The Sun
The Sun is the star at the centre of our solar system. It is a giant ball of hot, glowing gas that gives Earth its light, warmth, and the energy that keeps every living thing alive. Without the Sun, plants couldn't grow, the oceans would freeze solid, and Earth would be a dark, lifeless rock.
- Type Yellow dwarf star main-sequence G-type
- Diameter 1.39 million km 109 times wider than Earth
- Mass 333,000 Earths in one Sun
- Surface temp 5,500°C hot enough to melt anything
- Core temp 15 million°C where fusion happens
- Age 4.6 billion years about halfway through its life
What kind of object is the Sun?
The Sun is a star, just like the ones you see twinkling at night. It only looks bigger and brighter because it's so much closer. The next nearest star (Proxima Centauri) is more than 250,000 times further away than the Sun. The Sun is classed as a "yellow dwarf" or G-type main-sequence star. It's pretty average. There are stars in our galaxy that are a thousand times bigger and stars a hundred times smaller.
The Sun vs Earth
A star compared to its third planet
If the Sun were a beach ball, Earth would be a small marble. You could fit 1.3 million Earths inside the Sun.
The Sun is so massive that all the planets, moons, asteroids and comets in the solar system together only make up 0.14% of its mass.
If you could stand on the Sun, you'd weigh 28 times more than on Earth. A 30 kg child would weigh 840 kg.
The Sun's visible surface (the photosphere) is hot enough to vaporise every material we know. Its corona (outer atmosphere) is even hotter at over a million degrees, for reasons scientists still don't fully understand.
The Sun spins, but because it's gas, not solid, different parts spin at different speeds. The equator goes round once every 25 days, the poles take 35.
Sunlight travels at 300,000 km per second. It takes just over eight minutes for it to cover the 150 million km from the Sun to Earth.
What the Sun is made of
The Sun is almost entirely hydrogen (approx. 73%) and helium (approx. 25%). The remaining 2% is everything else: oxygen, carbon, iron, neon and traces of all the other elements.
The Sun's layers
Even though the Sun looks like a smooth glowing ball, it has distinct layers, just like Earth:
- The core is the centre, where temperatures hit 15 million°C and nuclear fusion turns hydrogen into helium. This is where the Sun's energy is made.
- The radiative zone is a thick layer where the energy slowly works its way outwards as light. A single packet of light can take 170,000 years to cross this zone.
- The convective zone is the outer 30% of the Sun, where huge bubbles of hot gas rise to the surface and cool gas sinks down again.
- The photosphere is what we see as the Sun's "surface". It's not solid, just the layer where the gas finally becomes thin enough for light to escape into space.
- The chromosphere is a thin layer above the photosphere, normally invisible except during a total solar eclipse.
- The corona is the Sun's outer atmosphere, stretching millions of kilometres into space. It's strangely much hotter than the surface (over a million degrees) and is what we can see streaming away from the Sun during an eclipse.
How the Sun makes energy
The Sun's energy comes from nuclear fusion. Deep in the core, hydrogen atoms are squeezed together so hard by the Sun's gravity that they fuse to form helium, releasing enormous amounts of energy in the process. Every second the Sun converts about 600 million tonnes of hydrogen into helium, releasing as much energy as billions of nuclear bombs.
Where does the energy come from in fusion?
When four hydrogen atoms fuse into one helium atom, the helium atom weighs slightly less than the four hydrogen atoms did. That tiny missing mass has been converted into pure energy, using Einstein's famous equation E=mc². Even though only approx. 0.7% of the mass is converted, the speed of light squared (c²) is such a huge number that the energy released is enormous. This is the same process that powers hydrogen bombs.
Solar weather: flares, spots and storms
The Sun isn't a quiet place. Its surface boils and churns, and every now and then it sends out enormous bursts of energy.
Sunspots
Sunspots are darker, cooler patches on the Sun's surface, caused by twisted magnetic fields. They look black in photographs but are actually still glowing white-hot. They just look dark compared to the bright surface around them. A single sunspot can be bigger than Earth.
Solar flares and CMEs
Sometimes the Sun releases sudden bursts of energy called solar flares, or huge clouds of charged particles called coronal mass ejections (CMEs). When these reach Earth they can cause beautiful auroras (the Northern and Southern Lights) but also disrupt satellites, GPS, and power grids. In 1989 a CME knocked out power for six million people in Quebec, Canada.
How the Sun was born and how it will end
The Sun formed about 4.6 billion years ago when a huge cloud of gas and dust collapsed under its own gravity. Most of the material formed the Sun; the leftovers became the planets, moons and everything else in the solar system.
The Sun has enough hydrogen fuel to keep burning for about another 5 billion years. After that, it will swell up into a "red giant" so huge it will swallow Mercury and Venus, and possibly Earth too. Eventually the outer layers will drift away into space and the core will shrink down into a small, dense, slowly-cooling object called a "white dwarf". Don't worry though, none of this will happen for billions of years.
Studying the Sun safely
Many spacecraft have been sent to study the Sun, including NASA's Parker Solar Probe which is currently flying closer to the Sun than anything humans have ever built. It has to use a thick heat shield to survive the temperatures it experiences.