The Sun is an average, middle-aged, main sequence star belonging to the spectral class G2. Like all other stars, it is composed of hot gases, predominantly hydrogen and helium. It does not have any solid surface. The observed surface simply corresponds to the diffuse spherical region from where we receive the solar radiation. Due to its vicinity, we know a lot more about the Sun in comparison with all other stars. We have detailed observations in a wide range of frequencies, such as radio, infrared, visible, ultraviolet, and x-rays. Furthermore, we are also able to study the pressure (or sound) waves propagating in the solar interior. These produce oscillations in the solar surface that lead to Doppler shifts in the absorption spectral lines in the solar radiation. By measuring these Doppler shifts, astronomers can deduce the nature of the pressure waves in the solar interior. This phenomenon is similar to the seismic waves in Earth’s interior and hence its study is called helioseismology. These observations provide a wealth of information about the Sun’s interior. Besides these, we can also study neutrinos from Sun, which originate in the solar core due to nuclear reactions. These neutrinos provide direct information about the fusion reactions occurring in the core.