How does Astrophage actually drain the Sun and threaten Earth?

Astrophage is a microorganism that consumes solar energy by absorbing and storing it, effectively draining the Sun's energy output. This reduction in solar energy causes the Sun to dim, which threatens Earth by drastically lowering the amount of heat and light it receives. As a result, Earth faces a severe cooling that could lead to freezing conditions and ecological collapse. The mission in Project Hail Mary is to understand why the star system Tau Ceti is resistant to Astrophage and find a way to protect Earth before it succumbs to this energy drain.