Almost every bit of energy you used today started in the Sun. The light that grew the wheat in your toast, the warmth that keeps lakes from freezing solid in summer, even the wind that turns turbines on the shores of Lake Huron: all of it traces back to one star about 150 million km away. The Sun is so far away that its light, moving at 300 000 km every second, takes about 8.3 minutes to reach you. It is also huge: about 109 Earths would fit side by side across its face, and it holds about 99.8% of all the mass in the solar system. Yet among the stars it is quite ordinary. It looks special only because it is the closest star to us by far.
What the Sun is and how it shines
A star is a huge ball of hot gas held together by its own gravity and giving off energy from nuclear fusion. The Sun is mostly hydrogen (about three quarters of its mass) and helium (about one quarter), with small amounts of other elements. It is so hot that its atoms are torn apart into charged particles, a state of matter called plasma. In the core, the temperature is about 15 million °C and the pressure is enormous. There, hydrogen nuclei smash together so hard that they join (fuse) to make helium. A helium nucleus has slightly less mass than the hydrogen that formed it, and that "missing" mass is turned into a huge amount of energy. Every second the Sun fuses about 600 million tonnes of hydrogen and turns about 4 million tonnes of mass into energy. It has enough hydrogen to keep doing this for billions of years.
The layers of the Sun
| Layer | What happens there | Approximate temperature |
|---|---|---|
| Core | Hydrogen fuses into helium, releasing energy | 15 million °C |
| Radiative zone | Energy moves outward as light, absorbed and re-emitted over and over, so it takes a very long time to get through | about 7 million °C down to 2 million °C |
| Convective zone | Hot plasma rises, cools and sinks again, like a pot of boiling soup, carrying energy to the surface | about 2 million °C down to 5500 °C |
| Photosphere | The visible surface; the light we see leaves from here | about 5500 °C |
| Chromosphere | A thin reddish layer above the surface, seen during total solar eclipses | about 4000 °C rising to tens of thousands of °C |
| Corona | The faint outer atmosphere, stretching millions of kilometres into space | over 1 million °C |
Energy made in the core does not shoot straight out. In the radiative zone, light is absorbed and given off again by particles countless times, each time in a random direction, so the energy takes tens of thousands of years (some estimates say over 100 000 years) to work its way out. In the convective zone, energy is carried faster by moving plasma. Once energy reaches the photosphere, light escapes into space and crosses the 150 million km to Earth in just 8.3 minutes.
Solar activity: sunspots, flares, prominences and the solar wind
- Sunspots: darker patches on the photosphere where strong magnetic fields block the hot plasma from rising. They are about 3500 to 4500 °C, still extremely hot and bright, but they look dark next to the hotter surface around them. Many sunspots are bigger than Earth.
- Solar flares: sudden, powerful bursts of radiation (X-rays, ultraviolet and radio waves) near sunspots, when twisted magnetic fields snap and reconnect.
- Prominences: huge loops of glowing plasma that follow magnetic field lines above the surface, often many times taller than Earth is wide.
- Solar wind: a constant stream of charged particles (mostly protons and electrons) flowing out from the corona in all directions, typically at about 400 km/s (fast streams reach about 800 km/s).
- Coronal mass ejections (CMEs): giant clouds of plasma blasted off the Sun. When one hits Earth, it can cause a geomagnetic storm with bright auroras.
Why the Sun matters for life
- Light for photosynthesis: plants, algae and some bacteria capture sunlight to make food, which feeds almost every food chain on Earth.
- Warmth: sunlight keeps most of Earth's surface warm enough for liquid water.
- Weather and the water cycle: the Sun's uneven heating makes winds and ocean currents and evaporates water that later falls as rain and snow.
- Stored energy: fossil fuels (coal, oil, natural gas) are the remains of ancient living things that grew using sunlight millions of years ago.
- Gravity: the Sun's gravity holds Earth and the other planets in their orbits.