The Sun: Our Star

A star you can see in the daytime

Every clear day, one star is so bright that it lights up the whole sky and hides all the others. That star is the Sun.

This can be a surprise. We often think of stars as the tiny twinkling dots we see at night. But the Sun is exactly the same kind of object: a giant ball of glowing, super-hot gas. The only reason the Sun looks huge and dazzling while the other stars look like pinpricks is distance. The Sun is our closest star, "only" about 150 million kilometres away. The next nearest star is so far off that its light takes more than four years to reach us.

So the Sun is not just a star — it is our star, the one our whole Solar System belongs to. Every planet, moon, asteroid and comet circles around it.

What the Sun is made of

The Sun is not solid. There is no surface you could ever stand on. It is made almost entirely of two gases: hydrogen and helium. Hydrogen is the lightest, simplest material in the whole Universe, and the Sun is packed with an unbelievable amount of it.

The Sun is also enormous. You could fit more than one million Earths inside it. Its gravity is so strong that it holds all eight planets in their orbits, even faraway Neptune, which is billions of kilometres distant.

Deep in the centre of the Sun, called its core, the temperature reaches about 15 million degrees Celsius and the pressure is crushing. In these extreme conditions, hydrogen is squeezed together so hard that it changes into helium. This process is called nuclear fusion, and it releases a colossal amount of energy. That energy is what makes the Sun shine. The Sun is, in a sense, a giant natural power station running on hydrogen.

How the Sun's energy reaches us

The energy made in the core slowly works its way outwards until it reaches the Sun's glowing surface, the photosphere, which is about 5,500°C. From there, it streams out into space in every direction as light and heat.

A tiny slice of that energy travels towards Earth. Even though light is the fastest thing in the Universe, the distance is so vast that sunlight takes about 8 minutes to reach us. That means when you feel the Sun on your face, you are feeling energy that left the Sun 8 minutes earlier!

The Sun does more than light our days. It:

• Warms the planet so that water can be liquid and life can survive.
• Drives the weather by heating air and oceans, which makes wind, clouds and rain.
• Powers the water cycle by evaporating water from seas and lakes.
• Feeds nearly all life through plants, which capture sunlight to make food.

The Sun and life on Earth

Almost every living thing depends on the Sun. Plants use sunlight in a process called photosynthesis to turn light, water and air into food. Animals then eat plants (or eat other animals that ate plants). So the Sun's energy flows through almost every food chain on Earth. Even the coal, oil and gas we burn today are really ancient sunlight, stored inside plants that lived millions of years ago.

This is why the Sun sits at the heart of so much science. Take the Sun away and Earth would become a frozen, dead rock within days.

Spots, flares and space weather

The Sun is not a calm, unchanging ball. Its surface bubbles and churns. Sometimes cooler, darker patches appear, called sunspots — they look dark only because they are a little cooler than the gas around them. Some sunspots are bigger than the entire Earth.

The Sun can also fling out bursts of energy and particles called solar flares and storms. When these particles reach Earth, they are mostly steered safely around us by Earth's magnetic field. But near the North and South Poles, they crash into the air and make it glow in beautiful curtains of colour — the auroras, also called the Northern and Southern Lights. So those glowing skies are a direct gift from our star.

A very important safety rule

The Sun gives us so much, but it must always be treated with respect. Never look straight at the Sun. Its light is so intense that staring at it, even for a moment, can damage your eyes — and ordinary sunglasses are not nearly enough protection. You do not need to look at the Sun to study it. Scientists use special filtered telescopes and spacecraft, and you can learn loads just from the Sun's effects, like shadows, warmth and the changing length of the day.

Try it yourself: track the Sun with shadows

You can study the Sun safely by watching its shadows move — without ever looking at it.

1. On a sunny day, find a flat, open spot outdoors. Stand a stick upright in the ground, or use a tall object that casts a clear shadow.
2. Mark the very tip of the shadow with a stone or chalk, and write the time.
3. Come back every hour and mark the shadow tip again.
4. By the end of the day, you will have a curve of marks. Notice how the shadow moves and changes length.

What you are really seeing is the Sun appearing to move across the sky as the Earth spins. The shadow is shortest around the middle of the day, when the Sun is highest. Ancient people used exactly this idea to build sundials, the world's first clocks. You have just built one too!

Want to keep exploring space? See The Sun, the Moon and the Stars for a gentle first look, and Planets of the Solar System to meet the worlds that orbit our star.