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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.
至少80年前，我们就了解了所有自然元素，从我们熟知的铁和碳，到元素周期表中的最后一个元素钫。
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:
这些元素中的大多数都是通过巧妙的化学方法发现的，但宇宙里第二多的元素也有着十分独特的渊源：
Helium was discovered in space before it was found on Earth.
它是先在宇宙里发现的，然后才在地球发现的。
And it took nearly three decades for scientists to accept that it could actually exist.
科学家花了近30年的时间才接受氦的存在。
The now-famous balloon filler and squeaky-voice-maker was first discovered in the atmosphere of the sun, back in the 1860s.
现在众所周知氦可以填充气球，可以发出吱吱的响声。人们当初首次发现氦是在19世纪60年代太阳的大气层中。
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.
大概那个时候，俄罗斯化学家德米特里·门捷列夫正在制作标准元素周期表的雏形版，它制作的方法是根据元素的化学特性来对其进行归类。
He even left gaps in his table for elements he predicted would be discovered someday.
他甚至还在表中留下了空位，给那些他认为有朝一日会发现的元素。
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.
门捷列夫的元素周期表并不包括一组元素，如今我们称之为惰性气体。门捷列夫甚至没有给惰性气体留出空位，因为没有人见过惰性气体。
Helium is one of these noble gases: elements that are incredibly unreactive.
氦就是一种惰性气体，也就是不与其他物质起反应的元素。
It's a struggle to do any chemistry with them at all, making them hard to detect.
要跟惰性气体起反应可要费一番工夫，这就使得惰性气体很难为人所发现。
It doesn't help that Earth's atmosphere is only about five parts per million helium, either.
虽然地球大气层中含有氦气，但含量微乎其微。
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.
但在太空中就不一样了：如果将宇宙视作一个整体，那么你会发现宇宙的75%是氢，25%是氦，其他都可以忽略。
The sun's composition is similar.
太阳的组成与宇宙类似。
So how can you detect an unknown element that doesn't react with anything and basically only exists in space in the 19th century?
所以，我们要怎样探测不与任何物质反应的未知元素呢？毕竟它存在于19世纪的太空啊。
The answer lies in a technique called spectroscopy.
答案取决于一种名为光谱学的技术。
If you put sunlight through a prism, you get a spectrum of light, with the visible part showing up as a rainbow.
将日光扫过棱镜的时候，会得到一个光谱的光线，其可见部分是一条彩虹。
In 1815, a German physicist named Joseph von Fraunhofer discovered something unexpected: the spectrum had holes in it!
1815年，德国一位名为约瑟夫·夫琅和费的物理学家发现了一件意想不到的事情：光谱里面有洞！
Fraunhofer had seen dark lines at very precise points in the spectrum that looked kind of like a barcode.
夫琅和费在光谱的特定位置可以看到暗色的线条，看起来很像是条形码。
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.
这些只会出现在日光中，所以它们看起来很像条形码：只需要查验光谱，就能看出日光和其他光的区别。
Fraunhofer labeled these lines A, B, C, and so on.
夫琅和费给这些线条标记了A、B、C等符号。
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.
50年后，2位科学家，即古斯塔夫·基尔霍夫和罗伯特·本生，对这些线条做出了里程碑式的发现，他们用了本生的新发明——本生灯。
By burning different elements, Kirchhoff and Bunsen discovered that each one had a unique collection of dark lines: a unique spectrum.
通过燃烧不同的元素，基尔霍夫和本生发现，每一个都有独特的暗色线条集合，也就是独特的光谱。
They also worked out that this spectrum was due to elements absorbing light, but only at specific wavelengths.
他们还发现，这个光谱之所以形成是因为有元素可以吸收光线，但只吸收特定波长的光线。
And what's more, some of the elements' lines matched the lines that came from sunlight.
更重要的是，一些元素的线条与来自日光的光线吻合。
The sun's spectrum was composed of the spectrums of other elements.
太阳的光谱由其他元素的光谱组成。
For instance, the two lines Fraunhofer labeled D1 and D2 were in the yellow region of the solar spectrum, and they also appeared in the spectrum of sodium.
比如，夫琅和费标注的两条线叫D1和D2，在太阳光谱的黄色区域。也会出现在钠的光谱中。
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.
所以本生和基尔霍夫得出结论：来自日光的线条D一定是由太阳中的少量钠放出的。而他们的想法确实是正确的。
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.
一旦他们发现可以通过光谱学来识别太阳中的元素，其他科学家就开始研究太阳光谱，寻找夫琅和费错过的其他线
There are lots of solar spectrum lines, but one line would soon stand out.
有很多太阳光谱线，但有一条线很突出。
In 1868, two researchers independently studied a solar eclipse.
1868年，2名科学家分别研究了日食现象。
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.
日食会阻碍太阳发出的主要光线，这样他们就可以获得太阳外层——日冕的清晰光谱。
From this they both detected a line near the two well-known sodium D lines, called D3.
通过这种方法，他们2人都在2条知名的钠D线——D3线上检测到了一条线。
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.
这些科学家中，有一人随后发现这条线并不是钠发出的，也不是来自其他任何一种已知元素。所以，他大胆宣称这种元素一定来自一种未知元素。
He named it helium, after Helios, the Greek Sun god.
他将这种元素称之为氦，这是以希腊太阳神赫利俄斯的名字命名的。
He'd just discovered a new element without ever getting his hands on the stuff!
他就是这样连碰都没碰到就发现了一种新元素。
For a while this discovery was controversial.
有那么一段时间里，这个发现引发了巨大争议。
How could you detect an element without a sample?
在没有样本的情况下，怎么可能检测到元素呢？
Besides, Mendeleev's periodic table had no room for a new element like this.
而且，门捷列夫的元素周期表也没有为这样一种新元素预留位置。
Some said the new line was just a hydrogen line that they'd previously missed.
一些人认为，这条新的线只是之前没有发现的一条氢线。
Because helium is so rare and unreactive, it was hard to isolate a sample.
因为氦十分稀少，又不是很活跃，所以很难隔离出样本。
Eventually, in 1895, a chemist at University College London isolated an element formed in the radioactive decay of uranium.
最后，在1895年的时候，英国伦敦大学学院的一名化学家在铀放射性衰变的过程中隔离出了一种元素。
This element had the distinctive D3 line, so he concluded it had to be helium.
这种元素有与众不同的D3线，所以他下结论称这种元素一定是氦元素。
He was actually looking for a different noble gas, argon, at the time, which he eventually found.
他当时其实在寻找一种惰性气体——氩，最终也得偿所愿。
After the discovery of helium and argon, Mendeleev was convinced to add the two noble gases to a new grouping on his periodic table.
在发现了氦和氩之后，门捷列夫终于被说服，将这两种惰性气体加到了元素周期表的新组别里。
All these discoveries were made before scientists knew why spectrums worked this way.
发现这些的时候，科学家还不知道光谱是如此运作的。
The answer turns out to be our old friend, quantum mechanics.
而它的答案却是我们所熟知的——量子力学。
We now know that atoms can only absorb and emit particles of light, aka photons, if those photons are at certain specific wavelengths.
我们现在知道，原子只会吸收并释放光粒子，即光子，但只会是特定波长的光子。
The precise wavelengths are unique to each type of atom, so every atom has a different spectrum that can be used to identify it.
每类原子的波长都是独特的，所以每个原子都有不同的光谱，可以据此进行识别。
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.
日食期间，科学家发现太阳外层的氦原子会吸收内层发出的光，而且只有特定波长的光才会被吸收。
Today, we can use spectroscopy to learn about the composition of all kinds of things we wouldn't know much about otherwise.
如今，我们可以用光谱学来了解所有事物的组成，从而对宇宙了解更多。
In some ways, we have more information about the composition of distant galaxies than about the stuff in the core of our own planet.
从某些角度来看，我们对遥远星系组成的了解多过度地球内核的了解。
Telescopes are also starting to be advanced enough for us to use spectroscopy to study the atmospheres of planets orbiting other stars.
如今十分先进的望远镜也让我们能够凭借光谱学来研究环绕其他恒星运行的行星大气层。
Maybe we've found all the natural elements, but we've barely scratched the surface of what we can learn with spectroscopy.
或许我们已经发现了所有自然元素，但对于光谱学，我们可能只了解了冰山一角。
Thanks for watching this episode of SciShow Space, which was brought to you by our patrons on Patreon!
感谢收看本期的《太空科学秀》，这离不开粉丝通过Patreon对我们的支持！
For more awesome stories about the history of space research and exploration, just go to youtube.com/scishowspace and subscribe.
要了解更多有关太空研究与探索的历史，欢迎订阅youtube.com/scishowspace。
