Energy and luminosity

The Sun produces a hundred million times more energy than all the planets combined. Just over half this energy is in the form of visible light, with the rest being infrared (heat) radiation. Only about a billionth of the Sun's energy reaches us here on Earth.

The Sun's energy comes from the 'burning' of its hydrogen gas via the process of nuclear fusion. In this process four hydrogen atoms combine to make one helium nucleus. During this process some mass is lost, and it is this mass that is converted into energy. Every second the Sun converts over 600 million tonnes of hydrogen into helium, and this results in 4.5 million tonnes of mass being converted into energy every second.

Energy generated in the core is carried outward to the surface by radiation and convection processes. Core temperature is about 15 million degrees Celsius, while at the surface the temperature is around 5500 degrees Celsius. The surface and interior temperatures are too high for the Sun to have any liquid or solid material.

The luminosity of a star is an indication of the total amount of energy it produces every second. This rate depends on the core temperature and pressure of the star, which in turn depends on its mass and age. The Sun's luminosity is 3.9 x 1026 joules per second.

Throughout its life the Sun has increased its luminosity by about 40 per cent and it will continue to increase for some time.

The Sun has several different layers. At the centre is the core, which is where energy is produced via nuclear fusion reactions. Above this is the radiative zone, where energy travels very slowly upwards. Closer to the surface is the convective zone, where heat is transported much faster to the surface, or photosphere. The photosphere is a thin shell of gases about 200 km thick and forms the visible surface of the Sun. Most of the energy radiated by the Sun passes through this layer. It has a temperature of about

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