The Sun

It is so familiar that we forget what it is. The Sun is a star, the same kind of object as the thousands of points of light in the night sky, only ten thousand times closer than the next nearest. It is a ball of glowing gas, mostly hydrogen, so large that a million Earths would fit inside, and so heavy that it holds every planet in orbit from billions of kilometres away.

A nuclear furnace

At the centre of the Sun, the weight of everything above squeezes the core to 250 billion times the pressure of Earth's atmosphere and heats it to about 15 million degrees. Under those conditions hydrogen nuclei, which normally repel each other, are slammed together hard enough to stick. Four hydrogen nuclei become one helium nucleus. The helium weighs slightly less than the hydrogen that made it, about 0.7 percent less, and that missing mass becomes energy, exactly as Einstein's famous equation describes.

The Sun converts around 600 million tonnes of hydrogen into helium every second, losing four million tonnes of mass in the process. It has been doing so for 4.6 billion years and has used only about half its core hydrogen. It has roughly another five billion years to go.

The long journey out

The energy made in the core does not leave quickly. It emerges as gamma rays, which are absorbed and re-emitted by the dense gas countless times, drifting outward in a random walk. A single packet of energy can take something like 100,000 years to travel from the core to the surface, a distance light could cover in two seconds in empty space. In the outer third of the Sun the gas boils like water in a pot, carrying heat upward in vast rising columns. That boiling gives the surface its grainy texture, each granule about the size of France, lasting ten minutes before sinking.

The visible surface, the photosphere, is about 5,500 degrees. From there the light takes just over eight minutes to cross 150 million kilometres to Earth.

Spots, flares and storms

The Sun is threaded with magnetic fields, and because different parts rotate at different speeds, those fields get twisted and tangled. Where strong field bursts through the surface it blocks the rising heat, creating sunspots, patches that look dark only because they are cooler than their surroundings. The number of sunspots rises and falls on an eleven-year cycle.

Twisted magnetic fields can snap and reconnect, releasing in minutes as much energy as billions of hydrogen bombs: a solar flare. Often a flare hurls a billion tonnes of charged gas into space, a coronal mass ejection. When one of these clouds hits the Earth a day or two later, it compresses our magnetic field and funnels particles toward the poles, where they light up the sky as the aurora. A large enough storm can knock out satellites and power grids. In 1859 one set telegraph wires sparking across Europe and America.

The corona and the wind

Above the surface lies the corona, a halo of thin gas glowing at over a million degrees, far hotter than the surface below it. Why the atmosphere is hotter than the surface remains one of the great puzzles of solar physics. The corona is normally invisible, swamped by the glare, but during a total eclipse it appears as a pearly crown around the black disc of the Moon.

The corona is so hot that it constantly leaks into space as the solar wind, a stream of particles racing outward at hundreds of kilometres per second. The wind fills the solar system and pushes against the gas between the stars, carving a bubble, the heliosphere, that extends far beyond Pluto. Two Voyager probes crossed its edge after more than thirty-five years of flight.

The Sun's future

In about five billion years the core's hydrogen will run out. The Sun will swell into a red giant, engulfing Mercury and Venus and scorching the Earth, then shed its outer layers into space and shrink to a white dwarf the size of our planet, slowly cooling for ever. Long before then, over the next billion years, its gradual brightening will make Earth too hot for oceans.

Why it matters

Nearly every form of energy on Earth is sunlight in disguise. Plants capture it; animals eat plants; coal, oil and gas are sunlight stored by ancient forests and plankton. Wind blows because the Sun heats the air unevenly. Rivers flow because it lifts water into clouds. Only nuclear and geothermal power owe nothing to the star overhead. Understanding the Sun is understanding the engine of our world.