The hotness or coolness of a body in regards to its sensible heat and is measured by a thermometer.
because since its very hot the hotter it gets it starts to melt the color on the star.For Example blue stars. there the hottest star.
That heat is absorbed or given free by the systems matter.
its color
colour. red - cool blue - very hot think of heating a piece of metal, it goes from red to orange to white to blue the hotter it gets.
Look up blackbody radiation and blackbody spectrum.