Hello everyone, I have a theoretical question about an animation in the documentary I've linked in this post.
At timestamp 05:01, you can see an animation of Venus, suffering the effects of a change in gravitational forces. After being close enough to the expanding sun, Venus begins to lose material on its surface due to the stronger gravitational pull of the sun.
For me, this whole animation doesn't seem right because the mass and the center of mass of the sun doesn't change. I can imagine how the mater of the sun's surface could change the gravitational pull that the material on Venus' surface is experiencing, but would it be that strong at that point? I would've assumed that more and more of the sun's mass will be concentrated in the core as the red giant is expanding and not leaving enough mater to overcome Venus' gravity at its surface.
Once, Venus enters the denser parts of the suns' atmosphere, I wouldn't doubt that the atmospheric drag or the fall towards the denser core would scatter the material on the surface and eventually destroy Venus completely. Shortly before that, however, while Venus is still roughly maintaining an orbit similar to its current one, this doesn't seem right to me.
Did they mess up this animation or will the sun, at its surface, have more gravitational pull than Venus?
This is the key point of my question because the mass m of the "bunch of mass" that is close to Venus will be less when you consider the density p with m = p * V, where V is just the volume of mass that is close enough to Venus. Assuming the Suns density is uniform (which it isn't, most of the mass is far away from Venus), p and by extension m and by extension F (the gravitational force) are proportional to 1/r³.
Unfortunately, the actual calculations are far too hard for me, but my intuition would suggest that the 1/r³ relationship of density and volume would outdo any other factors here.
Good point. The Sun's radius would have to increase over 150 times to reach Venus orbit, so it's safe to assume the density at that point would be less than 1/millionth the average 1.4g/cc, and once the Sun started crossing the Sun-Venus L1 point, it would start "raining Sun" onto Venus.
So a better visualization would be, that once the Sun grew past L1, a cloud of Sun would start enveloping Venus.
I wonder what temperature would that be at, but the Sun-Venus L1 gets about 2800kW/m², and increasing the surface of the Sun by 150² times would definitely leave it much cooler than the current 5700K, so it might not be all that dramatic. Then again, a Sun flare can get up to 100 million K... probably not a good idea to get one straight in the face, even at 1000K.