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 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?

Show and explain your reasoning.

You may assume the surface area of a sphere is A = 4r².

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

Why do fusion reactions stop at iron?

What happens to the star when the fusion reactions stop?