|Once upon a time, in the German town of Bavaria, a 14-year-old boy named Joseph Fraunhofer was working as an apprentice glassmaker. It was 1801. He had been orphaned 3 years before, and expected to spend his life working very hard making spectacles. Suddenly the roof of his workshop collapsed, burying him in the rubble.|
The local Prince Elector, who later became Maximilian I of Bavaria, led the rescue effort and was delighted to find the boy alive. He gave young Joseph some books and money and ordered his harsh employer, Philipp Anton Weichelsberger, to give him time off work to study.
Some 13 years later, after much study and during a very successful glassmaking and lens crafting career, Fraunhofer decided to study light in more detail. Hans Lippershey had invented the telescope two hundred years previously, and Galileo had made it famous. But telescopes had a problem. When you shine light through a prism, it splits into rainbow colours, and that is what was happening in magnifying lenses too. Isaac Newton had tried to find out a way to get around this. Ironically, he succeeded, but didn't realise it! He had passed light through a prism and showed that blue light bent more than red - and that if you pass the spectrum through another, inverted, prism, it will revert back into white light again. An optician named John Dolland had hoped to use this technique to rescue lenses, and Fraunhofer wanted to perfect this.
Lenses, however, are curved - less straightforward than prisms. Fraunhofer was going to have to do some serious testing, to make sure each colour was "bent" by exactly the right amount. To do this, he was going to have to examine the refractive index - for his purpose, the light-bending power of different types of glass. Since each type of glass bent light to a different extent, Fraunhofer needed a source of light that was always a single colour. He chose sunlight.
Galileo's telescope. Credit: http://www.edinformatics.com/math_science/solar_system/solar_system.htm
Sunlight, of course, splits into a huge range of colour:
So Fraunhofer invented the spectroscope
, to pass it through a very thin slit before entering a prism. He then looked for some feature which he could use as a colour standard.
To his surprise, what he saw was this:
The whole spectrum was crossed with fine black lines. It seemed that tiny but particular shades of rainbow colours were missing. These lines had been noticed before by a metallurgist named William Wallaston, but Wallaston hadn't thought they were anything more important than the borders between the individual colours. Fraunhofer's sighting clearly demonstrated that this was not the case. He measured their precise positions and labelled the clearest ones with letters of the alphabet - they were always in exacatly the same place when he came back. Over the years he discovered 574 dark lines, which are now called Fraunhofer lines
It was known by the early 19th century that light is made of waves and, therefore, the dark lines meant a missing wavelength. Fraunhofer set about trying to discover what that might mean. One thing he did was apply his spectroscope to starlight through a telescope. Starlight also produced dark lines. Some were the same as in sunlight. Others were not.
Fraunhofer never found out what caused the dark lines. Like many glassmakers of his day, who were poisoned by heavy metal vapours, he died very young, aged only 39. But today we do know. That story of how we found out will come next time. But what is happening is that those particular wavelengths of light are being absorbed by atoms.
An electron can be thought of orbiting the nucleus of an atom (this is of course an oversimplification, but it'll work for this). As each planet has an orbit, so do electrons. But being struck by a photon of light - at exactly the right energy, i.e. exactly the right wavelength - can temporarily boot them up into a "higher" one. This absorbs the photon of light. And that wavelength will therefore be missing from the spectrum.
A "spectrum" not only means a rainbow, but a graph showing how much light is and isn't present at each wavelength. Here is an example. Each "dip" means that there is less of that particular wavelength because it is being absorbed. What kind of elements
do you think are absorbing light here?
Yup . . . hydrogen, sodium, magnesium and calcium. (H, Na, Mg and Ca.) The "G" region is one of Fraunhofer's own labels, and it in fact shows the presence of the molecule CN.
That means that in a beautiful ringed spiral galaxy known as UGC 6968, nominated by Romulous, there are the elements hydrogen, sodium, magnesium and calcium in high enough quantities to absorb a lot of light, and also the CN molecule.587741603103768670
For those who I haven't quite exhausted enough with chemistry and astrophysics yet, the presence of CN means the stars must be relatively cool, as such a molecule would be broken up in hotter stars. That probably means the spectrum comes from the middle of the galaxy, not the starforming trailing arms. The reddish colour of the inner galaxy is another indicator of a relatively low temperature.
The H is in fact H-gamma. NGC3314 tells me:
H-gamma is the third line in the visible-light series of hydrogen lines, all of which end up in the n=2 state (first excited state) of the atom. So H-gamma is the transition between electron energy states n=2 and n=5, and show up most strongly in stars with temperatures hot enough for atomic collisions to put lost of hydrogen atoms in n=5 (stars broadly like Vega).
That means that the electron (hydrogen, H, only has one) has already been booted up one orbit - and has gone up another three!
For those who wish to know more about Fraunhofer lines, NGC3314 also says:
H and K are the violet calcium lines near 3933/3968 A, D is the pair of sodium lines at 5890/5896, C is H-alpha, A and B come from oxygen in our own atmosphere and have been calibrated out in the SDSS spectra.
Many thanks NGC3314 for checking and adding to my chemistry, to Geoff Roynon for lending me the marvellous book "The Magic Furnace"
which gave me the idea for this Object of the Day, to you lovely people for putting up with rather a long one, and to Romulous for an Object of the Day nomination which came along at rather a good time! Next time - the second half of the story
Update: Next half of the story is here