▁the ▁Sun ▁will ▁become ▁hot ▁enough ▁to ▁trigger ▁hydro gen ▁fusion ▁in ▁its ▁surrounding ▁shell . ▁This ▁will ▁cause ▁the ▁outer ▁layers ▁of ▁the ▁star ▁to ▁expand ▁greatly , ▁and ▁the ▁star ▁will ▁enter ▁a ▁phase ▁of ▁its ▁life ▁in ▁which ▁it ▁is ▁called ▁a ▁red ▁giant . ▁Within ▁ 7 . 5   b ill ion ▁years , ▁the ▁Sun ▁will ▁have ▁expanded ▁to ▁a ▁radius ▁of ▁ 1 . 2   AU — 2 5 6 ▁times ▁its ▁current ▁size . ▁At ▁the ▁tip ▁of ▁the ▁red ▁giant ▁branch , ▁as ▁a ▁result ▁of ▁the ▁vast ly ▁increased ▁surface ▁area , ▁the ▁Sun ' s ▁surface ▁will ▁be ▁much ▁cool er ▁( about ▁ 2 6 0 0   K ) ▁than ▁now ▁and ▁its ▁lum inos ity ▁much ▁higher — up ▁to ▁ 2 , 7 0 0   current ▁solar ▁lum inos ities . ▁For ▁part ▁of ▁its ▁red ▁giant ▁life , ▁the ▁Sun ▁will ▁have ▁a ▁strong ▁stell ar ▁wind ▁that ▁will ▁carry ▁away ▁around ▁ 3 3 % ▁of ▁its ▁mass . ▁During ▁these ▁times , ▁it ▁is ▁possible ▁that ▁Sat urn ' s ▁moon ▁T itan ▁could ▁achieve ▁surface ▁temper atures ▁necessary ▁to ▁support ▁life . ▁ ▁As ▁the ▁Sun ▁exp ands , ▁it ▁will ▁sw allow ▁the ▁plan ets ▁Mercur y ▁and ▁Ven us . ▁Earth ' s ▁fate ▁is ▁less ▁clear ; ▁although ▁the ▁Sun ▁will ▁en velop ▁Earth ' s ▁current ▁orbit , ▁the ▁star ' s ▁loss ▁of ▁mass ▁( and ▁thus