Discovery of Helium and the Rise and Fall of Coronium

Scientists know that our sun and other stars contain certain elements. How was this information obtained?

In the early nineteenth century, the German physicist Josef Fraunhofer studied the emission spectrum of the sun and noticed certain dark lines at specific wavelengths. We interpret the appearance of these lines by supposing that originally a continuous band of color was radiated and that, as the emitted light moves outward from the sun, some of the radiation is reabsorbed at those wavelengths by the atoms in space. These dark lines are therefore absorption lines. For atoms, the emission and absorption of light occur at the same wavelengths. By matching the absorption lines in the emission spectra of a star with the emission spectra of known elements in the laboratory, scientists have been able to deduce the types of elements present in the star.

Another way to study the sun spectroscopically is during its eclipse. In 1868 the French physicist Pierre Janssen observed a bright yellow line (see Figure) in the emission spectrum of the sun’s corona during the totality of the eclipse. (The corona is the pearly white crown of light visible around the sun during a total eclipse.) This line did not match the emission lines of known elements, but did match one of the dark lines in the spectrum sketched by Fraunhofer. The name helium (from Helios, the sun god in Greek mythology) was given to the element responsible for the emission line. Twenty-seven years later, helium was discovered on Earth by the British chemist William Ramsay in a mineral of uranium. On Earth, the only source of helium is through radioactive decay processes—a particles emitted during nuclear decay are eventually converted to helium atoms.

Fraunhofer’s original drawing, in 1814, showing the dark absorption lines in the sun’s emission spectrum. The top of the diagram shows the overall brightness of the sun at different colors.

The search for new elements from the sun did not end with helium. Around the time of Janssen’s work, scientists also detected a bright green line in the spectrum from the corona. They did not know the identity of the element giving rise to the line, so they called it coronium because it was only found in the corona. Over the following years, additional mystery coronal emission lines were found. The coronium problem proved much harder to solve than the helium case because no matchings were found with the emission lines of known elements. It was not until the late 1930s that the Swedish physicist Bengt Edlén identified these lines as coming from partially ionized atoms of iron, calcium, and nickel. At very high temperatures (over a million degrees Celsius), many atoms become ionized by losing one or more electrons. Therefore, the mystery emission lines come from the resulting ions of the metals and not from a new element. So, after some 70 years the coronium problem was finally solved. There is no such element as coronium after all!

During the total eclipse of the sun, which lasts for only a few minutes, the corona becomes visible.