The inner core is a spherical ball, whose radius is about 70% of that of the Moon! It is mostly made of an iron-nickel mixture. Its temperature is about $$5500^{\circ} \, \text{Celsius}$$, the same as the temperature of the Sun's surface! Even though the temperature is so high, the iron-nickel mixture remains a solid in the inner core. We shall understand the reason behind this at the end of the module.