Encyclopædia Galactica Rami T. F. Rekola Tuorlan Astronomical Society Tuorla Observatory University of Turku

Star

A light emitting gaseous sphere, which at some stage of its life produces energy by nuclear fusion (primarily: hydrogen -> helium). The Sun is a rather dim average sized star. Stars are classified by their surface temperature into spectral types of which the hottest O type is half a million times brighter than the Sun and faintest M type only a thousandth of the Sun's brightness. Lifetime of a star depends on its brightness (hottest ones live only some millions of years before they explode into supernovae, coldest ones live tens of billions of years and wither into oblivion). The nuclear fusion, and pressure of supernova explosions, produce elements heavier than helium - the building blocks of all matter in the Universe.

See also spectral types of stars.