Spectroscopy and Astronomy

Astronomers generally do not look through their large telescopes. Most of the time large telescopes are collecting light for a spectrograph, which spreads the light out into a rainbow. Each kind of atom or ion has certain special wavelengths which it can absorb or emit. Atoms in the cool outer layers of a star absorb light coming from the hotter regions within, producing dark absorption lines across the spectrum. These can be used to identify the atoms that make up a star. In the spectrum at right, the dark lines are 2 from ionized calcium at about 390 nm wavelength, hydrogen lines at 410, 434, 486 and 656 nm, a line from ionized magnesium at 518 nm, and a line from sodium at 590 nm. To see the lines produced by any element, use this Java applet.

And astronomers do not actually use color film to take a color picture of the spectrum, for several reasons:

Color film or CCDs are less sensitive than black and white.
Color film does a very poor job of representing the continuous range of colors in a spectrum. It takes a lot of work to make a decent printout or display of a spectrum.
While a picture may be worth a thousand words, a graph showing brightness vs. wavelength has much more quantitative information than a picture of the spectrum.

Thus astronomers make graphs of their spectra, with the y-axis showing the brightness, while the x-axis shows the wavelength. In the figure this graph is aligned just below the picture of the spectrum.

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