The Sun
The heart of the system: our 4.6-billion-year-old star
Summary
The Sun is the star at the centre of the Solar System. It produces energy through nuclear fusion reactions that turn hydrogen into helium. It is 4.6 billion years old and has 333,000 times the mass of Earth. It is the energy source of the whole system: life feeds on it.
In depth
The Sun is a G2V yellow dwarf star: middle-aged and middle-sized. It is made entirely of plasma: 73% hydrogen, 25% helium and 2% other elements. In the core the temperature is 15 million °C and the pressure 340 billion times that of Earth's atmosphere. Under these conditions nuclear fusion takes place: 4 hydrogen nuclei combine into 1 helium nucleus, and the mass difference is released as enormous energy, following Einstein's equation E=mc². Every second 600 million tonnes of hydrogen become helium; this energy reaches Earth as light in 8 minutes. The Sun is about halfway through its life: in 5 billion years, when its hydrogen fuel runs out, it will swell into a red giant, swallowing Mercury, Venus and perhaps Earth, then shed its outer layers and leave a small white dwarf at its centre.
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