Sunspot: Definition, Cycle & Theory

Instructor: Jeff Fennell

Jeff has a master's in engineering and has taught Earth science both domestically and internationally.

Sunspots are dark regions on the sun that have been observed since the time of Galileo. It is believed that sunspots are areas of high magnetism on the sun. This lesson will cover information about sunspots.


Sunspots are regions on the photosphere of the sun that appear dark because they are cooler than the surrounding photosphere. Dark spots move across the surface of the sun, contracting and expanding as they go. Sunspots have a temperature of about 4500 K, which is about 1500 K cooler than the rest of the photosphere.


Large sunspots can have diameters of about 50,000 km. Sunspots this large can be seen with the naked eye. Sunspots can number up to 100 in a group, but normally sunspot groups are in the single digits. Sunspots develop and persist for periods ranging from hours to months.

Scientists have a theory as to what causes sunspots. It is believed that the rotation of the sun causes distortion in the magnetic field. These distortions cause magnetic areas to break through the photosphere, resulting in what we see as sunspots.

Sunspots on the sun

Sunspot Cycle

Galileo famously determined that the Sun's rotation period is about a month by watching sunspots as they passed. Their short-term and long-term cyclical nature has been well observed.

Astronomers use a number called the sunspot number to determine how many sunspots there are at any given time on the photosphere. To calculate this number, they count the number of sunspot groups and multiple it by ten, they then add any individual sunspots that are visible. Sunspot groups are multiplied by ten because on average there are ten sunspots in a sunspot group.

Plotting the total number of sunspots observed in a year reveals a pattern showing the number of sunspots in a cycle. The number of sunspots increases and decreases over an 11-year cycle, known as the sunspot cycle.

Plotting the sunspot number per year shows an 11-year cycle
sunspot cycle

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