The Sun: Our Star

Remember from Grade 6

In Grade 6 you learned that the Sun is a star at the centre of our solar system, that it is about 109 Earths across, that its surface is about 5500 °C, and that its gravity holds the planets in their orbits. You also learned that stars emit light while planets and moons reflect it. In Grade 9 you will look inside the Sun, find out where its energy comes from, and see why it matters so much to life on Earth.

A question to start

Every second, the Sun gives off more energy than humans have used in all of history. It has been shining for about 4.6 billion years and it has not run out. A campfire burns out in a few hours. What could possibly keep a star going for billions of years?

The answer is not burning (there is no oxygen fire in the Sun). The answer is nuclear fusion, and you will learn how it works today.

The Sun by the numbers

• Type of object: a medium-sized, middle-aged star. Astronomers call it a yellow dwarf (a G-type main-sequence star). Many stars are bigger and hotter; many are smaller and cooler.
• Diameter: about 1.39 million km, about 109 times Earth's diameter.
• Mass: about 333 000 times Earth's mass. The Sun holds about 99.8% of all the mass in the solar system.
• Composition: mostly hydrogen (about three quarters of its mass) and helium (about one quarter), with small amounts of other elements.
• Age: about 4.6 billion years. It is about halfway through its life as a main-sequence star.
• Temperature: about 5500 °C at the photosphere and about 15 million °C in the core.
• Distance from Earth: about 150 million km. Its light takes about 8 minutes and 20 seconds to reach us.

Inside the Sun: the layers

The Sun has no solid surface. It is made of plasma, a state of matter that forms when gas is so hot that electrons are pulled off the atoms. Scientists describe the Sun in layers, from the inside out:

1. Core. The centre. The temperature is about 15 million °C and the pressure is enormous. Here nuclear fusion produces the Sun's energy.
2. Radiative zone. Energy moves outward as radiation, bouncing from particle to particle. This is so slow that energy can take many thousands of years to cross this zone.
3. Convective zone. Here the plasma moves in huge convection currents: hot plasma rises, cools near the top and sinks again, like boiling soup.
4. Photosphere. The visible 'surface', about 5500 °C. Most of the sunlight we see comes from here.
5. Chromosphere and corona. Thin outer layers of the Sun's atmosphere. The corona is strangely hotter than the photosphere, over 1 million °C. Scientists are still researching exactly why. You can see the corona with your eyes only during a total solar eclipse.

Where the energy comes from: nuclear fusion

In the core, the temperature and pressure are so high that hydrogen nuclei (protons) crash together hard enough to fuse. Through a series of steps, four hydrogen nuclei become one helium nucleus.

Here is the amazing part: the helium nucleus has a tiny bit less mass than the four hydrogen nuclei that made it. That missing mass has been turned into energy. Albert Einstein's famous equation E = mc² tells us that even a tiny amount of mass (m) becomes an enormous amount of energy (E), because c (the speed of light) is a huge number.

The Sun converts about 4 million tonnes of mass into energy every second. It is so massive that it can keep doing this for billions of years.

Fusion is not the same as burning. Burning is a chemical reaction that needs oxygen and rearranges atoms. Fusion is a nuclear reaction that changes the nuclei themselves and releases millions of times more energy per kilogram of fuel.

An active star: sunspots, flares, CMEs and solar wind

The Sun's surface is always changing because of its powerful, tangled magnetic field.

• Sunspots are darker patches on the photosphere. They are not really dark; they are a bit cooler than the area around them (but still thousands of degrees), so they look dark next to it. Strong magnetic fields slow the flow of heat there. The number of sunspots rises and falls in a cycle of about 11 years.
• Solar flares are sudden, powerful bursts of radiation, often near sunspots.
• Prominences are huge loops of plasma that follow magnetic field lines above the surface.
• Coronal mass ejections (CMEs) are giant clouds of plasma blasted out of the corona into space. If one heads toward Earth, it can arrive in one to a few days.
• The solar wind is a steady stream of charged particles flowing out from the Sun all the time, in all directions.

You will see in the next lesson how these cause the northern lights and space weather on Earth.

The Sun's role in the solar system

• Gravity: because the Sun has almost all of the solar system's mass, its gravity holds the planets, dwarf planets, asteroids and comets in orbit.
• Light and heat: the Sun is the only object in the solar system that makes its own light. Planets and moons shine only by reflecting sunlight.
• The heliosphere: the solar wind blows a giant 'bubble' around the solar system called the heliosphere. It helps shield the planets from some of the high-energy particles coming from outside the solar system.

The Sun's role in sustaining life on Earth

• Energy for food chains: plants, algae and some bacteria use sunlight for photosynthesis. Almost every food chain on Earth starts there.
• The right temperature: sunlight keeps most of Earth's surface warm enough for liquid water, which all known life needs.
• Weather and the water cycle: the Sun's heat evaporates water and drives winds (next lesson).
• Day, night and seasons: living things use the Sun's daily and yearly rhythms for sleep, migration, flowering and hibernation.
• Vitamin D: our skin makes vitamin D using ultraviolet (UV) light. Too much UV, though, damages skin and eyes, which is why we use sunscreen and never look directly at the Sun.