Wikipedia on the inner core:
https://en.wikipedia.org/wiki/Inner_core
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The Earth's inner core is the Earth's innermost part. It is primarily a solid ball with a radius of about 1,220 kilometres (760 miles), which is about 70% of the Moon's radius.[1][2] It is composed of an iron–nickel alloy and some light elements. The temperature at the inner core's surface is approximately 5,700 K (5,430 °C), which is about the temperature at the surface of the Sun.[3
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