However far away an object is, if we can gather enough light from it, we can find out what it is made of. In its spectrum are the fingerprints of the atoms that emitted or absorbed the waves. The unlocking of this secret of the Universe was stated by the great English scientist William Wollaston. We are particularly proud of Wollaston because he was born in our town, Dereham. In 1802 Wollaston tried to measure the refractive properties of different transparent materials. He noticed a number of dark lines in the spectrum of the Sun. He wrongly decided that they were boundaries between the colours that Newton had identified in the spectrum. Perhaps if he had taken more time to experiment he would have identified their true nature and had the lines named after him, but instead Wollaston turned his attention to other things.
William Wollaston
Fingerprints of light in the spectrum of a fluorescent lamp reflected in a CD
Joseph Fraunhofer
IJoseph Von Fraunhofer was a master optician who made telescopes (he invented the German equatorial mount). He needed to accurately test his lenses but he had problems with the light splitting into spectra. So in1814 he tried making a pure coloured light with different sorts of lamps. He magnified their spectra and was surprised to find that whatever the lamp burnt, he always got two bright yellow lines very close together standing out from the ribbon of rainbow colours.
The mystery pair of yellow lines seen in the spectrum from a candle
Fraunhofer then used his instrument to look at the spectrum of sunlight to see if he could see the two yellow lines there. What he discovered in his excellent optics were not the few lines that Wollaston has seen but hundreds of them throughout the rainbow of colours. Where he had expected to see the two bright yellow lines he saw the opposite - two dark black lines. The yellow lines seemed to correspond exactly to each other, enhanced in burners and missing in the Sun. He never could work out why but because he correctly concluded that the lines were missing parts of the spectra, rather than where colours met as Wollaston had thought. These lines are known as Fraunhofer lines.
In 1857 two friends in Germany, Robert Bunsen and Gustav Kirchhoff, made a spectrometer out of two old telescopes a prism and a cigar box.
Kirchhoff (left) and Bunsen (right)
Bunsen was a chemist and had been looking at the colours of different chemicals in a flame. He had even invented a special gas burner that made a colourless flame. It was such a good design that nearly 250 years later we still use Bunsen's Burner. Kirchhoff was a physicist so he set up the spectroscope and got Bunsen to put some salt (sodium chloride) into the flame of his burner as Gustav peered at it through one of the telescope eyepieces. So what did the spectrum look like? There wasn't one; just two bright yellow lines next to each other. They tried some other salts. Potassium gave a lilac coloured flame but in their spectroscope there was a violet line and a red one. Each metal gave its own special mixture of lines. They even developed a game where Bunsen would make a mixture of different salts and Kirchhoff would work out what they were. They got so good at it that they could analyse unknown materials. For example they found the element lithium, an element that people then thought was rare, in the ash of their cigars!
Kirchhoff's spectrometer and Bunsen's burner
In 1860 they discovered a new element using their technique. Because of its characteristic colour in a flame they called it after the Latin for grey/blue caesium In 1861 they found another and because of its red flame called it using the Latin for red - rubidium.
Bunsen, being a chemist, was happy to keep on looking for new elements but Kirchhoff wanted to solve physics problems. Kirchhoff had noticed that the twin yellow lines of sodium showed up in all but the purest samples of other substance. He puzzled over the connection between the bright lines he and Bunsen had discovered and the dark lines Faunhofer had discovered in the Sun's spectrum.
Kirchhoff needed a source of a continuous spectrum, so he set up a piece of kit called a Drummond light. This uses a hydrogen/oxygen burner to heat a stick of lime and makes a spectrum without enhanced or missing lines. He passed this light through a bunsen flame coloured with sodium and looked at it using his spectroscope. Kirchhoff expected to see an effect like looking at a candle, with the two yellow line sodium enhanced. What he discovered were the yellow lines missing! The flame was absorbing much more yellow light than it was making. Kirchhoff was very excited, he had made artificial Fraunhofer lines! Suddenly it all made sense. The elements could both emit and absorb their characteristic lines. Elements left their fingerprints in light just like a criminal at the scene of a crime. The mystery of Fraunhofer lines was solved and Kirchhoff had become the first person to find an extraterrestrial chemical - he had discovered sodium in the Sun.
Kirchhoff told Bunsen of his discovery. Bunsen warned that it could be a coincidence. After all, there were hundreds of Fraunhofer lines, there was a possibility that sodium matched up by chance. He helped Kirchhoff produce the spectral lines of iron. They counted 60 distinctive bright lines. When they checked them against the Fraunhofer lines in the Sun they found everyone in precisely the right position. If ever their was 'cast iron' evidence this was it! They followed this discovery with the identification of nearly 30 more elements in the Sun, including Hydrogen which we now know makes up about 75% of the Sun's mass. Bunsen wrote about there work - "At present Kirchhoff and I are engaged in an investigation that doesn't let us sleep. Kirchhoff has has made a wonderful and unexpected discovery in finding the cause of the dark lines in the solar spectrum"
There was a Solar eclipse in 1868 and the French astronomer Janssen and the English astronomer N. Lockyer, took the opportunity to look at the spectra of the Chromosphere and corona of the Sun. They discovered a mystery emission line that had never been seen before because they had been swamped by the intensity of the Photosphere. Lockyer and Janssen knew that they had discovered a new element as it did not match any of the known patterns. They named the element after the Greek god of the Sun - Helios. It was 17 years before Helium was found on Earth in an ore of Uranium by William Ramsay. We now know that it is the second most abundant element in the Universe and makes up nearly 25% 0f the mass of the Sun.
Since those times, the absorption and emission lines in the spectra of all bands in the EM spectrum have been used to gain information about the chemical composition of every part of space. The fact that the lines can be Doppler shifted by movements relative to the observer have also been invaluable in working out the dynamics of the Universe including its origins.
Other highlights of spectral study discoveries are:
1904 Johannes Franz Hartmann spectral absorption lines from interstellar gas.
1912 Vesto Melton Slipher uses shifted spectral lines to show that most galaxies are rushing away from us (recession).
1929 E. P. Hubble links the increase in recession velocities of galaxies to their distance from us indicating an expanding Universe which could be extrapolated back to an origin.
1963 Hydroxyl group found by radio engineer team at Lincoln Laboratory MIT
1968 Water, Ammonia found.
1969 The first organic molecule -Formaldehyde (H2CO) discovered
1970 Methanol CH3OH discovered
1982 Cyano-decapenta-yne HC11N found.
2000 Glycolaldehyde (a sugar) found in a giant cloud in Sagittarius.
2003 Over 130 different compounds have now been found in interstellar space. There is some evidence that glycine, the simplest amino acid, has been found. There is much research going on trying to reproduce the conditions of interstellar space both with real chemicals and in computer programs. A few scientists theorise that interstellar chemistry could be rich enough to be the origin of life.