The ABC of Spectroscopy. Part C

Ok, this is the 3rd part in the Spectroscopy overview. Hopefully you've read Part A and B so you already know that hot bodies emit radiation, and that the wavelength of the radiation relates to its colour.

To understand a spectrogram we need to know a little bit about Quantum Physics. It looks complicated but it's not.

As David said in his comment in the previous post: "As well as being a supremely useful way of gathering information from the entire visible universe, spectroscopy was directly involved in the formulation of quantum theory; which radically changed the way we understand everything."

When you were at school you were probably given the impression that atoms are like little solar systems, with electrons flying in orbit like planets around the nucleus. This is totally wrong in so many ways, in particular, electrons don't exist as tiny "balls" of stuff, but rather exist as fuzzy wave-like structures which don't have an exact point-like location in time and space, at least not until they are looked at. It's been calculated that if they were orbiting the centre of the atom in a classical sense they would lose all their energy and fall into the centre in far less than a billionth of a second. Instead it turns out that they exist (if they can be said to exist at all) in well defined "shells" surrounding the nucleus.

When they have more energy they move up to a higher shell but they always want to return to the lowest shell as soon as possible. Here's a funny thing: when they move from a lower shell into a higher shell, they don't move through the space in between; they instantaneously disappear from one place and appear in another.

The simplest element, and the most abundant one in the universe, is Hydrogen. Here's a model of the atom:

http://en.wikipedia.org/wiki/Hydrogen_spectral_series

We're interested in the Balmer series in the centre. These are the distances of the various "orbits" of the electrons in Hydrogen atoms.

A nanometer (nm) is one billionth of a metre, by the way. How big is that? Not very big. While you read the sentence "A nanometer (nm) is one billionth of a metre, by the way" your hair has grown several nanometers.

Here's the great thing; every element has its own orbital distances. In other words, if you find electrons at the distance from the nucleus of 656nm, 486nm, 434nm and 410nm, you know it's a Hydrogen atom. Suddenly the whole visible Universe becomes accessible. You know what everything is made of simply by seeing how far the electrons are from the centre of the atom.

So how can you measure orbital distances of electrons in atoms of stars countless billions of miles away? With spectroscopy!

All you do is:

Create a spectrum of the starlight:

Then calibrate it. In other words identify the wavelengths of the visible light.

Then look closely and you should see dark lines in the spectrum:

These lines are the characteristic lines of the 4 visible hydrogen lines.

http://en.wikipedia.org/wiki/Spectral_line

As soon as you see these lines you know you are looking at hydrogen, whether it's in a laboratory, or from light for a star millions of light years away. What causes the lines? This wasn't understood until the theory of Quantum Physics was developed less than 100 years ago.

The light created by the star moves up to the surface. Then it passes through the gaseous outer layers of the star. Most of the light passes through and comes to us as a continuous spectrum, the electrons in the atoms take the energy from the light at at the specific frequencies corresponding to the distances of the various shells mentioned above, and they use it to jump to higher levels. So each element in the gas will "absorb" certain frequencies of light, and by looking at these "missing frequencies" which show up as dark lines - called Fraunhofer lines, after the guy who first saw them - we can identify the element. Magic!