What is the Sun made of and how does it shine?
The Sun turns 4 million tonnes of itself into light every second, yet its core makes about as much power per cubic metre as a compost heap.
Guess first
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Which one is it?
The Sun is a giant ball of hot, glowing gas, mostly hydrogen with about a quarter helium, and it shines because deep in its core hydrogen is being fused into helium. It is a massive sphere of hot , and it is so heavy, about 330,000 times the mass of Earth, that it holds roughly 99.86 percent of all the mass in the Solar System.
Every second, the core fuses about 600 billion kilograms of hydrogen into helium and turns four billion kilograms of matter into energy. That energy works its way out to the surface, where it leaves as visible light and infrared radiation. Light from the surface crosses the roughly 150 million kilometres to Earth in about 8 minutes, so the Sun you see is the Sun of 8 minutes ago.
What makes this weirdly human is the argument that came before. In the nineteenth century, Lord Kelvin and Hermann von Helmholtz suggested the Sun was slowly shrinking under its own gravity, and that would have kept it shining for only about 20 million years, too short for the rocks geologists were studying. The real answer, fusion, was not understood until later.
The Sun is not a calm lamp. Its surface is about 5800 , yet the thin outer atmosphere above it, the , reaches one to two million kelvins. Its activity reaches Earth as auroras, and sometimes as disruption to radio communications and electric power. That is the story of space weather.
Try it
How far does light go?
y = 6x + 0
Challenge · Light covers about 18 million km a minute. Set that speed and find when the line reaches the Sun's distance, about 150 million km: just over 8 minutes.
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Recap
Hydrogen fuses into helium in the core, and the energy takes thousands of years to reach the surface and eight minutes to reach us.
💡 A trick to remember it · Fuse in the core, wander for ages, race for eight minutes: the Sun's light has a slow start and a fast finish.
Surprising fact · Per cubic metre the core makes about as much power as a compost pile; the Sun shines because it is so big.
Connects to
- ⚛️ How does a nuclear power plant turn splitting atoms into electricity?
- 🌞 What are sunspots and why does the Sun have an 11-year cycle?
- 🌌 Why do the northern lights happen, and why were they seen so far south in 2024?
- 🌌 How did the Sun and the planets form from a spinning cloud of gas?
- ☀️ How does the Sun shine if its core is too cold for fusion?
- Space weather
Sources (1)
No source, no claim. Every fact in this lesson (13 claims) cites at least one of these.