Lifthrasil: Independently of that merger, the sun will run out of fuel, enter red giant state (in about 5 billion years) and finally explode.
GamezRanker: Nitpick: it is currently theorized/calculated/etc that the sun will actually become a red giant. Of course, the life currently on the planet then (if any) is screwed either way**, so it's mainly an academic distinction.
(**and even before that, after about 1 billion years the changes in the sun will very likely make the surface unlivable)
I am pretty sure that even long before that, the same will happen to us as has happened to the poor Mars: the magnetic field will weaken enough so that our atmosphere will slowly but surely just evaporate to the vast space. (in Mars this happened as its core became inactive over time, not producing the magnetic field anymore). And on Earth the effect will be even more pronounced because we are closer to Sun than Mars is, as it is the Sun's radiation which will rip the atmosphere away from us. No more seas or atmosphere for you, buddy.
Who knows, possibly this "leaking" is happening all the time, and our atmosphere is already much much thinner than it was e.g. a million years ago. I recall reading somewhere (sorry I don't recall the exact figures) that the magnetic field would have already weakened by like 10% since the 1800s or so. News like that freak me out much more than seeing Thurnberg whine about "climate change". (Naturally this weakening of the magnetic field could be just temporary and related to the magnetic poles switching places... hopefully.)
Who knows, when we learn that our atmosphere is leaking to the space all the time, maybe we will all scramble to burn all the coal and oil we can find, in order to strengthen the surrounding atmosphere? Screw the climate change, do it for our atmosphere!
This is also why I find the idea of trying to "terraform" and populate Mars quite silly. The only advantage to that would be that it is a bit farther away from the Sun so it will "survive" (as much as a dead planet can "survive") a bit longer before being engulfed by the expanding Sun.
My only hope will be that over millions of years, humankind will evolve to learn to live without oxygen and withstand sun's radiation, so that we can live only by the energy we get from the sun. You know, like those big creatures in the Farscape TV-series (the organic spaceship where the protagonist and the crew travels, and which becomes pregnant).
Or then the humankind just ceases to be anymore, and oh well. At least I will not be around to see its demise, unless hindus and buddhists were right after all with their born-again theories.