Increase tidal force?
Gongjie Li [...] says: first, the expanding star will heat the planets, boiling off their atmospheres, while increasing solar tides will stretch the planets into egg-shapes.
I really don't get this. Tidal force is a result of gravitational force. Gravitational force is not in any way affected by the radius of the body that exerts it. The only two factors that matter is the mass of the body and the distance you are from it's center of mass. If you are moving into something that can be considered uniformly distributed along each distance to the center. As you can when you dig into an actual body or as you progress towards the center of a spiral galaxy. You can discard all the mass that is further out than yourself and only consider the mass that is closer than yourself to the center to calculate the net gravitational force you are experiencing. It does not matter whether all the matter on the inside of yourself is condensed into a perfect hollow sphere* you are a millimeter away from or that if all of the matter is condensed into a point in the center. The gravitational force you experience is the same.
Since tidal force is something you experience since a body actually has volume and thus parts of it will be further away from the center of mass of another close by body. This means the gravitational force on the particles it is made up of will be slightly different and this is in effect the tidal forces.** But again, as long as there is space*** between the bodies and the mass remains the same the gravitational force will remain equal on each of those particles with regard to the volume of the both of them and thus the tidal forces will remain equal.
Please enlighten me if you see the flaw in my reasoning or what I didn't get from what Gongjie said.
*Assume a theoretical hollow sphere that has the same mass as the theoretical body we started with and that is infinitely thin. If you are unable to, assume it to be 1 mm thick. Both are equally impossible.
**Almost, but not exactly.
***geddit?