# eli5. How did humans first determine what outer space is made of?

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eli5. How did humans first determine what outer space is made of?

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Space is not “made of” anything. Are you asking when did we first know that space is a vacuum?

The answer is “spectrography” – now, let me explain that:

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Suppose you take a perfectly clear tube, and fill it with a pure gas – say neon. Then you run electricity through it. If you did it right, the gas will glow – that’s what a neon light is. If you then put this light through a prism, something interesting happens: you don’t get a full rainbow. Instead, you get a few specific lines of light at specific colors – Neon gives mostly red light, but also a yellow line, a green line, and two blue lines.

If you use a different gas, you get different lines of light. Helium has one red band, one yellow band, a couple where green turns into blue, three blue bands, and 5 purple bands. Hydrogen has one red, one light blue, one dark blue, and 4 purple bands. Every gas has a different set of these lines of light. Which means that if I have an unknown gas in a tube, and put electricity through it, I can tell what the gas is by seeing what colors of light show up when I put that light through a prism.

It turns out, it also kinda works in reverse: if I take that same clear tube, and shine light through it, the gas blocks the exact same colors of light that it would have lit up with when I put electricity through it. I’m going to skip the explanation of why this is, but it has to to with atomic physics – with how electrons behave in an atom and in a molecule.

It doesn’t block ALL the light though – just a little. You can see this with water: just a little water looks clear, but as you get more of it, it looks blue. That’s because water doesn’t block blue light, so that’s what gets through. Gasses block less light than water, but it’s the same idea. Which means I can also tell how dense the gas is in the tube by seeing how much light gets blocked.

That’s spetrography.

Stars give out “white” light – light in every color. But we don’t see that light directly – we see it through space. It’s pretty easy to figure out what light a star is giving out – there’s math for that. So when we look at a star, and look at all the specific colors we see, if there’s any difference between what the math says the star is giving out, and what we see, then we know something is blocking those colors of light. Which means there’s some gasses between us and the star.

At that point, it’s just a matter of figuring out what combinations of gasses and how much of each would block out the exact amount of light that we see is being blocked.