Solar System information #2: The Sun

The Sun - Centre of the Solar System

The Sun is the nearest star to us, about 150 million km from Earth. Light from the Sun takes about 8 minutes to reach us. The mass of the Sun is 745 times the combined mass of all the planets, and the planets, asteroids, and comets are all held in their orbits by its strong gravity. Without the Sun, the Earth would be a dark, dead, frozen world. Sunlight warms our planet, powering the weather systems. Plants use sunlight to photosynthesize ("make food in the light"), and oxygen is produced as a by-product.

Our Sun is an almost constant furnace of burning gas: its energy output averages 390 million billion billion Watts of power. That output only varies by a tiny fraction of a percent, which is good for us – even a small change could make life on Earth difficult. The energy of the Sun (and the other stars) comes from nuclear fusion reactions deep in the core, at unimaginable pressures and temperatures of around 15 million degrees Celsius. These fusion reactions combine hydrogen atoms to produce helium atoms plus energy. About 600 million tons of hydrogen is converted to helium every second.

The surface of the Sun, which is the visible part that we see, is much cooler and has no nuclear reactions going on. At only 5800°C, the Sun’s surface is about the same temperature as the yellow part of a candle flame. Some spots on the surface (‘sunspots’) are about 100°C cooler, and while still very bright, look dark in comparison. Sunspots are caused by tangled magnetic fields and come and go, with the average number of sunspots varying in an 11 year cycle.

The Sun's mass is about 333,000 times that of the Earth. In fact, it contains about 99.8% of all the matter in the Solar System. Even so, the Sun is quite an "ordinary" yellow star; there are many stars larger, many smaller; many brighter and many fainter. It would take about 340 Earths placed side by side to encircle the Sun, and about 1,290,000 Earths packed tightly to make up its volume.

The sun is around 4.5 billion years old, and expected to last another 4 to 5 billion years. In the later stages of its life it will cool and swell to become a red giant star, incinerating the planets Mercury, Venus and Earth. Then it will shrink to become a white dwarf star, gradually cooling and fading until it disappears from view.