Sunspots are as a result of intense magnetic field activity, where magnetic field lines criss-cross and tangle with each other, on the surface of the Sun, the photosphere. The development of Sunspots begin way in the centre of the Sun, where heat is generated and is transported through various layers of the Sun before reaching the Sun’s surface. If the heat encounters a strong magnetic force, a sunspot shall develop.
Sunspots consist of a cool and dark nucleus known as the umbra, meaning Shadow in Latin; surrounding this, a lighter patch known as the penumbra; in Latin meaning nearly shadow. The temperature of the umbra is around 4,000 degrees centigrade; the temperature of the Photosphere is around 6,000 degrees centigrade. This reduction in temperature, obviously, means that the centre of the sunspot is cooler, and appears darker. The reason for the dark appearance is because of the intensity of the magnetic force: the magnetic force lines are so contorted and twisted that the energy from the convection layer underneath the sunspot is reduced, therefore very little, if any, light escapes.
The umbra is the part of the Sunspot where the magnetic force is at its most intense, where the magnetic field lines are at their most tangled and contorted, that light can not escape therefore giving its really dark appearance. The magnetic force around the umbra, the penumbra, is not so intense, meaning that the magnetic lines are not so twisted and contorted, so some light can escape, therefore making the penumbra appear slightly lighter than the umbra but still darker than the surrounding photosphere.
The coolness and darkness of the Sunspot is relative to the Sun. If a sunspot was extracted from the surface, it would actually appear just as bright if not brighter than the Moon orbiting planet Earth, and at 5,000 degrees it is too cold for any nuclear fusion to take place but it would also be too hot for any life to exist on it. If a sunspot was a star, it would be too cold and too small to produce enough heat and light for any of the orbiting planets.
Appearance of both the penumbra and umbra differ: the umbra appears to be a solid black spot, whereas the penumbra appears fuzzy and lighter.
The size of a Sunspot can vary widely. Sometimes the bigger sunspots observed without an aid of a telescope can give an illusion of a single sunspot, when in fact there’s actually many sunspots that are simply formed very close to each other. The average size of a Sunspot is nearly twice the size of planet Earth. Massive relative to us; tiny relative to the Sun!