“The sun is a mass of incandescent gas
A gigantic nuclear furnace
Where hydrogen is built into helium
At a temperature of millions of degrees” -They Might Be Giants
It’s so ingrained in us that the Sun is a nuclear furnace powered by hydrogen atoms fusing into heavier elements that it’s difficult to remember that, just 100 years ago, we didn’t even know what the Sun was made out of!
The conventional wisdom at the time, believe it or not, was that the Sun was made out of pretty much the same elements that the Earth is! Although that might seem a bit absurd to you, consider the following piece of evidence.
Every element in the periodic table — which was well-understood back then — has a characteristic spectrum to it. When those atoms are heated up, the transitions back down to lower-energy states cause emission lines, and when a background, multi-spectral light is shone on them, they absorb energy at those very same wavelengths. So if we observed the Sun at all these individual wavelengths, we could figure out what elements were present in its outermost layers by its absorption features.
That technique is known as spectroscopy, where the light from an object is broken up into its individual wavelengths for further study. When we do this to the Sun, here’s what we find.
Basically, there are the same elements that we find on Earth. But what is it, exactly, that causes those lines to appear with the relative strengths that they appear. For example, you may notice that some of these absorption lines are very narrow, while some of them are very, very deep and strong. Take a closer look at the strongest absorption line in the visible spectrum, which occurs at a wavelength of 6563 Ångströms.
What determines the strength of these lines, as well as the relative weakness of the lines surrounding it? It turns out that there are two factors, one of which is obvious: the more of an element you have, the stronger the absorption line is going to be. That particular wavelength — 6563 Å — corresponds to a well-known Hydrogen line.
But there is a second factor that must be understood in order to get the strength of these lines right: the level of ionization of the atoms present.
Different atoms lose an electron (or multiple electrons) at different energies. So not only do different elements each have a characteristic spectrum associated with them, they can exist in a number of different ionized states (missing one electron, or two, or three, etc.) that each have their own, unique spectrum!
Because energy is the only thing that determines the ionization state(s) of atoms, this means that different temperatures will result in different relative levels of ionization, and therefore, different relative levels of absorption!
So when we’re looking at stars — like the Sun — we know that they come in a huge variety of different types, as a look through any telescope or binoculars will immediately show you, if it isn’t clear to your naked eye.
These stars, very notably, come in strikingly different colors, which tells us that — at least at their surfaces — they exist at vastly different temperatures from one another! Because hot objects all emit the same type of (blackbody) radiation, when we see stars of different colors, we’re really detecting a temperature difference between them: blue stars are hotter and red stars are cooler.
After all, this is why we classify stars the way we do in modern times, with the hottest, bluest stars (O-type stars) at one end and the coolest, reddest stars (M-type stars) at the other.
But this was not how we always classified stars. There’s a hint in the naming scheme, because if you had always classified stars by temperature, you might expect the order to go something like “ABCDEFG” instead of “OBAFGKM,” right?
Well, there’s a story here. Back before this modern classification scheme, we instead looked at the relative strengths of absorption lines in a star, and classified them by what spectral lines did or did not show up. And the pattern is far from obvious.
Different lines appear and disappear at certain temperatures, as completely ionized atoms have no absorption lines! So when you measure an absorption line in a star, you need to understand what its temperature is (and hence its ionization properties are) in order to rightfully conclude what the relative abundances of the elements are within it.
And if we go back to the Sun’s spectrum, with the knowledge of what the different atoms are, their atomic spectra, and their ionization energies/properties, what do we learn from that?
That, in fact, the elements that are found on the Sun are pretty much the same as the elements found on Earth, with two major exceptions: Helium and Hydrogen were both vastly more abundant than they are on Earth. Helium was many thousands of times richer on the Sun than it is here on Earth, and Hydrogen was about one million times more abundant on the Sun, making it the most common element there by far.
Know who the scientist was who put this all together? I’ll give you a hint: it was a 25-year-old woman who was never fully given the credit she deserved.
Meet Cecilia Payne (later Cecilia Payne-Gaposchkin), who did this work for her Ph.D. thesis way back in 1925! (Astronomer Otto Struve called it “undoubtedly the most brilliant Ph.D. thesis ever written in astronomy.”) Just the second woman to earn her Ph.D. in astronomy through Harvard College Observatory (where she had to move to earn one; her original alma mater, Cambridge, didn’t award Ph.D.s to women until 1948), she wound up having a remarkable astronomy career, becoming the first female chair of a department at Harvard and an inspiration to generations of astronomers, both male and female.
Historically, Henry Norris Russell (of Hertzsprung-Russell fame) was often given the credit for the discovery that the Sun is primarily composed of hydrogen, as he dissuaded Payne from publishing her conclusion and stated it himself four years later. Let that be the case no longer! This was Cecilia Payne’s brilliant discovery and she deserves full credit for making it. The strength of the absorption lines combined with the temperature of the stars and the known ionization properties of atoms leave you with the inescapable conclusion: the Sun is a mass of primarily Hydrogen! Thanks to Cecilia Payne, now you know how we know that.