Georgina Dawson-Perry, of Ilam School, asks :-
Why is the sun so hot?
Murray Matthews, a nuclear chemist with the Ministry of Health's National Radiation Laboratory, Christchuch, responded.
The Sun is so hot because of energy released by nuclear reactions taking place in the region of extremely high gas pressure near its centre.
The Sun is a huge ball of gas, mainly hydrogen. Because of its size, the gas within it is compressed due to the pressure exerted by gravity. The extreme pressure near the Sun's centre heats the gas to a temperature of 14 million degrees Celsius, which is sufficient to start nuclear reactions like those which take place in a "hydrogen bomb". The Sun can thus be imagined as a continuously exploding nuclear bomb and the energy released maintains its heat.
The main nuclear reaction in the Sun is the combining ("fusion") of hydrogen atoms to form helium atoms. Some nuclear material is converted to energy during this process. If you destroy mass you produce energy as summed up in the expression made famous by Albert Einstein, E = mc2. The Sun is thus "nuclear powered" and functions as a huge thermo-nuclear reactor converting 4 million tonnes of matter into energy every second. No wonder it's so hot!
We see only the cooler outer part of the Sun where the gas is about 6000 degrees Celsius which is hot enough to emit visible light (daylight), and infra-red radiation (heat). These sustain life as we know it on Earth.