- Know the size, distance and age of the Sun
- Name the layers of the Sun
- Explain nuclear fusion as the source of the Sun’s energy
- Calculate how long sunlight takes to reach the Earth
The morning sunbeam coming through your window has travelled 150 million kilometres. It gives plants energy for photosynthesis, warms the Earth and drives winds and rain. But what is the Sun, and how has it kept shining for billions of years without going out?
The Sun is a star
The Sun is the star closest to the Earth — a huge, glowing ball of hot gas that makes its own light. Its diameter is about 1.39 million km, 109 times that of the Earth. The Sun holds about 99.8% of all the mass of the Solar System. It is about 4.6 billion years old and is made mostly of hydrogen (about ¾) and helium (about ¼).
The average distance from the Earth to the Sun: about 150 million km (149.6 million km more precisely). It is used to measure distances in the Solar System.
- ttravel time of light, in seconds (s)
- sdistance, in km
- cspeed of light ≈ 300,000 km/s
The distance to the Sun is about 150,000,000 km and the speed of light is 300,000 km/s. How long does light take to travel from the Sun to the Earth?
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500 s = 8 · 60 s + 20 s = 8 min 20 s.
So we always see the Sun as it was about 8 minutes 20 seconds ago.
Inside the Sun
| Layer | What happens there? |
|---|---|
| Core | The temperature is about 15 million °C. Nuclear fusion takes place here and energy is produced. |
| Radiative zone | Energy moves outward as light very slowly — over tens of thousands of years or more. |
| Convective zone | Hot gas rises like boiling water, cools and sinks back down. |
| Photosphere | The visible “surface”; about 5,500 °C. |
| Chromosphere and corona | The Sun’s atmosphere. The corona is hotter than 1 million °C and can be seen during a total solar eclipse. |
Where does the Sun’s energy come from?
The Sun does not burn like a fire. In its core, under enormous temperature and pressure, hydrogen nuclei join together to form helium nuclei. This process is called nuclear fusion. In the reaction a small part of the mass disappears and turns into a huge amount of energy, as Einstein’s famous formula describes.
- Eenergy released, in joules (J)
- mmass turned into energy, in kg
- cspeed of light, 3 · 10⁸ m/s
Because c² is a huge number, even a small mass gives an enormous amount of energy.
Sunspots and the solar wind
Sunspots are areas of the photosphere that look dark. They are about 1,500–2,000 °C cooler than their surroundings, so they appear darker; strong magnetic fields there stop hot gas from rising. The number of sunspots rises and falls in a cycle of about 11 years. In years with many sunspots, solar flares are also more frequent.
A constant stream of charged particles (protons and electrons) flows from the corona into space — this is the solar wind. It usually travels at about 400 km per second. The Earth’s magnetic field shields us, but near the poles particles enter the atmosphere and create the aurora (northern and southern lights). Strong bursts cause magnetic storms that can disturb satellites and communications.
The Sun’s diameter is about 1,392,000 km and the Earth’s is about 12,760 km. How many Earths could be lined up across the Sun’s diameter?
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About 109 Earths fit across the Sun.
Key points
- The Sun is the nearest star; its average distance is about 150 million km (1 AU).
- Sunlight reaches the Earth in about 8 minutes 20 seconds.
- The Sun’s energy comes from nuclear fusion of hydrogen into helium in its core.
- Layers: core, radiative zone, convective zone, photosphere, chromosphere and corona.
- Sunspots are cooler regions; the solar wind causes auroras.
Check yourself
10 questions. Every correct answer earns XP.