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We’ve known about all of the naturally occurring elements for at least 80 years, from the familiar ones like iron and carbon to the very last one we found: francium. Most of these elements were discovered by doing clever chemistry, but the second-most abundant element in the universe also has one of the most unique stories:.

Helium was discovered in space before it was found on Earth. And it took nearly three decades for scientists to accept that it could actually exist. The now-famous balloon filler and squeaky-voice-maker was first discovered in the atmosphere of the sun, back in the 1860s.

Around the same time, Russian chemist Dmitri Mendeleev was making what would soon become the standard periodic table by categorizing the known elements by their chemical properties. He even left gaps in his table for elements he predicted would be discovered someday. But Mendeleev’s table didn’t include the group of elements we now call the noble gases, or even a gap for them, because no one had ever seen one.

Helium is one of these noble gases: elements that are incredibly unreactive. It’s a struggle to do any chemistry with them at all, making them hard to detect. It doesn’t help that Earth’s atmosphere is only about five parts per million helium, either.

But in space it’s different. If you could look at the universe as a whole, you would find that 75% of it is hydrogen and 25% is helium, and everything else is negligible. The sun’s composition is similar.

So how can you detect an unknown element that doesn’t react with anything and basically only exists in space in the 19th century? The answer lies in a technique called spectroscopy. If you put sunlight through a prism, you get a spectrum of light, with the visible part showing up as a rainbow.

In 1815, a German physicist named Joseph von Fraunhofer discovered something unexpected: the spectrum had holes in it! Fraunhofer had seen dark lines at very precise points in the spectrum that looked kind of like a bar code. These lines only appeared in sunlight, so they also acted like a bar code: you could distinguish sunlight from other types of light by looking at the spectrum.

Fraunhofer labeled these lines A, B, C, and so on. And 50 years later, two scientists: Gustav Kirchhoff and Robert Bunsen, made a revolutionary discovery about these lines using Bunsen’s new invention: the Bunsen burner. By burning different elements, Kirchhoff and Bunsen discovered that each one had a unique collection of dark lines: a unique spectrum.

They also worked out that this spectrum was due to elements absorbing light, but only at specific wavelengths. And what’s more, some of the elements’ lines matched the lines that came from sunlight. The sun’s spectrum was composed of the spectrums of other elements.

For instance, the two lines Fraunhofer labelled D1 and D2 were in the yellow region of the solar spectrum, and they also appeared in the spectrum of sodium. So Bunsen and Kirchhoff concluded that the D lines from the sunlight must have been caused by small amounts of sodium in the sun. And they were right.

Once they realized they could identify elements in the Sun using spectroscopy, other scientists got to work studying the solar spectrum, looking for more lines that Fraunhofer missed. There are lots of solar spectrum lines, but one line would soon stand out. In 1868, two researchers independently studied a solar eclipse.

The eclipse blocked light from the main part of the sun, allowing them to get a clear spectrum from the sun’s outermost layer, the corona. From this they both detected a line near the two well-known sodium D lines, called D3. One of these researchers later realized that the line wasn’t from sodium, or from any known element, and so he made the bold claim that it must have been from an unknown element.

He named it helium, after Helios, the Greek Sun god. He’d just discovered a new element without ever getting his hands on the stuff! For a while this discovery was controversial.

How could you detect an element without a sample? Besides, Mendeleev’s periodic table had no room for a new element like this. Some said the new line was just a hydrogen line that they’d previously missed.

Because helium is so rare and unreactive, it was hard to isolate a sample. Eventually, in 1895, a chemist at University College London isolated an element formed in the radioactive decay of uranium. This element had the distinctive D3 line, so he concluded it had to be helium.

He was actually looking for a different noble gas, argon, at the time, which he eventually found. After the discovery of helium and argon, Mendeleev was convinced to add the two noble gases to a new grouping on his periodic table. All these discoveries were made before scientists knew why spectrums worked this way.

The answer turns out to be our old friend, quantum mechanics. We now know that atoms can only absorb and emit particles of light, aka photons, if those photons are at certain specific wavelengths. The precise wavelengths are unique to each type of atom, so every atom has a different spectrum that can be used to identify it.

During the eclipse, researchers were seeing helium atoms in the Sun’s outer layer absorbing light emitted from the lower layers, and the absorption was happening only at distinct wavelengths. Today, we can use spectroscopy to learn about the composition of all kinds of things we wouldn’t know much about otherwise. In some ways, we have more information about the composition of distant galaxies than about the stuff in the core of our own planet.

Telescopes are also starting to be advanced enough for us to use spectroscopy to study the atmospheres of planets orbiting other stars. Maybe we’ve found all the natural elements, but we’ve barely scratched the surface of what we can learn with spectroscopy. Thanks for watching this episode of SciShow Space, which was brought to you by our patrons on Patreon!

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