▁into ▁stell ar ▁populations ; ▁Population ▁I ▁stars ▁are ▁the ▁young est ▁stars ▁and ▁have ▁the ▁highest ▁metal ▁content ▁( our ▁Sun ▁is ▁a ▁Pop ▁I ▁star ), ▁while ▁Population ▁III ▁stars ▁are ▁the ▁oldest ▁stars ▁with ▁a ▁very ▁low ▁metal ▁content . ▁ ▁Tem per ature ▁and ▁size ▁ ▁In ▁ 1 8 6 0 ▁Gustav ▁Kirch hoff ▁proposed ▁the ▁idea ▁of ▁a ▁black ▁body , ▁a ▁material ▁that ▁em its ▁elect romagnet ic ▁radiation ▁at ▁all ▁w avel ength s . ▁ ▁In ▁ 1 8 9 4 ▁Wilhelm ▁Wien ▁derived ▁an ▁expression ▁rel ating ▁the ▁temperature ▁( T ) ▁of ▁a ▁black ▁body ▁to ▁its ▁peak ▁emission ▁w avel ength ▁( λ max ). ▁ ▁b ▁is ▁a ▁constant ▁of ▁proportional ity ▁called ▁Wien ' s ▁disp lacement ▁constant , ▁equal ▁to ▁▁ ▁This ▁equation ▁is ▁called ▁Wien ' s ▁Law . ▁By ▁meas uring ▁the ▁peak ▁w avel ength ▁of ▁a ▁star , ▁the ▁surface ▁temperature ▁can ▁be ▁determined . ▁ ▁For ▁example , ▁if ▁the ▁peak ▁w avel ength ▁of ▁a ▁star ▁is ▁ 5 0 2   nm ▁the ▁corresponding ▁temperature ▁will ▁be ▁ 5 7 7 8 ▁k el v ins . ▁ ▁The ▁lum inos ity ▁of ▁a ▁star ▁is ▁a ▁measure ▁of ▁the ▁elect romagnet ic ▁energy ▁output ▁in ▁a ▁given ▁amount ▁of ▁time . ▁ ▁L um inos ity ▁( L ) ▁can ▁be ▁related ▁to ▁the ▁temperature ▁( T ) ▁of ▁a ▁star ▁by ▁ ▁, ▁ ▁where ▁R ▁is ▁the ▁radius