Because a star's temperature is an indication of the energy passing through each unit of its surface, it follows that for cool red and hot blue stars of identical luminosities, the cooler red star must be considerably smaller so that the energy escapes through a reduced surface area and has a far greater heating effect. Hence, knowledge of a star's colour and luminosity can reveal its size.
In particular, I can't understand the relation between energy and the heating effect. Of course, I searched for the word "the heating effect" in my own language. Also, I Googled in English.
In this context, what is the heating effect? Could you explain to me? If you give me any help, it will be very useful to me.