A sunspot is a temporary dark patch on the Sun’s surface where intense magnetic fields block heat from reaching the top. Sunspots appear darker than the surrounding surface because they’re roughly 1,500 degrees Celsius cooler, around 4,300 Kelvin compared to the photosphere’s average of 5,800 Kelvin. They range widely in size, but large sunspots average about 25 times the area of Earth. Some last only hours, while complex groups can persist for weeks or even months as the Sun rotates.
Why Sunspots Form
The Sun generates energy deep in its core, and that energy travels outward through a process called convection, essentially hot plasma rising and cooler plasma sinking, much like water boiling in a pot. Sunspots form when strong magnetic fields punch through the surface and suppress this convection. The magnetic field acts like a plug, preventing hot material from rising normally. With less heat reaching the surface in that spot, the area cools and darkens relative to its surroundings.
This creates what solar physicists describe as a “cool cone” above the blocked region. Energy that would have risen through that area gets diverted around it instead, which is partly why the region immediately surrounding a sunspot can appear slightly brighter than normal.
Anatomy of a Sunspot
A fully developed sunspot has two distinct parts visible through a telescope. The center, called the umbra, is the darkest region where the magnetic field is strongest and most vertical. Surrounding the umbra is the penumbra, a lighter ring of streaky, filament-like structures where the magnetic field fans outward at an angle. The penumbra forms through the interaction of strong magnetic fields with the thermal forces trying to push energy upward in a region where light alone can’t carry enough heat.
Not all sunspots develop a penumbra. Small, simple spots without one are called pores. These are essentially magnetic concentrations that suppress convection over a limited area but haven’t grown large or complex enough to develop the surrounding filamentary structure.
The 11-Year Solar Cycle
Sunspot activity follows a roughly 11-year cycle of rising and falling numbers. At solar minimum, the Sun’s surface may go days or weeks with few or no visible spots. At solar maximum, dozens can be present at once. The current cycle, Solar Cycle 25, was predicted to peak around July 2025, with the maximum potentially falling anywhere between November 2024 and March 2026, according to NOAA’s Space Weather Prediction Center.
Sunspots don’t appear randomly across the Sun’s surface. Each cycle begins with spots forming at mid-latitudes, around 30 to 40 degrees north and south. As the cycle progresses toward maximum and then declines, new spots appear closer and closer to the equator. Plotting this pattern over time produces what’s known as the butterfly diagram, because the shape of each cycle’s sunspot distribution resembles a pair of wings. Sunspot maximum occurs when coverage peaks at about 15 degrees latitude. Spots almost never form within a few degrees of the equator itself, and they never appear above about 40 degrees latitude.
At the tail end of one cycle, the last spots from the dying cycle linger near the equator while the first spots of the new cycle are already appearing at higher latitudes. The two cycles briefly overlap.
How Sunspots Drive Solar Storms
Sunspots mark magnetically active regions, and those regions are the launch points for the Sun’s most dramatic outbursts. The Sun’s equator rotates faster than its poles, which stretches and twists magnetic field lines over time. When these twisted fields get stretched to a breaking point, they snap and reconnect, releasing enormous amounts of energy in the process.
This magnetic reconnection can produce several types of eruptions. A solar flare is an intense flash of light and radiation. A radiation storm sends high-speed particles streaming into space. A coronal mass ejection, or CME, launches a massive cloud of solar material outward from the Sun. A single event can trigger one, two, or all three of these simultaneously. Solar storms are most frequent and intense during solar maximum, when the Sun is covered in the magnetically complex sunspot groups that give rise to these eruptions.
Effects on Earth and Technology
When solar storms reach Earth, they interact with our planet’s magnetic field and upper atmosphere in ways that can disrupt modern technology. Coronal mass ejections can trigger geomagnetic storms that induce extra electrical currents in the ground, which can degrade or damage power grid equipment. The same geomagnetic disturbances interfere with GPS and satellite navigation signals, reducing their accuracy.
Solar flares produce strong X-rays that can degrade or completely block high-frequency radio waves, causing communication blackouts for aircraft and ships that rely on those frequencies. Energetic particles from radiation storms can penetrate satellite electronics and cause electrical failures, posing a risk to the fleet of satellites that modern communications, weather forecasting, and financial systems depend on. These same particles are a radiation hazard for astronauts.
On the more enjoyable side, geomagnetic storms push auroras to lower latitudes than usual. During strong storms near solar maximum, people in the northern United States, southern Europe, and equivalent southern latitudes sometimes see vivid displays of the northern or southern lights that are normally confined to polar regions.
What Sunspots Look Like Through a Telescope
With proper solar filters, sunspots are one of the easiest solar features to observe. The largest spot groups are occasionally visible to the naked eye at sunrise or sunset when the Sun is dimmed by the atmosphere (though staring at the Sun without proper protection at any other time risks permanent eye damage). Through a filtered telescope, the dark umbra and lighter penumbral filaments are clearly visible, and you can track a sunspot group’s movement across the solar disk over several days as the Sun rotates, completing one full rotation roughly every 27 days as seen from Earth.
Sunspot counts have been recorded since the early 1600s, making them one of the longest continuously tracked astronomical phenomena. That long record is what allowed scientists to identify the 11-year cycle and notice longer-term patterns, including the Maunder Minimum, a period from roughly 1645 to 1715 when sunspots nearly vanished for decades.