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the one million degree rainbow


When astronomers first looked at the spectrum of light from the Sun's corona during a solar eclipse in 1869, it looked very strange. Unlike the Fraunhofer spectrum of the photosphere with its dark absorption lines, the spectrum of the corona showed bright, emission lines. What was even more puzzling was that they didn't appear at wavelengths connected with any known element.


Since scientists knew that every atom and ion emits light at a set of very particular wavelengths did that mean there was a new element in the Sun?


For a long time many scientists thought the answer was ‘Yes!’ and they gave it the name coronium. It wasn't until 1939, 70 years after the first eclipse observations, that the mystery was solved by two Swedish scientists called Edlen and Grotian. They found that the emission lines did in fact come from common elements, such as iron and calcium.


How could this be? Simple - once you know the answer! The temperature in the corona is so high that the atoms lose many of their electrons and become ions, which have different spectral lines. No need to invent coronium then!


Professor B. Edlen
Professor B. Edlen

The reason it took so long to understand what the emission lines were telling us about the corona was that until then no one had even considered the possibility that the Sun's atmosphere could be so hot, a lot hotter than its surface. Some lateral thinking was required!

I have to be very careful when investigating the Sun's spectrum. I need to know how hot that part of the Sun I'm studying is before I can identify the lines for certain. Only then can I measure the brightness of the emission lines and work out what the Sun is made of.





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