▁ 2 0 0   AU ▁and ▁form ▁a ▁hot , ▁dense ▁proto star ▁( a ▁star ▁in ▁which ▁hydro gen ▁fusion ▁has ▁not ▁yet ▁begun ) ▁at ▁the ▁centre . ▁ ▁At ▁this ▁point ▁in ▁its ▁evolution , ▁the ▁Sun ▁is ▁thought ▁to ▁have ▁been ▁a ▁T ▁Ta uri ▁star . ▁Studies ▁of ▁T ▁Ta uri ▁stars ▁show ▁that ▁they ▁are ▁often ▁accompanied ▁by ▁disc s ▁of ▁pre - plan et ary ▁matter ▁with ▁masses ▁of ▁. ▁These ▁disc s ▁extend ▁to ▁several ▁hundred   AU — the ▁Hub ble ▁Space ▁T eles cope ▁has ▁observed ▁pro top lan et ary ▁disc s ▁of ▁up ▁to ▁ 1 0 0 0   AU ▁in ▁diameter ▁in ▁star - form ing ▁regions ▁such ▁as ▁the ▁Or ion ▁Neb ula — and ▁are ▁rather ▁cool , ▁reaching ▁a ▁surface ▁temperature ▁of ▁only ▁about ▁ 1 0 0 0 ▁k el v ins ▁at ▁their ▁h ott est . ▁Within ▁ 5 0   mill ion ▁years , ▁the ▁temperature ▁and ▁pressure ▁at ▁the ▁core ▁of ▁the ▁Sun ▁became ▁so ▁great ▁that ▁its ▁hydro gen ▁began ▁to ▁f use , ▁creating ▁an ▁internal ▁source ▁of ▁energy ▁that ▁counter ed ▁gravit ational ▁contra ction ▁until ▁hydro static ▁equilibrium ▁was ▁achieved . ▁This ▁marked ▁the ▁Sun ' s ▁entry ▁into ▁the ▁prime ▁phase ▁of ▁its ▁life , ▁known ▁as ▁the ▁main ▁sequence . ▁Main - sequence ▁stars ▁derive ▁energy ▁from ▁the ▁fusion ▁of ▁hydro gen ▁into ▁hel ium ▁in ▁their ▁cores . ▁The ▁Sun ▁remains ▁a ▁main