The Sun – The Engine That Runs Everything

Stand in the sun on a winter morning and feel the warmth soak into your back. It seems like the simplest thing in the world. But the heat on your skin has come a very long way, and in a sense it has been travelling for tens of thousands of years. The light itself left the Sun only about eight minutes ago. The energy it carries was made deep inside the Sun long before that, back when people were living through the last ice age.

The Sun is the source of very nearly all the energy that has ever mattered to life on Earth. The food you ate today was built by plants using sunlight. Petrol and coal are ancient sunlight, captured by living things long ago. Even the wind that turns a turbine is air pushed around by the Sun's uneven heating. It is worth understanding, because it is both utterly ordinary and quietly astonishing.

An Ordinary Star Next Door

The Sun is a star, and a thoroughly average one. It is middle-aged and one of hundreds of billions in our galaxy alone. It is a vast ball of mostly hydrogen and helium gas, held together by its own gravity and big enough to fit well over a million Earths inside.

It looks special only because it is close. The Sun is about 150 million kilometres away, while the next nearest star is tens of trillions of kilometres off. Our whole existence depends on an unremarkable star that happens to live in the neighbourhood. Its place among other stars, from birth to eventual death, is traced in The Life and Death of Stars.

The Puzzle of a Fire That Never Goes Out

For most of history nobody could explain how the Sun shines. It has been pouring out colossal amounts of energy for billions of years. If it were simply burning like a bonfire, even the best fuel imaginable would run out within a few thousand years.

The answer, worked out in the twentieth century, is nuclear fusion.

How Fusion Works

Deep in the Sun's core, gravity squeezes the hydrogen to enormous pressure and heats it to around fifteen million degrees. Normally the nuclei of hydrogen atoms keep apart, because their positive charges repel each other. Under those extreme conditions, they are slammed together hard enough to stick, or fuse. Step by step, four hydrogen nuclei end up as one helium nucleus.

Here is the trick. The helium weighs very slightly less than the four hydrogen nuclei that made it. That missing mass has not vanished. It has been converted into energy, following Einstein's famous equation, which says mass and energy are interchangeable and that even a speck of mass is worth a huge amount of energy.

The Sun fuses roughly six hundred million tonnes of hydrogen every second. A few million tonnes of mass go missing in the process, every second, and become the light and warmth that cross space to us. The Sun has enough fuel to keep this up for around another five billion years. Efforts to copy the process here on Earth are explored in Nuclear Fusion – The Star in a Bottle.

Why the Sun Does Not Explode

With all that energy being released, why does the Sun not simply blow itself apart? The answer is a permanent tug of war. Gravity is always trying to crush the Sun inward, while the outward pressure from all that fusion energy pushes back. The two are almost perfectly matched.

Better still, the balance corrects itself. If the core fused a little faster, the extra energy would puff it up slightly, which lowers the pressure and slows fusion down again. If fusion slowed, the core would contract and heat up, speeding things back up. That is why the Sun has shone so steadily for about four and a half billion years.

From Core to Corona

The energy made in the core does not get out easily. The Sun's interior is so dense that energy cannot travel straight out. It is absorbed and re-emitted again and again, each time heading off in a random direction, like someone lost in an enormous crowd. Estimates vary, but it probably takes tens of thousands of years, and possibly more than a hundred thousand, for energy to zigzag from the core to the surface.

Once it reaches the visible surface, called the photosphere, the energy is finally free. The photosphere is around five and a half thousand degrees, cool compared with the core but still hot enough to glow brilliant white. From there, light crosses the gap to Earth in about eight minutes.

Above the surface the Sun does something scientists are still trying to explain. Its thin outer atmosphere, the corona, is far hotter than the surface below it, reaching millions of degrees. That seems backwards. Why the corona is so hot remains one of the real open questions in solar physics. Spacecraft such as NASA's Parker Solar Probe and the European-led Solar Orbiter are flying closer to the Sun than ever before partly to find out.

Solar Wind and Solar Storms

The Sun does not only send light and heat. It constantly blows out a stream of charged particles called the solar wind, which flows past all the planets. And it has violent weather. Its magnetic field becomes tangled and knotted, and when those knots snap it can release solar flares and enormous eruptions of material called coronal mass ejections, hurling billions of tonnes of charged particles into space.

When a blast like that heads our way, the most beautiful result is the aurora. Particles are funnelled by Earth's magnetic field towards the poles and make the upper atmosphere glow. In Australia that means the aurora australis, usually seen from Tasmania or the far south of the mainland, though during the powerful storm of May 2024 it was photographed much further north than usual. Earth's magnetic field and atmosphere protect us well, a story told in The Magnetosphere – Earth's Invisible Shield, so these storms pose no danger to your body.

An Old Star and a New Vulnerability

Technology is another matter. A powerful solar storm can drive electrical surges through power grids, potentially damaging transformers and causing blackouts. It can disrupt or damage satellites used for communications, navigation and weather forecasting, and it can interfere with radio and GPS.

In 1859 a huge solar storm, now called the Carrington Event, caused telegraph systems to spark and fail. A storm of that size hitting today's far more electrified, satellite-dependent world could cause severe and expensive disruption. That is why space weather forecasters, including a team at the Bureau of Meteorology, watch the Sun around the clock, much as other forecasters watch for cyclones.

Worth a Second Thought

The Sun is an ordinary star turning a sliver of its own mass into energy every second. It stays steady because gravity and fusion have fought to a near-perfect draw, and it will keep doing so for billions of years yet. Everything you have ever done has been powered by it, which makes that familiar warmth on a winter morning a little more remarkable.