The sun is described as a big star composed mainly of hot hydrogen and helium gas.
There are various layers in the Sun, including the core or core, the radiative zone, the convection zone, the photosphere (which emits most of the sunlight), the chromosphere and its outer part, the corona. Density decreases with altitude, from the nucleus to the atmosphere. The temperature in the nucleus is of the order of 15 million kelvin.
The solar atmosphere is a complex medium consisting of plasma subjected to an intense magnetic field, with solar flares and the solar wind. Sunspots, which appear dark because of their lower temperature, are, like the crown, radiosources. Solar activity related to these phenomena varies cyclically.
The observation of the solar spectrum is carried out thanks to spectroheliographs. This spectrum includes rays, known since Fraunhofer (1814), which signal the presence of hydrogen and helium mainly and about sixty other elements. It also includes bands that betray the presence of molecules in the solar atmosphere.
