Just how long does it take for the old obsolete theory to die out? xmatt's theory doesn't take into account these factors:
The first thing is Alpha Centauri. The gravity of the system will hit the Solar Oort Cloud, so that comets, asteroids and mini-planets of all size will be thrown towards the Sun, reaching the inner Solar system, where we are. Jupiter can deflect or absorb most, but not all of the matter. There will be a massive meteor shower.
The Sun has brightened 10% since the Earth formed. It will continue to brighten, reaching 30% before the expansion. The core is getting more compressed already. In one billion years it has brightened so hot that the oceans will boil, the steam will rise to high atmosphere and dissociate to hydrogen and oxygen because of the ultraviolet radiation. The Earth will be as dry and dead as the Moon.
In a simulation it was shown that the Sun loses a lot of mass as it expands. Its gravity will weaken so much that the orbit of Earth will be in the place where Mars orbits today. Still, it's so hot that the crust of the Earth will completely melt. Then the Sun will blow its outer layers out. The white dwarf in the center will eventually cool down to a cold ball of iron.
So, this new scenario is much worse than the old one! First the meteors, then oceans boil, and then the crust will melt. And only 800000 years to begin! The Last Day will be in the far future, when the remains of the Sun drift with to another star or a black hole. For the humans, the Last Day is coming much faster.
Source: Tiede. http://www.tiede.net/