Seeing the stars with spectroscopy
No one has ever been to the Sun, so how do we know that it's over 90% hydrogen gas with some helium, oxygen, nitrogen and a few other elements? The answer lies in the light that the Sun sends out. Speaking to Ginny Smith, Cambridge historian Josh Nall demonstrated a device that early scientists used to study this light: the spectroscope.
Josh - It's actually a spectroscope from the mid-20th century which hopefully, we can get someone up here to have a go with.
Ginny - What exactly is a spectroscope?
Josh - It's an instrument for looking at the spectrum of light. As we've known for a long time at least, since the time that Isaac Newton was working in this town, when you shine light through a prism, the prism refracts that light which is to say it splits it up into its constituent spectrum.
Ginny - So, this is why you get rainbows because the water droplets split up white light into all the different colours and we call that a spectrum.
Josh - Precisely.
Ginny - How can this help us find out what something like the sun is made of?
Josh - I think the easiest way to explain it here to our audience is, to begin with actually, our most modern instrument that we have here, a modern computer spectroscope.
Ginny - So, we've got a screen here with a kind of graph at the bottom and a rainbow at the top. It's connected to a long tube which has a thing in the end which I guess is the spectroscope. So, if you point this at different things, will we see different kinds of rainbow on the screen?
Josh - Indeed. So, what we've got here on the graph at the bottom is the wavelength of the light that is coming into the end of this tube and then we have intensity here. We can actually visualise at the top here the colours. So, if we shine white light we start to see pretty much contributions coming into the spectroscope from every part of the spectrum. This is what Isaac Newton first proposed was that actually, white light was composed of these more fundamental colours. We're now looking in the mid-19th century into scientists who start to want to investigate whether by looking at the nature of light, one can actually determine what substance it is that's giving off that light.
Ginny - But they didn't have a nice computer screen and a nifty little gadget like this. So, what did they have to do?
Josh - Crucial to the studies were two German scientists - Robert Bunsen of the Bunsen burner fame and Gustav Kirchhoff. One of their crucial insights was that they began to look at the spectrum that individual elements gave off. For some time, people had been aware that when you looked at the light through a prism that came off of substances, you started to get certain colours very prominently appearing in bright bands and certain colours not appearing at all.
Ginny - So, the same reason why if you've done that school chemistry experiment where you put different elements in a flame, they burn different colours, they also give off different coloured lights and you can pick that up with a spectroscope.
Josh - Absolutely. And so, they started to think, well maybe then we can actually use the colour of light to determine what different elements are giving off that light. So, if we take a specimen, a modern specimen that we have here as an example, this is a tube of neon. As many people will be aware when you pass a high electric voltage across a tube of neon, it will glow a kind of characteristic colour.
Ginny - Does anyone want to come and give us a hand? What's your name?
Minnie - Minnie.
Ginny - You can see the neon here and it's shining a beautiful pink colour. So, what we need you to do is just point the spectroscope at it and tell us what you can see on the screen.
Minnie - Well, it's red colours and yellow and orange.
Ginny - Exactly. Is that what you'd expect?
Josh - Yeah. So, you're starting to see very specific peaks at certain specific points in the spectrum. So, what I want to do here with our volunteer is see if we can recreate this with natural historical objects. So for this, we're going to have to get you to put on some gloves.
Ginny - So, this is kind of about the size of a lipstick. You could quite easily slip it in your pocket and you can look through one end and at the other end, is there still a prism in it?
Josh - There's actually a series, a chain of prisms throughout the tube which are positioned against each other so that the light passing through will be split into its spectrum by the time that it reaches your eye. You want to hold it and look down there.
Minnie - It's a rainbow.
Josh - Can you see any colours in particular, any prominent colours there?
Minnie - There's still this red, orange, yellow, lots of green, blue and purple.
Josh - What happens as you said there is you get many multiple bands. But Gustav and Kirchhoff noticed was that certain bands were particularly prominent. These, they could use as a kind of fingerprint for a given element. The next crucial discovery was to think about sunlight because people had noticed for some time when they were working that the spectrum that you get from sunlight has this continuous rainbow effect. But then it has this thick black bands across the spectrum and nobody really knew what these black bands were. And Gustav and Kirchhoff realised that you could map up the fingerprints of individual elements to these black bands in the spectrum of the sun.
Ginny - So, rather than seeing those bands standing out, they'd actually being kind of taken away.
Josh - Yes. So, this is what's called an absorption spectrum. What happens when sunlight passes out from the centre of the glowing sun, it actually has an outer atmosphere around it. as the light passes through that atmosphere, the elements in that atmosphere absorb at bands that are specific to that particular element.
Ginny - So, you can tell what that light has passed through on its way from its source to you.
Josh - Absolutely. The crucial discovery here is for the first time, you could actually chemically analyse the sun and you could determine what elements there are present in the sun.
Ginny - How did this change the way people thought about science or about the universe?
Josh - Well, I think it had a really profound impact on people's outlook on the nature of the universe because this discovery was made around 1859. And not that long before, in the 1840s, you've got people like August Comte writing a book in which he claims that it will be impossible for man to ever know what the sun is made. It would be pointless even to try studying because it's so far away. Ten years later, you've got William Whewell here in Cambridge, gets into a heated debate with other philosophers about whether or not there's life out there in the universe. And Whewell points out that there can't be possibly be life out in the rest of the universe because we don't even know whether the universe is made from the same stuff as we find on Earth. But then in the wake of these discoveries of the spectroscope, you've got proof that matter out in the universe is composed of all of the same fundamental elements as the materials that we find on Earth. Not just that the sun contains elements that we're familiar with, but crucially, that you can start to find substances like iron and water on planets like Mars which were discoveries that were soon made. That starts to make people go the other way to say, "Well, hold on. the universe might be full of planets just like the Earth." The other use of the spectroscope which we haven't mentioned is that chemists were also using them to actually discover new elements. So, a lot of the work that the spectroscope did in the hundred or so years that followed Bunsen and Kirchhoff's work was actually to discover new elements. Because of course, if you find a signature which you can't map to an element you already have, it's quite possible that you have new elements. So for example in 1868, Norman Lockyer discovers helium in the corona of the Sun.