What Are Sunspots?

A Strange Darkness

Early astronomers were baffled by mysterious dark areas that would appear, grow, and seem to move across the surface of the Sun. Were they planets orbiting our star? Did they even exist at all? Or were they just illusions caused by imperfect telescope lenses?

Today, we call these dark areas sunspots, and we know quite a bit about their origin. But there is a lot to learn about how they influence flares and plasma on the solar surface.

Admire the sunspots we captured during the 2024 total solar eclipse

Arcs, Loops, and Tunnels of Force

Thanks to advanced space probes and better telescopes, scientists have gathered enough solid data to start to unlock the mystery of sunspots.

Since the 1960s, we’ve known that magnetic fields drive the creation of these dark areas. The key seems to be the action of the Sun’s differential rotation—meaning the speed of the sphere’s rotation is faster at its equator than at its poles. This dynamic works to “wind up” magnetic fields rolling through the photosphere.

Solar magnetic waves react to the winding action by creating loops, arcs, tubes, and tunnels of force that keep super-hot gasses from forming on small patches of the surface. The result is a group of less dense, cooler areas that appear darker when we view them from Earth.

And these dark patches aren’t just slightly cooler than the other parts of the surface—the heat difference is dramatic. Sunspots are usually measured at around 3900 C (about 7000 F), while the surrounding photosphere typically holds a temperature of around 5500 C (about 10,000 F).

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Shape Shifting

Sunspots come in a wide variety of shapes and sizes, and they are dynamic: expanding, contracting, and morphing into different formations during their life cycles.

The first visible sign of a spot is known as the umbra. This is the darkest area of a sunspot, with the strongest magnetic field. In time, the umbra becomes more intense, and another structure becomes visible—the penumbra, a lighter zone surrounding the core, made up of elongated filaments.

Eleven-Year Cycles

These dark regions seem to wax and wane in an approximately eleven-year solar cycle. Each cycle starts after the photosphere reaches a period of minimal to no sunspot activity, or solar minimum. As the first spots of a new cycle appear, they are at the higher latitudes of the Sun, and increase in size and activity until solar maximum—the peak of sunspot activity—is registered.

Scientists label these cycles numerically. Solar cycle 1 started in 1755—the first year of reliable sunspot data. We are now in solar cycle 25, which began in 2019 and is expected to last until about 2030.

Sunspots don’t last for the entire cycle. Some dark patches last for just hours or days, others for months at a time.

Closer Encounters

NASA’s Parker Solar Probe and the European Space Agency’s (ESA) Solar Orbiter are giving astronomers a better view of the center of our solar system. In 2021, Parker became the first probe to fly through the Sun’s upper atmosphere.

On its record-breaking journey, Parker will come closer to the center of the Sun than any other probe, closing to just 6.2 million km (3.9 million miles), which is seven times closer than any spacecraft has come before.

And this exploration will happen at astounding speed. Solar winds and the enormous gravitational power of the Sun will turn Parker into the fastest object ever built, expected to reach a top speed of 690,000 km/h (430,000 mph), or 0.064% the speed of light, in 2025.

But Parker isn't alone in delivering new data to researchers. As the ESA Solar Orbiter circles the Sun, specialized cameras are examining sunspots with polarized light, and other onboard instruments will monitor plasma emissions. This may give scientists a better understanding of the dynamics of how matter moves on the solar surface, and how that impacts sunspot formation and behavior.

Topics: Astronomy, Sun