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  • [♪ INTRO]

  • 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 barcode.

  • These lines only appeared in sunlight, so they also acted like a barcode:

  • 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!

  • For more awesome stories about the history of space research and exploration,

  • just go to youtube.com/scishowspace and subscribe.

  • [♪ OUTRO]

[♪ INTRO]

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B1 spectrum helium bunsen element sun mendeleev

The Impossible Element Hiding in the Sun

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    林宜悉 posted on 2020/03/30
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