A sunspot is a relatively dark, cooler region of the Sun's photosphere associated with a concentrated magnetic field. It appears dark compared with the surrounding photosphere because its temperature is lower, although a sunspot is still extremely hot by terrestrial standards. A typical sunspot contains a darker central umbra surrounded by a lighter penumbra with filamentary structure. Sunspots can range from small transient features to enormous complex magnetic regions spanning tens of thousands of kilometres.
Their strong magnetic fields suppress some of the convective transport of heat from below the photosphere, causing the affected region to receive less energy and appear darker. Sunspots are therefore visible manifestations of the Sun's magnetic activity rather than holes or solid dark objects on the solar surface. They can persist for days or weeks, while large active regions containing groups of sunspots can evolve over longer periods.
The magnetic field associated with a sunspot is produced by the emergence of magnetic flux through the solar photosphere. The Sun is made of electrically conducting plasma, so motions inside the convection zone can stretch and twist magnetic fields. Differential rotation causes different latitudes to rotate at different angular speeds, contributing to the distortion of magnetic structures. Magnetic flux tubes or concentrated magnetic fields can rise through the convection zone and emerge through the photosphere as bipolar active regions.
Where magnetic field strength becomes particularly high, convection is inhibited, reducing the upward transport of heat. The visible result is a cooler dark region. The magnetic field can be thousands of times stronger than the typical solar surface field, making sunspots important indicators of the Sun's magnetic structure and activity.
The umbra is the darkest central part of a mature sunspot and contains the strongest, more vertically oriented magnetic field. Surrounding it, the penumbra contains more inclined magnetic fields and filamentary structures associated with complex plasma flows. Sunspots can occur individually, but many form groups containing several regions of positive and negative magnetic polarity. The arrangement of these polarities provides information about the underlying magnetic field.
Sunspot groups can become magnetically complex as they evolve, and regions with strong magnetic shear are often associated with solar flares and coronal mass ejections. The visible shape of a sunspot is therefore only one aspect of a much larger magnetic system extending from the solar interior through the photosphere into the corona.
Sunspots are closely connected with the approximately 11 year solar activity cycle. Near solar minimum, relatively few sunspots may be visible for extended periods. As activity increases, sunspot numbers rise and active regions become more common, eventually reaching solar maximum before declining again. Sunspot observations have provided one of the longest continuous records of solar activity. The cycle is associated with changes in the Sun's magnetic field and culminates in a reversal of the global magnetic polarity. The complete magnetic cycle is therefore approximately 22 years even though the most visible sunspot number cycle is approximately 11 years. Sunspot groups also display systematic changes in latitude over the course of a cycle, producing the pattern historically known as the butterfly diagram.
Sunspots have played an important role in the history of astronomy. Early telescopic observers in the seventeenth century recorded sunspots and demonstrated that the Sun was not an unchanging perfect sphere. Galileo Galilei and other observers used sunspot motion across the solar disc to infer that the Sun rotates. Later systematic observations revealed long term changes in sunspot abundance. In the nineteenth century, astronomers established a relationship between sunspot numbers and the solar activity cycle.
Modern instruments can measure not only the number and area of sunspots but also their magnetic field strength, polarity and motion. Spectropolarimetry is particularly valuable because magnetic fields alter spectral lines through effects such as Zeeman splitting. Sunspots therefore connect historical observational astronomy with modern solar magnetohydrodynamics.
Sunspots themselves are not cold. Typical sunspot umbra temperatures are several thousand kelvin, lower than the surrounding photosphere, which is roughly 5800 kelvin. Because thermal radiation depends strongly on temperature, the cooler umbra emits less visible light per unit area and appears dark by comparison. If a sunspot could somehow be isolated from the surrounding photosphere and viewed against a completely dark background, it would still be intensely luminous. Its apparent darkness is therefore a contrast effect. The penumbra has intermediate brightness and temperature and exhibits distinctive filamentary structure. The combination of magnetic suppression of convection and radiative processes determines the observed brightness. This explains why sunspots can appear visually striking despite being regions of extremely hot plasma.
Sunspots matter beyond the visible appearance of the Sun because they identify active magnetic regions capable of producing space weather. Complex sunspot groups can store substantial magnetic energy in the solar atmosphere. Magnetic reconnection in these regions can produce solar flares, while large scale magnetic restructuring can contribute to coronal mass ejections. The resulting radiation and particles can disturb Earth's ionosphere and magnetosphere, affecting radio communication, satellites, navigation systems and electrical infrastructure. Sunspot counts are therefore used as one indicator of solar activity. However, sunspot number alone does not perfectly predict the strength or consequences of an individual eruption because magnetic topology, active region complexity and eruption geometry are also important.
A sunspot is distinct from a solar flare, although the two are often associated. A sunspot is a comparatively long lived magnetic feature visible in the photosphere, whereas a flare is a transient burst of energy produced by rapid magnetic reconfiguration. Sunspots are also distinct from solar prominences, filaments and coronal holes, which occur primarily in the solar atmosphere and have different physical characteristics. Modern understanding treats sunspots as manifestations of the Sun's magnetic dynamo and flux emergence processes. Their study provides information about convection, magnetic field generation, solar rotation and the solar activity cycle. By observing how sunspots form, move, merge, decay and produce eruptions, scientists can investigate the coupling between the Sun's interior, visible surface and outer atmosphere.
