The other alternative is, we all die due to radiation rather than  heat, but if we all, somehow, survive the radiation, we will all  be cooked, no doubt.    Reason: The sun is a star, so sooner or later it would die out.  What would happen is the centre of the sun will get smaller and  hotter. Outer layers will be pushed out further by this increased  heat, so it'll grow in size. Becoming a Giant Red Star, it will  grow to about a hundred times its size and will swallow up  Mercury!