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.