What are Fraunhofer lines?

Fraunhofer lines, also known as absorption lines or dark lines, are dark bands or lines observed in the spectrum of the Sun or other stars. They were first discovered by the German physicist Joseph von Fraunhofer in the early 19th century.

When sunlight or starlight passes through the outer layers of a star's atmosphere, known as the photosphere, certain wavelengths of light are absorbed by the atoms and ions present in the atmosphere. These absorption lines appear as dark lines superimposed on the continuous spectrum of light emitted by the star.

Each absorption line corresponds to a specific wavelength of light that has been absorbed by a particular element or molecule in the star's atmosphere. The exact wavelengths of the absorption lines are characteristic of the elements or molecules involved. By analyzing the positions and intensities of these lines, astronomers can determine the composition of the star's atmosphere and study its physical properties, such as temperature and density.

Fraunhofer lines are designated by letters, such as the well-known lines named after the letters A, B, C, D, etc. The most prominent and easily visible Fraunhofer lines in the solar spectrum are the lines of hydrogen (the Balmer series) and the lines of elements like sodium (the D lines) and calcium (the H and K lines).