Twinkle, twinkle little star

by Nicholas Mee on October 8, 2012

Twinkle, twinkle little star
How I wonder what you are!

A few of the stars at the heart of the globular cluster Omega Centauri. (copyright NASA)

In 1857, a whole new world of science was opened up by a sensational discovery made by two German scientists, Robert Bunsen and Gustav Kirchhoff. Bunsen excitedly announced to a colleague:

“At present Kirchhoff and I are engaged in a common work which doesn’t let us sleep … Kirchhoff has made a wonderful, entirely unexpected discovery in finding the cause of the dark lines in the solar spectrum … Thus a means has been found to determine the composition of the sun and fixed stars with the same accuracy as we determine sulphuric acid, chlorine, etc., with our chemical reagents. Substances on the earth can be determined by this method just as easily as on the sun, so that, for example, I have been able to detect lithium in twenty grams of sea water.”

Chemical Fingerprint

Bunsen and Kirchhoff had discovered that each chemical element has a unique fingerprint that would allow them to track down its presence anywhere in the universe. If an element is heated in a flame until it glows, the light it emits has a characteristic colour. And when that light is passed through a prism, the resulting spectrum appears as a number of bright lines corresponding to light at a series of sharply defined wavelengths. For instance, sodium will burn with an intense yellow glow. Many street lamps contain sodium vapour and this is why they shine with a yellow hue. The light that sodium emits, when passed through a prism, splits into a pair of sharp lines in the yellow region of the spectrum. Bunsen and Kirchhoff took each element in turn and heated it in a flame until it glowed. They then passed this light through a prism and recorded its unique fingerprint.

A schematic solar emission spectrum and absorption spectra of four elements. From top to bottom: emission spectrum of the Sun, and absorption spectra of sodium, hydrogen, lithium and mercury. The dark lines in the solar spectrum correspond to the bright lines in the spectra of sodium and hydrogen, but not lithium or mercury, demonstrating that the Sun’s solar atmosphere contains sodium and hydrogen, but not lithium or mercury.

Cosmic Barcode

If an element is present in the atmosphere of a star, it will absorb the light streaming out from its  core at exactly the same wavelengths. The star’s light is thus depleted at these wavelengths and so they appear as dark lines in the star’s spectrum. So the dark lines in the star’s spectrum caused by the presence of a particular element are at exactly the same wavelengths as the bright lines that form that element’s spectrum in the laboratory.  A star’s spectrum is like a cosmic barcode containing precise information about the identity of each element that is present in the star’s atmosphere. By matching the dark lines in the spectra of the Sun and the stars to the lines they were observing in the laboratory, Bunsen and Kirchhoff could recognise the elements that were present in a star. These telltale signs of each element are clear even at a distance of trillions of kilometres.

The illustration on the right gives some examples to show how spectroscopy could be used to determine which elements are present in the Sun.


How amazing it must have been to be the first to determine the composition of the stars?

In my book Higgs Force I also explain how the stars generate their huge energy output and shine so brilliantly.


Incidentally, talking of colour, the Kindle edition of Higgs Force now contains colour illustrations throughout which can be viewed on the Kindle Fire.

Further Information
If you fancy trying out a bit of your own spectroscopy, take a look at the following website:


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