The dark central region of a sunspot is called the umbra. It appears dark because it is cooler and emits less visible light than the surrounding solar surface.
Sunspots form where intense magnetic fields emerge through the Sun's photosphere, the visible layer of its atmosphere. These magnetic fields interfere with the movement of hot plasma from deeper layers, reducing the amount of heat reaching the surface. The affected area therefore looks darker by comparison, even though it is still extremely hot.
Around the umbra is a lighter, filament-like region called the penumbra. A complete sunspot may contain both parts: the umbra at the center and the penumbra surrounding it. Sunspots often occur in groups and can change in size and shape over days or weeks.
Sunspot activity follows an approximately 11-year cycle and is linked to the Sun's changing magnetic activity. Sunspots themselves are not holes or solid dark objects. They are regions of altered brightness, and their number can help scientists track solar activity and anticipate events such as solar flares.