A sun gear is a central gear in a gear system, often found in planetary gear sets. It is surrounded by one or more planet gears, which rotate around it while also engaging with a ring gear on the outside. This arrangement allows for efficient power transmission and speed reduction in various mechanical systems. Sun gears are commonly used in applications such as automatic transmissions in vehicles and in machinery that requires compact and efficient gear arrangements. Their design helps distribute load evenly, enhancing durability and performance in systems like automobiles and industrial equipment.