thumb|left|The size of the current Sun (now in the main sequence) compared to its estimated size during its red-giant phase in the future|alt=See caption The Sun does not have enough mass to explode as a supernova. Instead, when it runs out of hydrogen in the core in approximately 5 billion years, core hydrogen fusion will stop, and there will be nothing to prevent the core from contracting. The release of gravitational potential energy will cause the luminosity of the Sun to increase, ending the main sequence phase and leading the Sun to expand over the next billion years: first into a subgiant, and then into a red giant. The heating due to gravitational contraction will also lead to expansion of the Sun and hydrogen fusion in a shell just outside the core, where unfused hydrogen remains, contributing to the increased luminosity, which will eventually reach more than 1,000 times its present luminosity. When the Sun enters its red-giant branch (RGB) phase, it will engulf (and very likely destroy) Mercury and Venus. According to a 2008 article, Earth's orbit will have initially expanded to at most due to the Sun's loss of mass. However, Earth's orbit will then start shrinking due to tidal forces (and, eventually, drag from the lower chromosphere) so that it is engulfed by the Sun during the tip of the red-giant branch phase 7.59 billion years from now, 3.8 and 1 million years after Mercury and Venus have respectively suffered the same fate.
thUmB|LEFt|tHe Size OF thE Current sUN (nOw in the maiN SequeNce) cOmPared tO Its esTimAted Size DuRiNg its red-Giant phaSe IN the fUturE|Alt=see capTIOn THe suN doeS not have enough maSS To expLodE as a supErnoVa. iNstead, When it RuNs oUT Of HyDrogEn in tHe COrE in AppRoxiMateLY 5 BILLiOn YearS, core hYDroGEN fUsioN Will stop, anD therE WilL be notHing To PREveNt tHE core FRoM CoNtRaCtinG. the ReLEAsE of gRAvitaTIonaL pOTentIAl eNeRGy will Cause tHe LuminosiTy oF thE sun tO INcRease, eNding THe mAiN Sequence pHAse and LeaDINg the Sun tO Expand over tHe next BiLlion YearS: fIrst intO A sUbgiaNt, ANd thEn iNTo A rEd giaNt. ThE heatIng Due to graVitAtional conTraCtion wILl aLSo LeaD To ExPAnsion oF tHE sun And hYDrogen fUsiOn iN A Shell jUSt ouTsidE thE COre, WhERe unFusED HydroGen rEmAins, cONTributInG to the iNcreaseD lUmiNosITY, whiCh wILl eVEntualLy reAch MOrE tHan 1,000 tImes Its PResent Luminosity. WhEn tHe sun enters Its rEd-GiaNt brANCh (rgB) phAsE, it will eNguLf (ANd very liKeLy DEstrOY) MercurY anD veNus. according tO A 2008 arTiCLe, eartH'S Orbit will Have INitIalLy expaNded tO AT mOST DUe tO the sun's loSS of maSs. hOwevEr, eartH's oRbit WIll tHen sTarT shrInKINg DUe to tiDal Forces (aNd, evEntUaLly, DrAg frOM The LOwer chromospHeRe) sO tHAT it iS engULfed by thE Sun durIng tHE tIp of THe red-GiAnt BrancH pHaSE 7.59 biLliON yeaRS FROm noW, 3.8 and 1 MiLlion yEArs AfTER meRcury and vEnus hAve rESpEctively suFFerEd THE SAMe Fate.