L6. The Sun: Our Local Star

Unit 2 · The Solar System

R-report

L6. The Sun: Our Local Star

The Sun looks like a smooth yellow disc. It is actually a churning ball of gas, 1.4 million kilometres wide, converting four million tonnes of itself into pure energy every second — and it has been doing it for 4.6 billion years.

What the Sun is made of

The Sun is a star: an enormous ball of hot gas held together by its own gravity. It is roughly three quarters hydrogen and one quarter helium, with small amounts of everything else. It has no solid surface. If you could fly toward it you would never land on anything — the gas would simply grow denser, hotter, and more crushing until it destroyed your spacecraft. What looks like a surface is the photosphere, the layer where the gas finally becomes transparent enough for light to escape. The scale is hard to hold in your head. About 109 Earths could be lined up across the Sun’s face, and around 1.3 million Earths could fit inside it by volume. It contains about 99.8% of all the mass in the solar system. Everything else — every planet, moon, asteroid, and comet — shares the leftover 0.2%. And yet the Sun is a thoroughly ordinary star. Many stars are far larger and brighter; many more are smaller and dimmer.

Where the energy comes from

The Sun’s energy is made in its core, where the temperature reaches about 15 million °C and the pressure is crushing. Under those conditions hydrogen nuclei are forced together to make helium — a process called nuclear fusion. The helium that comes out weighs a tiny bit less than the hydrogen that went in, and that missing mass is released as energy. The Sun converts about four million tonnes of mass into energy every second. The layers work outward like this: - Core: fusion happens here, and nowhere else. - Radiative zone: energy travels outward as light, bouncing so often that it takes thousands of years to cross. - Convective zone: hot gas rises, cools, and sinks again, like boiling soup. - Photosphere: the visible surface, about 5,500 °C. - Corona: the thin outer atmosphere, strangely far hotter than the surface, visible during a total solar eclipse. From the photosphere, light takes 8 minutes and 20 seconds to reach Earth. You always see the Sun as it was 8 minutes ago.

A restless, changing star

The Sun is not calm. Its surface is covered in granules the size of countries, each one a column of rising gas, and it goes through an activity cycle roughly every 11 years. - Sunspots are cooler, darker patches where the Sun’s magnetic field is tangled. They still glow brightly — they only look dark against the hotter gas around them. - Solar flares are sudden bursts of energy near sunspot groups. - Coronal mass ejections hurl billions of tonnes of charged particles into space. When that material reaches Earth, our magnetic field steers it toward the poles, where it makes the auroras glow. Strong events can also disturb satellites, radio communications, and even power grids — which is why space weather is now monitored continuously. All of this matters directly to us. Sunlight drives photosynthesis, weather, and ocean currents, and the fossil fuels we burn are stored sunlight from ancient plants. Almost every energy source on Earth traces back to the Sun’s core.

Putting the investigation together

The Sun is a ball of hydrogen and helium big enough that gravity squeezes its core to 15 million °C, hot enough for fusion. That fusion releases the energy that works its way outward for thousands of years, escapes from the photosphere, and crosses to Earth in eight minutes.

Notice that studying the Sun means studying stars in general. It is the only star close enough to examine in fine detail, so everything we learn about its layers, its magnetic cycle, and its fusion becomes the model we apply to the billions of stars we can only see as points of light.