Special instruments known as spectrographs tell us that the Sun is made almost entirely of two gases, hydrogen and helium. The temperature at its surface is about 5 500 degrees Celsius – more than 20 times hotter than an oven on maximum. In the centre, the temperature soars to about 15 million degrees Celsius and the gas is squeezed to about 10 times the density of lead.
At such high temperatures and pressures, the Sun becomes a nuclear reactor, where hydrogen is converted to helium. At the same time, huge amounts of radiation are produced. This finds its way to the surface as visible, infrared and ultraviolet light, and X-rays.
Every hour, the Sun ejects a billion tons of gas into space at a speed of up to 2 million km per hour. Even so, the Sun is so massive that it can continue burning for billions of years.
The Sun becomes more active every 11 years. At such times, the number of dark sunspots on its surface increases. At the peak of the sunspot cycle, there are many more explosive solar storms. During the last solar maximum in 2001, many solar flares and coronal mass ejections blasted billions of tonnes of material into space at high speed.
If these storms head towards Earth, they can have serious side-effects. They may cause power cuts, damage satellites, and affect communications and navigation systems. They also cause the Earth's auroras – commonly known as the Northern and Southern Lights.