Spectroscopy

    Over one hundred years ago scientists made the amazing discovery that the light from stars could be spread into its component colors. They soon developed spectrographs that could be mounted on the back of their telescopes. With these new instruments, researchers could analyze stellar spectra in depth, and open up an entirely new discipline in science, spectroscopy. With this new tool of spectroscopy, we could now know, with some certainty, the intimate details of the structure and content of stars. Each chemical element has its own, distinct set of spectral lines. Can you correctly identify these elements?

    In the nineteenth century, physicists described three different types of spectra.


Central to this new science of spectroscopy were the three laws of Gustav Kirchoff which describe the three different types of spectra.
  1. A luminous, hot solid, or dense, hot gas, emits light of all wavelengths and so produces a continuous spectrum of radiation.
  2. A low density hot gas emits light, the spectrum of which consists of discreet, bright lines. This emission spectrum is indicative of the chemical composition of the gas.
  3. A cool, low pressure gas absorbs light at specific wavelengths or colors. This occurs in all normal stars. This absorption spectrum reveals the presence of chemical elements.

    Let's see how this all works. Here is a most useful applet that explains the three type of spectra.

    Stars typically show the third of the three types of spectra, the dark line or absorption spectrum, with hundreds of dark lines visible in a good spectrogram. Since stars are made primarily of hydrogen, we would expect to see the presence of that gas in a star's spectrum. In fact their are many series of energy transitions which occur in hydrogen. What virtually every one of these stellar spectra will show is the Balmer Series of lines due to hydrogen gas. In the Balmer series, electrons drop from higher energy levels down to the second energy level.

   Here are the spectra of some relatively common gasses.

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Updated May 6, 2010