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 ]