Galileo sketched them in 1610, arguing with contemporaries about what they even were. Four centuries later, sunspots are one of the oldest continuously tracked phenomena in science — and still the single best visual clue to where the Sun's next flare or CME is about to come from.

Sunspots are temporary dark patches on the Sun's visible surface, cooler than the surrounding photosphere by roughly 1,500°C — hot enough to glow, but dim enough by contrast to look black against the brighter surface around them. They form where bundles of magnetic field lines break through the surface and locally suppress the normal convective flow of heat from below, which is why they're cooler rather than an actual hole or blemish.

A sunspot typically has two parts: a darker central umbra, where the magnetic field is most concentrated and vertical, surrounded by a lighter penumbra, where the field lines fan outward at a shallower angle. Sunspots range from small, short-lived pores lasting a day to sprawling groups that can persist, rotating with the Sun, for multiple solar rotations.

Sunspots are tracked using the sunspot number (also called the Wolf number, after Rudolf Wolf, who standardized the method in 1848): R = k(10g + s), where g is the number of distinct sunspot groups, s is the total number of individual spots, and k is a correction factor accounting for differences between observers and instruments. Combining group count and individual spot count this way captures both how widespread activity is and how intense it is, in a single daily figure.

This gives solar physics an unusually long continuous record — the sunspot count has been tracked back to the first telescopic observations in 1610, longer than almost any other scientific measurement still in active use today, maintained today by the World Data Center SILSO in Brussels.

Individual sunspot groups are catalogued by NOAA as numbered active regions, written as AR followed by a number (for example, AR3664, the region behind the May 2024 superstorm, or AR4366, a notably prolific "flare factory" region from early 2026). The numbering simply increments as new regions are identified, restarting periodically, giving forecasters and the public a consistent shorthand for tracking a specific region's activity across the days it remains Earth-facing.

Plot the latitude of every sunspot group against time across a solar cycle, and a distinctive pattern emerges: spots from the outgoing cycle appear closer to the equator as the cycle winds down, while spots from the incoming cycle begin appearing at high latitudes, sometimes 40° or more from the equator, before drifting equatorward as the new cycle matures. Plotted over successive cycles, the overlapping pattern resembles a row of butterfly wings — hence the name, first published by British astronomer E. Walter Maunder in 1904.

The butterfly diagram remains one of the most useful visual summaries of where a cycle currently stands: watching whether the latest active regions are appearing at high or low latitude gives a rough read on whether a cycle is just beginning, at its peak, or winding down.

Between roughly 1645 and 1715, sunspot activity collapsed to a small fraction of its normal level for an extended stretch now called the Maunder Minimum — the best-documented "grand minimum" in the historical sunspot record, and a period that coincided with unusually cold winters in parts of Europe, though the precise causal relationship is still debated among researchers. Even the sparse sunspots that did appear during this period showed an unusually narrow butterfly pattern, clustered within about 15-20° of the equator rather than the 28°-plus range typical of a normal cycle — evidence of an especially weak underlying magnetic dynamo during those decades.

Not all sunspot groups are equally likely to erupt. The most magnetically complex regions — where north and south magnetic polarities are tightly interleaved rather than cleanly separated — are disproportionately responsible for major flares and CMEs, covered in detail elsewhere in this wiki. A large sunspot group isn't automatically a dangerous one; a smaller but more magnetically tangled region can outperform a much larger, simpler one.