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 werediscovered 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 spacebefore 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.
夫琅和费给这些黑线标上了A B C 以此类推
Fraunhofer labeled these lines A, B, C, and so on.
And 50 years later, two scientists:
古斯塔夫· 基尔霍夫和罗伯特· 本生
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 workstudying 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 researchersindependently 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 chemistat University College London
isolated an element formed in theradioactive 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 happeningonly 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.
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