Spectroscopy: The Starry Messenger, Vol. 1 issue 1

-- Fist Quarter, November 2003 --

FRONT PAGE

Volume 1, Issue 1

The Science of Spectroscopy

By Stanislav Kostarnov
What's a spectrum and how do we use it to tell the composition of distant celestial objects?

Determining the chemical composition of stars is a very complex and difficult task for astronomers, as you cannot simply take a sample of the star's makeup due to them being such an enormous distance away. Just a hundred and fifty years ago such a task would have been considered totally impossible, as it is to stop time in our age. However with what we now know about the behavior of light and emission spectrums we live in an age were the composition of any object in the sky can be measured through a brand new science of spectra analysis which is armed with the laws of Kirchhoff and Bunsen.

To learn more about spectral analysis we must first consider what is a spectrum. The word for spectrum comes out of the word "spectra" meaning phantom or apparition. It is an array of colors set up in accordance with their frequencies usually either in a line or a circle, not all of the spectra is visible to the human eye but rather most of it is invisible. For a perfectly blackbody the spectrum is continuous whilst for anything else it's either a broken continuum or a line spectra.

This is important because most bright astronomical objects shine because they are hot. In such a case, the continuum they emit tells us what the temperature is. It can also tell us what the celestial object is made of, and other important information such as their distance and sometimes its age.

One day in the 1850s persons by the names of Gustavo Kirchhoff and Robert Bunsen, who is most famous for the invention of a Bunsen burner, were playing around with their new gadget the Bunsen burner when they noticed that if you view the light formed by a heated element through a slit and through a prism you get a unique set of wavelength of color. To this day we know all the spectra of each element on the periodic table up to uranium and a few others that are even less stable. We also know also three laws that Kirchhoff, Bunsen had found which are now the basis of spectral analysis, these laws are:

Law 1 any object, which is hot and dense, is called a black body and will produce a continuous unbroken spectrum or rainbow
Law 2 a hot transparent gas produces an emission line spectra- a series of bright lines on a dark background.
Law 3 a cool dark gas produces dark lines on a continuous spectrum (rainbow if broken with a prism). More then that, those dark lines of the absorption spectrum, as this set of dark lines is called, take up exactly the same wavelength (spaces) as of the corresponding gasses emission spectrum

They then viewed the prism spectrum with an eye glass and found out that it was not continuous but instead had multiple holes which partially corresponded with a continuous spectra with the missing line spectra of some of the elements he had tested in the Bunsen burner experiments. They then realized that these lines were a unique set of wavelength for each element. They rationalized it via a model by which each electron of an atom could only jump a certain quantum, this quantum being the amount it needed to jump from one orbit to another. they called these continuums of wavlengths emission spectra.

Armed with Kirchhoff's and Bunsen's laws a new and flourishing science of spectral analysis was born. For the first time in human history scientists were able to tell what the composition of the celestial objects was. It is however a difficult and tedious job, which can often produce errors due to the inaccuracy of measurements. The spectral analysis of stars is strongly reliant on assumptions that certain lines are too faint to be visible or that others are blended together because of very small differences in wavelength. The need for multiple tests and reconfirmations of any information make the progress slow but it is the only way we can find out about the composition of stellar objects many light-years away.


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