How large will our Sun get during the red giant phase (ie which planets will it engulf) before the helium flash?
Low mass stars like the Sun do become very large prior to He ignition in the core. The exact value depends a bit on models for mass loss from the extended atmosphere (e.g. Guo et al. 2016), but estimates of 250 times its current size are possible (Schroeder &amp; Smith 2008; Spiegel &amp; Madhusudhan 2012).
At this radius both Mercury and Venus are engulfed. The mass loss from the Sun means Earth would orbit a bit further out than now. Nevertheless, it too would be engulfed in Schroeder and Smith's model at that point, because tidal drag dissipates energy from the orbit and it &quot;falls&quot; into the Sun very quickly.
Interestingly, the Sun achieves its maximum size at the tip of the RGB, rather than in the AGB phase.