 For at least a portion of its life , a star shines due to thermonuclear fusion of hydrogen into helium in its core , releasing energy that traverses the star 's interior and then radiates into outer space . Almost all naturally occurring elements heavier than helium are created by stellar nucleosynthesis during the star 's lifetime , and for some stars by supernova nucleosynthesis when it explodes . Near the end of its life , a star can also contain degenerate matter . Astronomers can determine the mass , age , metallicity ( chemical composition ) , and many other properties of a star by observing its motion through space , its luminosity , and spectrum respectively . The total mass of a star is the main factor that determines its evolution and eventual fate . Other characteristics of a star , including diameter and temperature , change over its life , while the star 's environment affects its rotation and movement . A plot of the temperature of many stars against their <unk> produces a plot known as a <unk> – Russell diagram ( H – R diagram ) . <unk> a particular star on that diagram allows the age and evolutionary state of that star to be determined . 
