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Sun-Earth Day Presents: Eclipse, In a Different Light

With the right tools, astronomers decode the Nature’s cosmic messages in a beam of light.

TECHNOLOGY THROUGH TIME ISSUE #39: Solar Sprectroscopy

Fraunhofer Stamp

The presence of dark lines in the spectrum of sunlight was first detected by a scientist named Fraunhofer, and can be seen in his original 1814 drawing.

For thousands of years, the sun and all celestial bodies were thought to be made from a Fifth Essence of eternal purity and luminescence. This idea persisted until the early 1800’s. Its downfall came with the rise of a new technology for studying light itself.

Isaac Newton (1643-1727) was the first to discover that sunlight was composed of multiple colors in 1666 by using a pair of simple prisms. It would be another 100 years before the Scottish physicist Thomas Melvill (1726-1753) discovered that putting different substances in flames, and passing the light through a prism, gave differently patterned spectra. Then in 1802, William Wollaston (1766-1828) in England discovered that the solar spectrum itself had many thin dark lines in the rainbow of colors. Joseph von Fraunhofer (1787-1826) investigated these lines in great detail starting in 1814. By using more than one prism to disperse the rainbow of light, he found an "almost countless number" of lines. He labeled the strongest dark lines A, B, C, D, etc. Scientists still call these the Fraunhofer lines today.

Some years later in 1849, while studying flame spectra, Jean Bernard Leon Foucault (1818-1868) saw a bright double yellow line at exactly the same wavelength as Fraunhofer's dark D line in the solar spectrum. A few years later in 1853, the spectrum of hydrogen was first observed by Anders Angstrom (1817-1874) in Sweden. Meanwhile, the first really systematic investigation of flame spectra was carried out by Robert Bunsen (1811-1899) and Gustav Robert

Modern Solar Spectrum

Modern spectrum of the sun taken by a high-resolution Echelle spectrograph. Note the numerous lines in the individual spectrum strips. Color has been added to the spectrum for effect.

Kirchhoff (1824-1887), in Heidelberg, between 1855 and 1863. They cataloged and measured thousands of spectral lines, including many new elements such as rubidium and cesium.

By further experimenting with sunlight, the spectra of different elements, and Fraunhofer's spectrum drawings, Kirchhoff was able to decode the sun's spectrum. The dark lines and their position indicated that the sun contained such familiar elements as sodium, magnesium, iron, calcium, copper, and zinc. Finally in 1869, Joseph Lockyer (1836-1920) studied the spectra of solar prominences (in eclipses) and found the spectra to be slightly Doppler shifted, so was able to deduce the speeds of the gases whirling around the sunspots. He also found a spectrum never seen before, and conjectured that it came from a new element he named Helium.

So, by the mid-1800’s, astronomers now had the key to what elements made up the farthest stars - they could find out just by using a spectroscope attached to the familiar telescope. Within a span of only a decade, scientists working in a lab, and astronomers working at their telescopes, were able to do what the famous philosopher Auguste Comte (1798-1857) had claimed was ‘impossible’:

"…Our knowledge concerning the gaseous envelopes [of stars] is necessarily limited to their existence, size … and refractive power, we shall not at all be able to determine their chemical composition or even their density … I regard any notion concerning the true mean temperature of the various stars as forever denied to us." (Cours de philosophie positive , vol. 2, 1835)

GALLERY

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