"So what", I hear you saying.
A remarkable thing happens because stars are black bodies. We can tell their temperature just by looking at them. We know that:
A blue star is hotter than a red star.
Because it is hotter, a surface patch of the blue star gives off more light than the same size surface patch of a red star.
Knowing This, Let's Build Some Stars ...
Now put together the surface patches like a jigsaw puzzle to make some stars. If a blue star, a yellow star and a red star all give off the same amount of light:
.....the red star is the largest (red surface patches are the dimmest so you need lots of them forming a very big star),
.....the yellow star is the next largest (yellow surface patches are brighter so you need less of them), and
.....the blue star is the smallest (blue surface patches are the brightest so you don't need many of them at all).