A sunspot is a temporary dark patch on the surface of the Sun, caused by intense magnetic fields that block heat from reaching the surface. Sunspots look dark only by contrast: they’re cooler than their surroundings, but still blisteringly hot at roughly 5,000 to 7,600°F. The surrounding solar surface sits around 10,000°F. These features can be as large as Earth or much bigger, and they play a direct role in the space weather that affects satellites, power grids, and radio communications.
Why Sunspots Form
The Sun’s visible surface is a churning layer of gas called the photosphere. Enormous convection cells constantly carry heat upward from the core, much like water rolling in a boiling pot. Sunspots appear where powerful magnetic fields punch through this surface and suppress that convection. With less hot material rising to the top, the area cools relative to its surroundings and appears darker.
These magnetic fields originate deep within the Sun, where electrically charged gas (plasma) flows and tangles magnetic field lines over time. When a bundle of field lines becomes concentrated enough, it breaks through the photosphere and anchors itself as a sunspot or, more commonly, a pair of sunspots with opposite magnetic polarities, like the two ends of a horseshoe magnet.
Structure of a Sunspot
A well-developed sunspot has two distinct zones visible through a filtered telescope. The central region, called the umbra, is the darkest and coolest part, with a typical temperature around 4,240 K (roughly 7,170°F). Surrounding the umbra is a lighter, striated ring called the penumbra, which is warmer at about 5,680 K (roughly 9,760°F). For comparison, the normal photosphere sits at about 6,050 K. Even the “cool” center of a sunspot is hot enough to glow brightly on its own; it only looks dark next to the even brighter surface around it.
In terms of scale, Earth is roughly the size of an average sunspot. Large sunspot groups, however, can stretch across many times Earth’s diameter and become visible to the (properly protected) naked eye.
How Long Sunspots Last
Sunspots are not permanent features. A typical sunspot forms in a matter of hours to a few days, usually less than five. Decay is slower, often taking weeks to months as the magnetic field gradually disperses and convection resumes. Small, simple spots may vanish in days, while large, complex groups can persist for several solar rotations (the Sun rotates roughly once every 27 days as seen from Earth), evolving in shape and size the entire time.
Sunspots rarely appear alone. They tend to form in groups within magnetically active regions, and solar scientists classify these groups by their size, the complexity of their largest spot, and how many smaller spots fill the interior of the group. A simple classification system identifies 60 distinct types of sunspot groups, ranging from small, isolated spots to sprawling clusters with tangled magnetic fields.
The Solar Cycle
Sunspot numbers rise and fall in a roughly 11-year pattern known as the solar cycle. At solar minimum, the Sun’s face may be nearly spotless for days or weeks. At solar maximum, dozens of sunspot groups can dot the disk simultaneously. The current cycle, Solar Cycle 25, was predicted to reach its peak around July 2025, with a maximum monthly sunspot number around 115. In practice, the peak could land anywhere between late 2024 and early 2026.
Not every cycle is equal. Between 1645 and 1715, a period known as the Maunder Minimum, sunspot activity nearly vanished. During the core of that era (1645 to 1700), annual sunspot counts rarely exceeded single digits. This period coincided with notably cold winters across Europe, though the exact relationship between low sunspot counts and Earth’s climate remains a subject of ongoing study. The Maunder Minimum serves as a reminder that the Sun’s magnetic activity can vary dramatically over centuries, not just the familiar 11-year rhythm.
How Sunspots Affect Earth
Sunspots themselves don’t directly reach Earth, but the magnetically active regions around them are the source of solar flares and coronal mass ejections (CMEs), both of which can have real consequences here.
Solar flares are sudden bursts of energy and radiation. Strong flares can disrupt high-frequency radio communications that bounce off the upper atmosphere, a problem for aviation and maritime operations that rely on those frequencies. Flares also accelerate high-speed particles that can pierce satellite hardware, degrading solar panels and damaging circuits over time.
CMEs are massive clouds of magnetized plasma hurled into space. When one hits Earth’s magnetic field, it can induce electrical currents in long conductors on the ground, including power lines. These currents can damage transformers, relays, and circuit breakers, potentially triggering widespread power outages. CMEs also heat the upper atmosphere, causing it to swell outward. That increased drag slows low-orbiting satellites and drops their altitude, which is why satellite operators closely monitor space weather forecasts tied to sunspot activity.
On the gentler side, the interaction between CME particles and Earth’s magnetic field produces the aurora borealis and aurora australis. During solar maximum, auroras can be visible at much lower latitudes than usual.
How to Observe Sunspots Safely
You should never look directly at the Sun without proper protection, and regular sunglasses are nowhere near sufficient. There are two safe approaches: filtering and projection.
- Solar filters: Special-purpose filters made from metal-coated glass, metalized polyester film, or black-polymer material fit over the front opening of a telescope or binoculars. These block all but a tiny fraction of the Sun’s light, producing a comfortably bright image in the eyepiece. Only filters designed specifically for solar viewing are safe.
- Projection: The simplest method is a pinhole projector. With the Sun at your back, you let sunlight pass through a small hole and project the image onto a flat surface behind it. You look at the projected image, never through the hole at the Sun. A telescope or binoculars can also project a magnified image of the Sun onto a white card, making sunspot groups clearly visible. Dedicated devices like the Sunspotter or Solarscope are designed for safe group viewing using this principle.
With either method, even modest equipment reveals the dark cores and lighter penumbrae of active sunspots, and during periods of high solar activity, you can watch groups change shape from day to day.