Dark Plots

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Astronomers had been puzzling over a mystery for almost half a century. In the early 1800s, Josef Fraunhofer discovered that when sunlight was split into its individual wavelengths, hundreds of dark lines appeared in the "rainbow" of colors. Fraunhofer spent decades trying to explain them, but he couldn't.

And neither could anyone else -- until 150 years ago this week. That's when two scientists reported that the dark lines were the "fingerprints" of chemical elements in the Sun's atmosphere. Their work allowed astronomers to begin probing the makeup of stars and galaxies throughout the universe.

Robert Bunsen had been studying the colors produced when he burned various chemicals in his new device, the Bunsen burner. A colleague at the University of Heidelberg, Gustav Kirchoff, kicked it up a notch by studying the flames with a spectrometer -- a device that splits light into its individual wavelengths.

Kirchoff found that when viewed against a dark background, the flame produced a series of bright lines. But when it was viewed against bright sunlight, the flame produced dark lines in the Sun's rainbow of colors. These lines were at the same wavelengths as the bright lines.

From that, Kirchoff deduced that the dark lines in the Sun's spectrum come from atoms that are above the Sun's visible surface. As light shines through them, they absorb some of the energy -- etching dark lines in the Sun's spectrum.

More about spectroscopy tomorrow.

Script by Damond Benningfield, Copyright 2009

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