Continuing from the story of absorption spectra . . .
(Click on the orange word if you need a reminder!)
We left our poor optician friend Fraunhofer
dead in 1826, without ever having discovered why thin dark lines appeared in the sunlight and starlight he'd put through his spectroscope. We now fast forward to 1859, and I'd like to introduce you to two German chemists, Bunsen
|We'll start with an adorable chap named Robert Bunsen, who evidently liked nothing better than playing in his chemistry laboratory and getting covered in dodgy chemicals - he managed to blow up his surroundings and nearly kill himself a few times! He perfected the Bunsen burner, our favourite piece of school equipment. He seems to have been a wonderful teacher (according to one website, Mendeleev was one of his students) and a perfect gentleman, even in aggressive academic debates. He was exceptionally modest about his discoveries and never took out a patent, although doing that could have earned him vast sums. The wife of another academic is reported to have said of him: "First, I would like to wash him, and then I would like to kiss him because he is such a charming man."|
It was known that different substances burnt in the flame of a Bunsen burner burnt with different colours, as the following picture shows:
a mine of information
about Robert Bunsen
(Credit: Polish Wiki
|Bunsen was trying to find out the exact colours of light that different chemicals gave off when hot. He was attempting to do this through different coloured glass slides, when a colleague, Gustav Kirchhoff, popped into his laboratory. Kirchhoff was an impressive scientist, discovering laws of circuits (see right) and black-body radiation - and something very important along with Bunsen. He seems to have been a very friendly person. When Bunsen moved to the University of Heidelberg, he arranged for Kirchhoff to come and teach there too. Anyway, he had a better idea than coloured glass for finding exact spectral lines. He suggested they do what Newton and Fraunhofer did: refract the light with a prism.|
Kirchhoff also formulated the
When the light angled through the prism, they saw something like this:
(Obviously they couldn't test hydrogen or neon, but this is the best Google Images could do.)
The chemicals burnt with light of only a very few specific wavelengths. After many experiments, the two chemists were able to confirm that each element burnt with its own unique bar code of colour. That made it possible to test any substance for any element by burning it, which had a huge impact on chemistry! The two of them discovered rubidium and caesium, two alkali metals
in the same group as sodium, in this way. Their names come from the colours of their spectral lines.
|In a sodium flame, Kirchhoff went on to notice 2 bright yellow lines in precisely the place where Fraunhofer had noted two dark lines in sunlight. He decided to see what would happen if he made both of them at once. He passed a beam of sunlight through a sodium flame. He expected to get a continuous spectrum, as the yellow lines superimposed themselves upon the absorption spectrum. They didn't. The black lines in the absorption spectrum got darker.|
This could only mean that the sodium in the flame was absorbing sunlight. Kirchhoff worked out that the source of sunlight was hotter than the sodium in the flame. He tested this by creating an extremely hot flame to represent sunlight, and passing its light through a cooler sodium flame. He got a sodium absorption spectrum, just like Fraunhofer's with sunlight!
Sodium absorption and emission lines.
At last, the Fraunhofer lines were understood. They represented the atoms that were present in the Sun. Nobody had been sure what the Sun was made of until Fraunhofer, Bunsen and Kirchhoff made these discoveries. (For many centuries it had been assumed that the Sun was made of iron.) When Niels Bohr
described the structure of the atom
, and showed that it was electrons jumping from orbit to orbit when kicked by photons of very specific energies (i.e. light of a very specific wavelength), it became clear what was happening.The galaxy of two weeks ago
had an absorption spectrum
. Recently, Tom started a thread
on galaxies which have a strong emission spectrum. Today's Object of the Day (whew, I hear you say - she's finally got round to the galaxy
) is not in it, but it is also a galaxy with strong emission lines:587738568167981186
and IC 3476, containing supernova 1970A according to NED.
Posted by LankyYankee, Waveney, Megalodon99, and Cicada.
Quite a different emission spectrum. The high peaks indicate specific wavelengths of light being given off by a few elements:
These huge peaks are single wavelengths being emitted in great quantities by a few very, very hot elements or molecules. When there's a dip, they're cooler atoms or molecules (i.e. in a cooler place, such as not
in a massive blue star) absorbing energetic light from their surroundings.
I must confess, I really don't agree with SDSS that it's a star.
Thanks again to Geoff for lending me Marcus Chown
's The Magic Furnace
, which gave me the idea for these two Objects of the Day. It also states that we began to understand atoms through our discoveries about stars, and we began to understand stars through our discoveries about atoms.