As a star runs out of hydrogen to fuel nuclear fusion in its core, changes within the star usually cause it to leave the main sequence, expanding and and cooling as it does so. Would a star with a radius 12 times that of the Sun, but a surface temperature 0.3 times that of the Sun, be more, or less luminous than the Sun ?

Assume surface area of a sphere to be A = 4 R²

Show and explain your reasoning.

(I have been advised to consider the proportional effect on stellar luminosity of both the change in surface area and the change in surface temperature of a star)

[Q2 and Q3]

For stars eight or more times the mass of our Sun the end of their life is characterised by a rapid succession of fusion that stop with the production of iron.

Why do fusion reactions stop at iron ?

What happens to the star when the fusion reactions stop ?