The Sun is a star. It is a rather ordinary star - not particularly big or small, not particularly young or old. It is the source of heat which sustains life on Earth, and controls our climate and weather. It is the closest star to Earth, and the most closely studied. From it we have learned a great deal about the physical processes which determine the structure and evolution of stars in general.

Active galaxies are galaxies which have a small core of emission embedded in an otherwise typical galaxy. This core may be highly variable and very bright compared to the rest of the galaxy. Models of active galaxies concentrate on the possibility of a supermassive black hole which lies at the center of the galaxy. The dense central galaxy provides material which accretes onto the black hole releasing a large amount of gravitational energy. Part of the energy in this hot plasma is emitted as x-rays and gamma-rays.

A star during most of its life is called a "main-sequence star". It consists of a central core, convective and radiative zones, the photosphere, the chromosphere, and the corona.