Starspots are stellar phenomena, so-named by analogy with sunspots. Spots as small as sunspots have not been detected on other stars, as they would cause undetectably small fluctuations in brightness. The commonly observed starspots are in general much larger than those on the Sun: up to about 30% of the stellar surface may be covered, corresponding to starspots 100 times larger than those on the Sun.

Detection and measurements

To detect and measure the extent of starspots one uses several types of methods.

Temperature

Observed starspots have a temperature which is in general 500–2000 kelvins cooler than the stellar photosphere. This temperature difference could give rise to a brightness variation up to 0.6 magnitudes between the spot and the surrounding surface. There also seems to be a relation between the spot temperature and the temperature for the stellar photosphere, indicating that starspots behave similarly for different types of stars (observed in G–K dwarfs).

Lifetimes

The lifetime for a starspot depends on its size.

Activity cycles

The distribution of starspots across the stellar surface varies analogous to the solar case, but differs for different types of stars, e.g., depending on whether the star is a binary or not. The same type of activity cycles that are found for the Sun can be seen for other stars, corresponding to the solar (2 times) 11-year cycle.

Maunder minimum

Some stars may have longer cycles, possibly analogous to the Maunder minimum for the Sun which lasted 70 years, for example some Maunder minimum candidates are 51 Pegasi, HD 4915 and HD 166620.

Flip-flop cycles

Another activity cycle is the so-called flip-flop cycle, which implies that the activity on either hemisphere shifts from one side to the other. The same phenomena can be seen on the Sun, with periods of 3.8 and 3.65 years for the northern and southern hemispheres. Flip-flop phenomena are observed for both binary RS CVn stars and single stars although the extent of the cycles are different between binary and singular stars.