In 1814 Joseph von Fraunhofer began a thorough study of the sun's spectrum. He found that it was crossed by many dark lines, now called absorption lines or Fraunhofer lines. Meanwhile, other scientists had been studying the light emitted and absorbed by elements in the gaseous state when they were heated in the laboratory. They discovered that each element always produced a set of bright emission lines associated with that element alone. The dark solar line that Fraunhofer had called D was shown to have the same position in the spectrum as the brilliant line that sodium gave off when it was heated in the laboratory.
Scientists now believe that the dark bands represent elements in the sun's atmosphere.
The lines are dark because the elements in the sun's atmosphere absorb the bright lines given off by the same element on the sun's disk. Linking the lines of the spectrum with the elements that emit or absorb them provided a way to study the composition of the sun's surface. Almost all elements known on Earth have been shown to exist on the sun. Studies of the solar spectrum have revealed that hydrogen makes up some 92 percent of the sun's atmosphere and helium about 8 percent. Carbon, nitrogen, oxygen, sodium, and other elements are also present.