According to the conventionally accepted definition, in order for an entity to be classified as a living thing, it must
(1) grow and develop,
(2) use energy,
(3) reproduce,
(4) be made of cells,
(5) respond to its environment, and
(6) adapt.

Technically, a star like the sun has a life cycle and uses energy. Although in some sense it reproduces and responds to its environment, the sun is certainly not made of cells. In this way, it is not a living thing like the living things on Earth. Cellular life, however, is just life the way we know it.

To think of the sun and its energy use and production system as living is an insightful application of the concept.