The Sun

The Sun is the closest star, which makes life on Earth possible. The sun is a sphere with a 1.4 million km diameter (109 times as much as the earth’s diameter) and has a mass approximately 300.000 times greater than the earth’s mass. 75% of it is hydrogen, 23% is helium and only 2% is formed by heavier elements. It produces its heat by transforming hydrogen into helium in its inner core, where the temperature reaches 15 million °C (the surface temperature is around 6000°C). The transformation reaction is called nuclear fusion and joins together 4 nuclei of hydrogen (protons) to create a helium nucleus, freeing a large quantity of energy, which, as photons, is irradiated towards the space. A solar constant is the radiation that perpendicularly hits a unit surface positioned at the top limit of the atmosphere and amounts to 1350 watts per square metre. This heat, multiplied by the surface of the earth’s section (the squared earth’s average radius multiplied by pi Greco) calculates the quantity of energy the earth receives from the sun every second, i.e. 173,000 TW.

Special reports

From the Multimedia section


  • 16 May 2011

    Bio-climatic architecture

    Solar energy is playing a vital role with reference to bioclimatic architecture...

  • The photovoltaic train

    In Italy the PVTRAIN project by Trenitalia (co-financed by the European Union) plans...

  • The plant of Almeria

    In the Tabernas desert, 30 km north-east of the Andalusia town of Almeria (Spain)...