structure implies more metals
- 🔎 Scientists are still working on updated opacity measurements and analyses to resolve the conflict
- 🤘 We don't know the Sun's composition, but it clearly has great musical taste
Q & A
What was the common belief about the composition of the universe 100 years ago?
-100 years ago, most astronomers thought that everything in the universe was made of the same stuff as the Earth - the same elements in basically the same amounts.
How did Cecilia Payne revolutionize our understanding of the composition of stars?
-Cecilia Payne perfected the science of spectrography and figured out that many of the gaps in the Sun's spectrum come from just the first two elements - hydrogen and helium. She realized stars were almost entirely made of these two elements.
Why are elements beyond hydrogen and helium called 'metals' by astronomers?
-Elements beyond hydrogen and helium are very un-abundant in the greater cosmos, so astronomers lump them together under 'metals', even though many would not normally be considered metals.
Why are the exact metal contents of stars important?
-The metals in stars are scattered when they die to become the building blocks of new stars, planets, and anything not made of hydrogen and helium. So knowing exact metal contents tells us about cosmic origins and potential for life.
Why is determining the exact metal content of the Sun difficult?
-Stellar spectra can be complicated. Atoms of elements absorb different wavelengths depending on temperature, charge and other factors. Also, absorption lines can overlap between elements.
How did a new spectral analysis method in 2009 challenge existing ideas about the Sun's composition?
-Using new precise models, a 2009 study found the Sun had only 1.34% metals, much less than the ~1.9% previously thought. This had big implications for expectations of metals and life in the wider universe.
How does helioseismology conflict with the 2009 spectral analysis results?
-Helioseismology shows a transition zone depth implying a higher solar metallicity around 1.9%, matching traditional ideas but conflicting with the new 1.34% finding.
What are some ways researchers are trying to resolve the conflict over measurements of solar metallicity?
-Updating spectral models with new data, re-evaluating opacity of metals like iron under solar conditions, and reconsidering convection zone behavior if solar metallicity is lower.
What is the current range of values found by different research methods for the Sun's overall metallicity?
-Different recent analyses have produced values ranging from about 1.34% to 1.6%, with no definitive answer yet.
What album name suggestions does the video request for the personified heavy metal rocker Sun?
-The video asks viewers to suggest potential album names for the metal rocker Sun in the comments.
Outlines
😲 How Cecilia Payne discovered stars are mostly hydrogen and helium
Cecilia Payne was a Harvard graduate student who analyzed the Sun's light using spectroscopy in the 1920s. She discovered that stars are made almost entirely of hydrogen and helium, with all other elements lumped together as "metals" despite not actually being metallic. Payne's finding challenged the assumption that stars have the same composition as Earth. Metals are important since they are formed in stars and become ingredients for planets and life.
😕 Conflicting estimates on exactly how metallic the Sun is
A 2009 study using a new model found the Sun has far fewer metals (~1.34%) than previous estimates (~1.9%), implying the whole universe has less metals available for planet formation. However, helioseismology showing the depth of the Sun's convective zone conflicts with a low metal amount. Astronomers are still investigating, with some new spectral analyses and opacity evaluations providing slightly higher metal estimates. But there is no consensus yet.
Mindmap
Keywords
💡spectrography
💡metals
💡opacity
💡convection
💡helioseismology
💡radiative zone
💡spectral analysis
💡composition
💡abundances
💡opacity physics
Highlights
Cecilia Payne pioneered the science of spectrography to determine that stars are made almost entirely of hydrogen and helium
Astronomers group all elements other than hydrogen and helium as "metals", including ones we don't normally think of as metals like carbon and oxygen
Metals in stars are crucial since they become the building blocks for new stars, planets, and life
The amount of metals in the Sun reveals information about the universe's capacity to support life
In 2009, new models found the Sun has far fewer metals than previously thought - only 1.34%
The lower metal content conflicts with helioseismology showing the Sun's interior structure depends on a higher metal content
The conflict suggests either the new models are wrong or our understanding of the Sun's interior is incorrect
Research since 2009 has updated metallicity estimates but not fully resolved the discrepancy
Some analyses of individual metals support the original higher estimates
Other studies have questioned if metals like iron are more opaque than expected, altering interior structure
If metals are more opaque, it could resolve differences between spectral data and interior models
But further analyses on opacities have been inconclusive and raised additional questions
Astronomers are still working to determine the Sun's composition from different angles
Knowing the Sun's exact metal content will reveal details about the universe's capacity for life
The Sun's composition remains an unsolved mystery, despite being close and constantly studied
Transcripts
Space, as a certain science fiction author once wrote,
is big.
Which means there’s a lot of room for mysteries.
But some of those mysteries seem like
we should have solved them by now.
Take our Sun, for instance.
It’s less than nine minutes away if you’re a ray of light.
That’s right under our noses
in the grand scheme of space.
And yet, after all this time,
astronomers still don’t know what the Sun is made of.
[intro jingle] A hundred years ago,
most astronomers thought that everything in the universe
was made of the exact same stuff as the Earth.
The same elements
in basically the same amounts.
But then Harvard grad student Cecilia Payne came along,
took a good hard look at the Sun,
and realized that was…
very much not the case.
As part of her research,
she perfected the science of spectrography,
where you look at all of the light coming off an object
to learn about its composition.
Here’s an example of the Sun’s spectrum.
See all those black bands?
They’re created when atoms in the Sun’s atmosphere
absorb some of the light that’s trying to make its way to us.
Or outer space more generally.
And you may have heard
that every element on the periodic table
has its own unique combination of wavelengths it can absorb.
But that’s only half true.
Different versions of an element
can also have their own version
of this light-based fingerprint.
Which is why in the 1920s,
the not-yet-Dr. Payne figured out
that many of the gaps in the Sun’s spectrum
come from just the first two elements
in the periodic table.
Our star, and all the stars in the known universe,
were almost entirely made of hydrogen and helium.
Everything else, at least by comparison,
was practically non-existent.
And because these elements
are so un-abundant in the greater cosmos,
astronomers lump them all together under a single name:
metals.
Yes, that includes the ones
you’d never think of as being metallic.
Carbon is a metal.
Oxygen is a metal.
I don’t like it, either, folks.
But we’re gonna have to roll with it.
Now, even if metals make up
a tiny fraction of a star’s atomic bits,
they mean a lot.
If not to the stars,
then to all the life forms looking up at them.
That’s because essentially
all of the metals in our universe
are made inside stars.
And as those stars die,
they scatter their atoms into interstellar space
to become the building blocks of new stars.
And planets.
And anything else that arises on those planets
that isn’t made of hydrogen and helium.
So knowing the exact amount of metals
inside our star can tell us
how we came to be,
and even suggest
how many “we”s could exist around other stars.
But if you’ve read the title to this episode,
you know that we don’t know
exactly how metal our Sun is.
And that’s because stellar spectra can be,
well, complicated.
As Cecilia Payne demonstrated,
the atoms of a given element
will absorb different wavelengths
depending on their temperature,
their charge, and other factors.
That’s why the Sun can have
so many absorption lines created
by just two elements.
But on top of that, individual lines can overlap
when you’re looking at something
made out of a bunch of different elements.
If you see a really dark line at one wavelength,
it could mean you have a lot of element X,
a lot of element Y,
or lesser amounts of both at the same time.
So in order to figure out
what the Sun is made of,
modern astronomers have to create a bunch of different models
for potential compositions,
and then see what fits the spectrum they actually observe.
Now, models can’t capture
all of the nuances in the Sun’s spectrum.
But back in 1989,
scientists thought they had found
a good approximation.
And together,
all the metals made up
about 1.9% of the Sun’s mass.
That value more or less stuck around for decades,
until a new spectral analysis
came onto the scene
and cast everything into doubt.
The research was published in 2009,
and the team behind it had been
developing a way to study ancient stars that were,
well, more like pop stars than heavy metal rockers.
In other words,
stars with super low metallicities.
And because of that,
the team’s models needed to be way more precise,
and use way fewer approximations
than what had come before.
For example,
they figured out
how to separate some overlapping absorption lines
coming from oxygen and nickel,
to get better measures of each.
And to get the models just right,
they tested them out
on the best sample of starlight we’ve got.
The Sun itself.
But doing so produced a completely unexpected result.
According to this brand new,
super fancy model,
the Sun is a lot less metal than we thought.
Instead of nearly 2% metal,
it was only around 1.34%.
For a value starting off as low as it did,
that’s a huge drop.
And it had big implications for the entire field of astronomy.
Because if our Sun had fewer metals,
it likely meant that the rest of the universe did, too.
And remember,
metals are needed to make things like planets and people.
So this updated number
threw a huge wrench into humanity’s expectations
for what we may or may not find
beyond our stellar neighborhood.
But let’s not be too hasty
in remaking the entire universe.
Because these new results
conflicted with another way
astronomers measure the Sun’s composition:
helioseismology.
Just like earthquakes help scientists
learn about the Earth’s different layers,
vibrational waves traveling through
the Sun have revealed its inner structure, too.
First, we’ve got the core.
That’s where the nuclear reactions happen,
and the resulting energy radiates outward
into the next layer up.
This is the radiative zone.
And here,
the energy is carried by photons bouncing
between atoms in random directions
as they slowly creep outward.
But eventually,
those photons and their energy
hit the Sun’s outer layer,
the convective zone.
Here, the temperatures
are cool enough for some atoms…
especially certain metal atoms…
to become opaque.
And if you’re a photon slamming into an opaque lump of Sun stuff,
you’re gonna get absorbed instead bounce.
And when you’re absorbed,
you’re gonna give your energy
to the lump and heat it up a little.
So in the convective zone,
the energy has to move via, well, convection.
You get big churning loops of plasma
that vaguely resemble water
moving in a pot on a stove.
Now, because it’s the metals that are responsible
for turning the Sun opaque,
the exact depth
where the radiative zone switches over
to the convective zones
depends on how metal the Sun is.
And using helioseismology,
scientists have pinpointed this boundary
to a very specific depth:
71.3% of the Sun’s total radius.
And based on our knowledge
of when and why elements turn opaque,
that depth is in line with the original estimates
for the Sun’s metallicity.
But if the Sun is less metal,
as that 2009 study suggests,
then we’d expect the boundary
to be closer to the surface.
Which it just…isn’t.
This conflict means
that either the new spectral analysis is wrong,
or the Sun’s interior doesn’t work
like we think it does.
Or possibly even both.
So ever since then,
scientists have been digging
deeper and deeper into the problem.
Some of the authors from that 2009 paper
published an update in 2021,
looking at several metals in more detail.
While most of the individual abundances
stayed pretty much the same,
there was an overall increase in metallicity.
A whopping 1.39%,
which is still too low for some astronomers’ liking.
Meanwhile, other researchers
have analyzed the Sun’s spectrum
with new models,
and wound up getting different numbers of their own.
For example, a study from 2022
found an overall metallicity that was about 1.6%.
But that’s not the only angle that astronomers
are attacking this problem from.
They’re also re-evaluating
what they think is happening inside the Sun.
Because if they’re wrong, t
he observed size of the convective zone
could still totally work with a lower metallicity.
One study published in 2014 looked
at the properties of iron…
which unlike oxygen,
actually fits into my definition of a metal.
And by exposing a bunch of iron
to the temperature and pressure conditions
you’d find inside the Sun,
they realized that it’s more opaque than we thought.
In other words, there may be less of it
than we thought there was,
but each atom packs more of a punch,
balancing everything out.
According to the research team,
this revelation could explain half of the discrepancy
between the helioseismology data
and the 2009 spectral analysis.
Which makes it seem like astronomers
are finally heading in the right direction.
Unfortunately, their follow-up analyses
of other metal opacities
haven’t produced such clear-cut results.
And instead,
they’ve raised more questions about exactly
how these studies should be done.
But maybe one day we’ll finally have the answer
to what might sound like a very simple question.
Now, astronomers might not know
exactly how metal the Sun is,
but we here at SciShow think it’s gotta be enough
for it to have awesome taste in music.
Behold, our heavy metal rocker Sun,
which you can get as a limited edition pin
by heading over to DFTBA.com/SciShow.
in addition to a lot of our other cool merch
including the shirt that I am wearing right now
And if you want to suggest names
for our Sun’s debut album,
feel free to leave them
in the comments below.
maybe the sun is a member of ekleipsis
Thanks for watching!
[ OUTRO MUSIC ]
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