The sun is the closest star to Earth. That gives all Earthlings, especially astronomers, a front-row seat to its activities. One of the most striking features of the sun’s activity is what astronomers call the solar cycle. This is an epic rise and fall in the sun’s level of activity that repeats every 11 years or so.

Astronomers owe the discovery of the solar cycle to sunspots. Ever since Galileo first pointed a telescope at the sun in 1610, people have witnessed the occasional emergence of these dark splotches on the sun. As the sun rotates, completing one spin every 27 days, those spots appear to move across our star’s surface.

Explainer: Stars and their families

Heinrich Schwabe regularly tracked sunspots from 1826 to 1843. This German astronomer is credited with discovering that sunspots’ frequency tends to vary every 11 years. During the most active time — or maximum — of this 11-year cycle, dozens of sunspots can be seen slowly crossing the sun at a time. At the least active point in the solar cycle — the solar minimum — our star may be sunspot-free.

Over the years, research has linked sunspots and the solar cycle to the sun’s magnetic field. Much like Earth, the sun has a magnetic field with a North Pole and a South Pole. But the sun’s magnetic field is at least 100 times as strong as Earth’s. It’s also much larger and more complex.

That mysterious magnetic field

The sun is a huge ball of super-hot gas. Temperatures within the sun are so high that electrons get ripped away from the cores, or nuclei, of their atoms. This creates a swarm of negatively charged electrons and positively charged nuclei. Such charged particles are known as ions.

As ions move around inside the sun, they create swirling magnetic fields. Those fields twist and turn, churning as the sun rotates. Sometimes, magnetic field lines come together and create points with extra-powerful magnetism. At the sun’s surface, those spots of intense magnetism cool off the surrounding gas, making sunspots appear.

The overall surface of the sun roils at roughly 5,500 degrees Celsius, (10,000 degrees Fahrenheit). Sunspots look so dark because they are much cooler — only around 3,500 ºC (6,300 ºF).

Over two weeks in 2017, NASA’s Solar Dynamics Observatory watched this storm on the sun’s surface. The storm appears to move across the sun because our star rotates, completing one spin every 27 days. As the observatory viewed this storm, the area of intense and complex magnetic fields grew. Eventually, the spot rotated out of view — but not before it was linked to intense auroras on Earth.

The sun’s powerful magnetic field affects much more than the star’s surface. In fact, our sun’s magnetic field is so strong that it reaches beyond the orbits of all the planets in the solar system. As a result, it can influence the movement of charged particles as much as 75 to 100 times as far from the sun as Earth is.

No one fully understands how the sun’s messy, intense magnetic field works. But astronomers do know that during each 11-year solar cycle, the magnetic field’s poles switch. What had been the sun’s magnetic North Pole becomes the South Pole, and vice versa. During the next solar cycle, the poles switch back again.

The switching of the sun’s poles doesn’t affect us much on Earth. But other events tied to the solar cycle do.

Why solar activity matters

As the sun approaches solar maximum, it can release powerful bursts of radiation from its surface. These outbursts are called solar flares and they occur when magnetic field lines coming out of the sun become too twisted. Like a rubber band that breaks when pulled too far, the twisted magnetic field lines snap, producing more than a million times the energy of a volcanic explosion. Flares travel outward at the speed of light, arriving at Earth eight minutes later.

A different type of burst known as a coronal mass ejection, or CME, is an explosion at the sun’s surface. CMEs hurl charged particles out into space. Their material can take one to three days to reach Earth. Like solar flares, CMEs are more common closer to solar maximum. But they can occur even during the sun’s supposedly quiet periods.

Tracking the solar cycle is important because flares and CMEs play a role in space weather. As they hit Earth’s magnetic field, flares and CMEs can trigger beautiful auroras in the sky near Earth’s North and South Poles. But not all their impacts are pretty. In fact, some can be quite harmful.

The charged particles and other types of radiation in solar flares and CMEs can disrupt radio signals. They can also damage satellites orbiting Earth and put astronauts in danger. To avoid exposure to these energetic blasts, astronauts must get behind specially designed shields in time.

A powerful space-weather storm called the Carrington Event showed just how dangerous CMEs can be. It occurred in 1859, shortly after Schwabe discovered the solar cycle. In that case, an extremely powerful CME caused widespread damage to power lines and telegraph wires on the ground. It even lit some telegraph stations on fire.

Scientists study the solar cycle to help predict when such events might occur. If a CME as strong as the Carrington Event happened today, it would likely knock out parts of the power grid. It could also damage satellites that make GPS systems work. Such a space weather disaster would likely also disrupt phone and internet service across the globe.

Studying the solar cycle

In 2018, NASA launched the Parker Solar Probe. The spacecraft was designed to get close to the sun and study its outer layers. Two years later, the European Space Agency launched its own spacecraft, Solar Orbiter, to do similar research. Together, the spacecraft are gathering data about the sun as it moves through the solar cycle.

Those data should help improve computer models that predict when hazardous flares and CMEs might threaten Earth. One of Solar Orbiter’s first discoveries was a phenomenon similar to solar flares called campfires.

The sun is now in its 25th solar cycle since astronomers started gathering accurate sunspot data more than 250 years ago. Solar Cycle 25 started in 2019 and will continue roughly through 2030. Solar maximum and the peak of sunspot frequency are expected in 2025.

Some solar cycles have more active solar maxima than others. Solar Cycle 25 was originally predicted to have a relatively weak maximum. But in the lead-up to solar maximum, sunspot numbers have been much higher than expected. The number of solar flares and CMEs has been surprisingly high, too.

During a total solar eclipse, like the one on April 8, 2024, the moon blocks out the sun’s bright disk for several minutes. At that time, our star’s outer atmosphere, or corona, briefly becomes visible to human eyes. If a big CME happens at the same time as the eclipse, even casual observers will be able to witness these stunning solar fireworks.