Some estimates are that as more of the hydrogen in the sun is converted to helium, the rate of reaction will increase so that in about 3 billion years the sun's output will have increased by about 40% and will have boiled all our "volatiles" (like all the water) off into space so that we'll be nothing but desiccated cinders well before the galaxies start to mingle and dance together.

Of couse we'll have resembled Venius for about a billion years already, before that, which isn't exactly a nice neighborhood to raise kids in from what I've heard.

John