Question

Why do stars shrink?

Answer 1

Stars loses their energy by radiation hence they shrink. I think this is the answer

Answer 2

The star's outer layers expand while the core is shrinking and as the expansion continues, the luminosity begins to increase. For a star with the mass of the sun, this expansion takes about a billion years and the star's radius increases 100 times, and its luminosity increases even more. The star is called a red giant.