Question

Why is the sun hotter than the earth?
I know that it is larger in size, but except that, what other factors contribute to it? My textbook says that the earth has more heat-generation mechanisms than the sun does, so it should be hotter, but it is not, why?

Answer 1

because of the shear size of the sun, you need to realize the sun is constantly producing energy through nuclear fusion. if you think about it, thats only like 1 or 2 heat generation mechanism. but the amount of energy produced from every single tiny nuclear fusion plus the shear size would dwarf earth's heat output significantly

so in this case, its the size of the sun

Answer 2

to put things into perspective
http://www.rayfowler.org/blog/wp-content/uploads/size_of_earth_3.jpg

Answer 3

The sun is an enormous crucible of hydrogen-fueled nuclear fire, and the earth is a rock ;) Really though, nuclear fusion, which powers the sun, is a tremendously energetic process that produces massive amounts of energy. The heating mechanisms within the earth (radioactive decay, tidal forces, etc.) are comparatively tame.