The Sun is more than just a fiery ball of burning gas. The solar energy that supports life on Earth comes from nuclear fusion, for the Sun is a giant nuclear reactor. Nor is this giant reactor a smooth golden sphere. It's a layered structure with complex magnetic fields generating dark sunspots and bright faculae. Magnetic loops can form filaments a million miles long. When magnetic energy builds up, it can erupt suddenly as a solar flare which may affect us on Earth.

F is for fusion
The alchemists of old, seeking to turn base metals into gold, were frustrated in their attempts to change one element into another. However the Sun does it all the time.

The nucleus of an atom contains one or more protons, the number of protons determining which element it is. The atoms of each element have a unique number of protons. But nuclear reactions can change elements from one kind to another. In nuclear fusion lighter nuclei join together (fuse) to produce a heavier element. This is how stars shine. About 85% of the Sun's energy comes from a nuclear reaction in which four hydrogen nuclei fuse to produce a helium nucleus.

Normally, you use energy to build something. However making a helium nucleus our of hydrogen gives out energy. Since energy can't be created or destroyed, where's this energy coming from? A clue is that the mass of a helium nucleus is slightly less than the total mass of four hydrogen nuclei. The explanation is in the most famous equation in physics, Einstein's

E = mc²
Einstein's theory says that matter and energy are interchangeable, and the equation shows how much energy (E) you'd get when a given mass (m) is converted into energy. Since c is the speed of light, when it's squared you have a huge number. This is why the Sun has been shining for five billion years and will go on doing so for a very long time.

F is for faculae [FAK.yuh.lee]
Most people have heard of sunspots, those dark spots on the Sun's surface that are related to solar activity. They're regions of intense magnetic activity. A sunspot is not actually dark. If you could see it apart from the rest of the Sun, it would seem as bright as a full Moon. However the Sun's surface temperature is 5500 °C (9900 °F), and that of sunspots is about 3400 °C (6200 °F). They seem dark because they're cooler than their surroundings.

But faculae – Latin for little torches – are not so obvious. Like sunspots, they're magnetic regions, but smaller and with more concentrated magnetic activity. Faculae are hotter than the surrounding region, which makes them bright spots. Although they're somewhat brighter than their surroundings, they're still difficult to pick out, as they're only about 30 °C (80 °F) hotter. Faculae are easiest to see near the edge of the Sun where the background isn't so bright.

Interestingly, a sunspot always has an associated facula, but faculae can exist in the absence of sunspots. During the solar activity cycle, the Sun is a teensy bit brighter at its maximum because the faculae brighten it more than the sunspots darken it.

F is for filament
Filaments and prominences are the same things, and they can be very dramatic. They're composed of cooler gas that's suspended above the Sun's surface by magnetic loops, and are often near sunspots. Their appearances differ only because of our perspective when viewing them.

Seen from above, a filament, which is long and snaky, looks dark. This is because, like sunspots, a filament is cooler than the surface underneath it. But they're still very hot. The filament shown here was photographed in February 2015. It was over 700,000 km (435,000 miles) long, which is greater than the radius of the Sun. This is unusually long, but not the longest known filament which was an amazing million miles long.

But we can often see filaments in profile. In that case it's clear that they're hanging above the Sun's surface. This is when they're called prominences.

F is for flares
All this magnetic energy building up in sunspots and filaments may be discharged explosively in a solar flare. A colossal amount of energy is released. NASA describes is as “the equivalent of millions of 100-megaton hydrogen bombs exploding at the same time.” But perhaps even more impressive is that every second the Sun gives out over ten times that much energy.

A coronal mass ejection (CME) may accompany a flare. A CME is the release of great clouds of gas and subatomic particles, accompanied by a huge amount of energy and a shock wave that accelerates the particles to very high speeds.

If a solar flare or a CME comes our way, Earth is likely to be treated to displays of aurorae. On the negative side, the solar storms may damage satellites and the electrical power grid, as well as interfere with communications.