A gravitationally bound ball of mostly hydrogen and helium gas which is self-luminous from internal nuclear fusion reactions. Stars can vary in composition and mass, with their radius and luminosity depending on mass and age. The progression of a star's luminosity, color, and radius over time is termed stellar evolution. Most stars fall on a well-defined region known as the main sequence when plotted on a so-called Hertzsprung-Russell diagram, which plots luminosity vs. temperature (or equivalently, color).

The composition of a star places it in what is known as a stellar population, with stars having a high content of heavy elements (collectively termed "metals" by astronomer) known as population I, and stars having a low metal content known as population II.

The mass of a star is extremely important in determining its evolution over time. Stellar masses are commonly given in terms of the solar mass, denoted and equal to kg (Astronomical Almanac 2000). The most massive star known is Melnick 42 in the Large Magellanic Cloud. It appears to have a mass of 80- and a luminosity of , where is the solar luminosity, equal to J s^-1.

Asymmetric Drift, Bright Stars, Constellation, Hertzsprung-Russell Diagram, Main Sequence, Nearby Stars, Star Cluster, Stellar Populations, Sun