The Sun

Without the Sun there would be no light, no warmth and no life on Earth. Yet it is a perfectly ordinary star, it just happens to be ours: the only star we can study up close. About 150 million kilometres separate us from it, and still you feel its power on your skin the moment you step into the sunlight.

A power plant made of hydrogen

The Sun does not burn like a campfire, because a fire would need oxygen and there is almost none inside it. Its energy comes from nuclear fusion: at the centre the temperature is about 15 million degrees Celsius under enormous pressure. There, hydrogen nuclei fuse into helium. The helium is a tiny bit lighter than the hydrogen it came from, and exactly this missing mass turns into energy according to the famous formula $E = mc^2$. Every second the Sun converts about 4 million tonnes of mass into radiation, and its fuel will still last for billions of years.

Bigger than you think

In the sky the Sun looks small, but that is only because of the distance. Its diameter is about 1.4 million kilometres. You could line up 109 Earths side by side to get across the Sun once, and more than a million Earth globes would fit inside it. The Sun holds about 99.8 percent of the entire mass of the solar system. All planets, moons, asteroids and comets together are just the leftover crumbs.

Spots on the Sun

The Sun's surface is not a quiet place. Dark sunspots keep appearing, some larger than the whole Earth. They only look dark by contrast: at about 4,000 degrees Celsius they are around 1,500 degrees cooler than their surroundings, because strong magnetic fields slow down the supply of hot gas there. The number of spots rises and falls in a rhythm of roughly 11 years. In active phases the Sun also hurls huge eruptions into space, which can trigger auroras on Earth.

Light running late

Sunlight travels at about 300,000 kilometres per second and still needs 8 minutes and 20 seconds to reach Earth. So you always see the Sun as it was a good 8 minutes ago. The journey before that is even crazier: the energy from the core wanders through the Sun's dense interior for many tens of thousands of years before it reaches the surface and is allowed to launch into space. The light on your skin is truly ancient.