We Don’t Know What the Sun Is Made Of

Summary

The 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

00:00

😲 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.

05:01

😕 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.

Keywords

💡spectrography

Spectrography is the science of analyzing the spectrum of light coming from an astronomical object to determine its chemical composition. It is key to understanding the Sun's composition. Cecilia Payne used spectrography in the 1920s to show that the Sun is made mostly of hydrogen and helium. Modern astronomers still use it but have to create complex models to interpret the complicated stellar spectra.

💡metals

In astronomy, 'metals' refers to any element heavier than hydrogen and helium. This includes elements we think of as metals, like iron, as well as things like oxygen and carbon. Metals make up a tiny fraction of a star's composition but are essential for life. Knowing the Sun's metallicity tells us about star formation in our galaxy and prospects for life around other stars.

💡opacity

Opacity refers to how easily light can pass through a material. Iron and other metals become opaque at the temperatures and pressures in the Sun's outer layer. This causes convection, which defines the boundary between layers seen in helioseismology data. But the metals may be more opaque than previously thought, changing our understanding.

💡convection

Convection occurs when opaque metals in the Sun's outer layer absorb photons, heating up and causing churning plasma loops that transfer energy. The depth at which the radiative zone transitions to the convective zone depends on opacity of metals, and should match the Sun's metallicity.

💡helioseismology

Helioseismology studies the Sun's interior through vibrations and waves, like seismology does for Earth. It gives the depth of the radiative/convective boundary, which conflicts with 2009 spectral analysis suggesting lower metallicity. This disagreement is a major part of the mystery over the Sun's composition.

💡radiative zone

The radiative zone is the Sun's middle layer, where energy from the core is carried outward by photons bouncing between atoms. In the outer convective zone, opaque metals absorb these photons instead, transferring energy through convection.

💡spectral analysis

Analyzing and modeling the Sun's light spectrum suggests it has only 1.34% metals, much lower than previous estimates. But this conflicts with helioseismology and opacity physics. Updated spectral analyses give different abundances, showing the mystery is unresolved.

💡composition

The Sun's composition by mass percentage is essential for understanding star formation in our galaxy. Key elements include hydrogen, helium, and metals like iron, carbon, oxygen. But despite centuries of study, we still don't know the exact composition.

💡abundances

Abundances refer to the amounts and fractions of different elements in the Sun. Key abundances include ~74% hydrogen, ~25% helium, and 1-2% metals. Newer spectral analysis gives different abundances for metals like iron and oxygen than old models.

💡opacity physics

Scientists are reconsidering the intrinsic opacity of iron and other metals under solar conditions, which affects the radiative/convective boundary seen in helioseismology. This may help resolve the conflict between old and new composition estimates.

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 ]