▁million ▁years ▁old . ▁ ▁This ▁class ▁is ▁named ▁after ▁the ▁prototype , ▁T ▁Ta uri , ▁a ▁young ▁star ▁in ▁the ▁T aur us ▁star - form ing ▁region . ▁They ▁are ▁found ▁near ▁mole cular ▁clouds ▁and ▁identified ▁by ▁their ▁optical ▁vari ability ▁and ▁strong ▁chrom os pher ic ▁lines . ▁ ▁T ▁Ta uri ▁stars ▁are ▁pre - main - sequence ▁stars ▁in ▁the ▁process ▁of ▁contract ing ▁to ▁the ▁main ▁sequence ▁along ▁the ▁Hay ashi ▁track , ▁a ▁lum inos ity – temper ature ▁relationship ▁obey ed ▁by ▁infant ▁stars ▁of ▁less ▁than ▁ 3 ▁solar ▁masses ▁() ▁in ▁the ▁pre - main - sequence ▁phase ▁of ▁stell ar ▁evolution . ▁It ▁ends ▁when ▁a ▁star ▁of ▁ ▁develop s ▁a ▁radi ative ▁zone , ▁or ▁when ▁a ▁larger ▁star ▁comm ences ▁nuclear ▁fusion ▁on ▁the ▁main ▁sequence . ▁ ▁History ▁While ▁T ▁Ta uri ▁itself ▁was ▁discovered ▁in ▁ 1 8 5 2 , ▁the ▁T ▁Ta uri ▁class ▁of ▁stars ▁were ▁initially ▁defined ▁by ▁Alfred ▁Harrison ▁Joy ▁in ▁ 1 9 4 5 . ▁ ▁Character istics ▁T ▁Ta uri ▁stars ▁compr ise ▁the ▁young est ▁visible ▁F , ▁G , ▁K ▁and ▁M ▁spectral ▁type ▁stars ▁( ). ▁Their ▁surface ▁temper atures ▁are ▁similar ▁to ▁those ▁of ▁main - sequence ▁stars ▁of ▁the ▁same ▁mass , ▁but ▁they ▁are ▁significantly ▁more ▁l umin ous ▁because ▁their ▁rad ii ▁are ▁larger . ▁Their ▁central ▁temper atures ▁are ▁too ▁low ▁for ▁hydro gen ▁fusion . ▁Instead , ▁they