Before continuing into the 20th century, it is required to stress the importance of spectroscopy as THE scientific tool which made it possible to advance knowledge on the source of the Sun’s energy by starting to give clues on the Sun’s composition. By analyzing the properties of the light reaching us, spectroscopy allowed scientists to identify elements that were found in abundance on the Sun instead of guessing its composition.
Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, by a prism. It is the measurement of radiation intensity as a function of wavelength.
In 1814, Fraunhofer discovered dark lines in the solar spectrum. These lines are typical spectral absorption lines. These dark lines are produced whenever a cold gas is between (1) a broad spectrum photon source and (2) the detector. In this case a decrease in the intensity of light in the frequency of the incident photon is seen as the photons are absorbed, then re-emitted in random directions, which are mostly in directions different from the original one. This results in an absorption line, since the narrow frequency band of light initially travelling toward the detector, has been turned into heat or re-emitted in other directions. By contrast, if the detector sees photons emitted directly from a glowing gas, then the detector often sees photons emitted in a narrow frequency range by quantum emission processes in atoms in the hot gas, resulting in an emission line (this knowledge was not yet available).
In the 1860s Kirchhoff and Bunsen noticed that several Fraunhofer lines coincide with characteristic emission lines identified in the spectra of heated elements. It was correctly deduced that dark lines in the solar spectrum are caused by absorption by chemical elements in the Solar atmosphere or the outer regions. So, in the Sun, Fraunhofer lines are seen from gas in the outer regions of the Sun, which are too cold to directly produce emission lines of the elements they represent.
During the solar eclipse of October, 1868, an English astronomer, Joseph Norman Lockyer, discovered an unknown element in the Sun, i.e. a set of spectral lines which did not correspond to elements in the lab. He named this element helium (Latin for Sun element). Lockyer observed a prominent yellow line from a spectrum taken near the edge of the Sun. With a wavelength of about 588nm, slightly less than the so-called “D” lines of sodium. the line could not be explained as due to any material known at the time, and so it was suggested by Lockyer that the yellow line was caused by an unknown solar element. He named this element helium after the Greek word ‘Helios’ meaning ‘sun’. Helium was discovered on Earth in 1895 more than 25 years after the discovery of the yellow line.
Nevertheless, it should be noted that lack of knowledge about the exact composition of the sun continued to be a roadblock to scientific progress. It was not until the 1920s and with the help of Quantum physics that astronomers, such as Cecilia Payne, using spectroscopic techniques established beyond a doubt that hydrogen is the most abundant element in the Sun. In her doctoral thesis “Stellar Atmospheres” published in 1925, Cecilia Payne, applied the new theory of Quantum Mechanics to show that the spectra of the stars were determined entirely by their temperatures, and that the abundances of the different chemical elements were essentially constant throughout the Galaxy. She correctly suggested that silicon, carbon, and other common metals seen in the Sun’s spectrum were found in about the same relative amounts as on Earth, but that helium and particularly hydrogen were vastly more abundant (for hydrogen, by a factor of about one million). Her thesis established that hydrogen was the overwhelming constituent of the stars (see Metallicity), and thus the most abundant element in the Universe. This central conclusion still stands today, but was not immediately accepted until 1929.
NOTE: In percentage of the total Sun’s mass, the composition of the Sun is 71% Hydrogen, 27% Helium, and among the remaining elements: Oxygen, Carbon, Nitrogen.