The sun is amazing. Long ago, the ancients thought the sun was a god--an ever-present, never-changing, all powerful ball of perfection. And they were partially right. The sun is the source of all life on Earth. Without it, we would wither and die (quickly). It holds the power of life and death in its metaphorical hands, i.e it essentially is a god (although not a sentient one).
It's also not static. Far from it. The sun is incredibly dynamic. It's a churning, writhing, pool of nuclear violence and has an intensely active magnetic field. Magnetic fields are created by moving electrical charges. On our own planet, the core is a hot ball of iron whose electrons get blown around by heat escaping the core. This creates the electrical current that makes our planet's magnetic field. On the sun, almost every atom is ionized--meaning that there is so much energy around that protons can't hold onto their electrons and they're all floating around independent of one another. Add to that that the sun is not sitting still, but constantly rotating like our planet, and now we're talking unthinkably huge electrical currents flowing across the sun all in the same direction. This creates extremely powerful magnetic fields.
Sorry. Only non-copyrighted image I could find. For better pictures of this, Google image search "differential rotation sun"
But it gets more complicated than that. The sun doesn't rotate in a fixed manner the way Earth does. It undergoes something called "differential rotation." This means that the lower latitudes of the sun (closer to the sun's equator) rotate much faster than those at the poles. For instance, if you stood on the sun's equator (and were not instantly incinerated), you would make a full revolution in 25 Earth days. But if you stood near the poles, it would take you 31 Earth days to make a full revolution.
Other than being nifty, why does this matter? Because of the magnetic fields. Magnetic field strength changes depending on flow speed and so the faster rotation at the equator drags the magnetic field lines along with it. As the material at the equator laps the material closer to the poles, the magnetic fields lines get twisted all around each other. They get more and more twisted and tangled and spun around each other until--POP! A dense coil of magnetic field lines erupts out of the surface of the sun like a telephone cord that's been too tightly wound.
Pair of sunspots on the surface of the sun
These erupted magnetic field lines shoot out of the surface of the sun and then dive back in a ways off. And where they exit the sun, they disrupt the normal convective flow of heat to the surface, making the point where they exit much cooler than the surrounding area. This point becomes dark and is called a sunspot. Because the exiting field lines always dive back in, sunspots come in pairs.
Over time, the field lines can become so severely twisted that they suddenly realign in an explosive release of energy and pinched off magnetic field lines. These magnetized energetic flourishes carry enormous amounts of energy out into space and are called solar flares. Occasionally, solar flares can be so energetic that they rip large chunks of mass off of the sun with them, sending piping hot plasma hurtling through space. This is called a coronal mass ejection, or CME. Solar flares and CMEs can both affect Earth's magnetic field in important ways, like disrupting the D layer in our own ionosphere, which in turn causes radio wave propagation decay and radio blackouts. They can also fry delicate electronic equipment in orbit as they pass over the poles (where energetic particles are deflected towards).
The previous sun cycle
But the sun's strange magnetic fields are even stranger, because every 11 years the magnetic poles on the sun flip sides! It's not known exactly why this happens, but it is thought to occur as an emergent property of the chaotic twisting and turning of the magnetic field lines near the surface. So every 11 years the chaotic twisted field lines smooth themselves out and become orderly. When this happens, there are fewer bundles and filaments of field lines erupting from the surface. This means fewer sunspots, fewer flares and fewer CMEs. This time is known as a solar minimum. But in the periods between nice orderly magnetic field lines and a well-defined north and south magnetic pole, the order dissolves into a bubbling frantic mess of wayward field lines like a tangled ball of yarn. This time of increased solar activity is known as the solar maximum.
The size of the maxima and minima also wax and wane over time in somewhat unpredictable ways. This most recent solar maximum was the weakest in a hundred years. And while it looks random, it seems to fit a larger scale periodic change in sun cycle activity--the cause of which remains tantalizingly unknown.
Why is any of this important? Many reasons. From a purely scientific standpoint, understanding the workings of the sun will help us understand how stars form, evolve and eventually die. But from a more practical standpoint, being able to predict solar activity will better prepare us for interplanetary travel. Outside of Earth's protective magnetosphere, astronauts would be exposed to a lot more radiation from the sun (commonly known as "solar wind" or "space weather") and even worse, galactic cosmic rays (GCRs, highly damaging radiation from distant supernovae). And while it may seem like a solar minimum would be a good time to travel, we can shield against solar wind pretty well and we cannot shield against GCRs as easily. So ironically, a solar maximum may offer a protective boost to the magnetic field around the solar system and deflect more GCRs. This does not bode well for a trip to Mars as we seem to be heading into a grand minimum.
It seems ironic that we know so little about the sun when it's the most prominent object in the sky. But we're learning more all the time. In a way, it's a little sad to be stripping the sun of its secrets. What at one time seemed like a god--the perfect, unblemished, unchanging sun--is finally having its delicate insides exposed and now we can see it for what it really is--sunspots and all.