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

Mysterious spectral lines in the solar corona led scientists in a hunt for extra-terrestrial elements.


Green Corona

The green color of the solar corona seen during a total solar eclipse was once thought to be cause by the element coronium.

Thanks to the advent of the spectroscope by two German scientists, Robert Wilhelm Bunsen (1811-1899) and Gustav Robert Kirchoff (1824-1887) in 1859, astronomers began experimenting almost immediately with this new way of analyzing light. By splitting light into its component wavelengths much as prism splits light into its colors, astronomers soon detected familiar elements like calcium and sodium in the light from distant stars and nebulae. In 1868, Sir Norman Lockyer (1836-1920) discovered a new line in the solar spectrum called Helium. This element was not detected on Earth until about 25 years later. Meanwhile, the solar corona was revealing itself as an equally mysterious region.

Charles Augustinus Young (1834-1908) and William Harkness (1837-1903) independently discovered a new bright (emission) line in the spectrum of the Sun's corona. When they pointed their telescopes at the solar corona during the August 7, 1879 eclipse, they saw several bright lines, including a particularly strong green line at a wavelength of 5303 Angstroms (1 Angstrom unit = 10-8 centimeters). This green line is so intense that, when eclipses are photographed (like the image above) the green color can easily be seen.

Norman Locklyer & Spectroscope

Norman Lockyer was the first astronomer to attach a spectroscope to a telescope to study the sun.

Like Lockyer’s helium, this new atomic line in the solar corona was considered to be from a new element unlike anything seen under laboratory conditions, so they called it Coronium. Young even went so far as to identify it as Iron Line Number 1474, though it didn’t fit the expected pattern of lines from this element. It wasn’t until 60 years later that Swedish astronomer Bengt Edlin(1906-1993)finally determined that the Coronium lines were caused by the element iron seen under very high temperatures. The atom had lost 13 of its 26 electrons (a state that physicists call call ‘Iron-14’ or Fe XIV). One of the remaining 13 electrons, when excited, gives up its energy and produces this green line. A weaker red line also seen in the corona at 6374 Angstroms, was produced by iron atoms stripped of 9 electrons (Fe X). It would take temperatures near a million degrees C to make iron atoms behave this way, but how could the corona possibly be that hot? Discovering the source of the Coronium line only opened up another perplexing question that astronomers were not able to answer until the start of the 21st Century.

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