Janssen detects an unknown solar spectral line

Background. Jules Janssen, a French astronomer, participated in a long series of solar eclipse expeditions, including one to Guntur, India, in 1868. During this expedition, he observed the solar chromosphere. The observation revealed bright lines in the spectrum, indicating the chromosphere was gaseous. This work contributed to the discovery of the gaseous nature of the solar chromosphere.

Observation. On 18 August 1868, Janssen observed a bright yellow spectral line in the solar chromosphere during a total eclipse in Guntur, British India. The line had a wavelength of 587.49 nanometers. Janssen realized the chromospheric spectrum could be observed without an eclipse due to the brightness of the lines, leading to his method for observing prominences in daylight.

Identification. Norman Lockyer observed the same yellow line from London in October 1868. He named it D3 because it was near the known D1 and D2 Fraunhofer lines of sodium. Lockyer suggested the line was caused by an unknown solar element, as it could not be explained by known materials. This element was later named helium.

Detection Context. The first evidence of helium was observed as a bright yellow line in the solar chromosphere. While Janssen recorded the line during the eclipse, the initial detection of the spectral line signature is attributed to Georges Rayet, Captain C. Pogson, and Lieutenant John Herschel, with Janssen subsequently confirming the observation.