3. (4 pts) Thermal radiation.

a) Our Sun is a white star. Its thermal radiation covers all wavelengths of visible light, and it is

emitting approximately the same amount of light at all wavelengths in the visible light range. Is a

blue star hotter or colder than the Sun? Is a red star hotter or colder than the Sun? (Ignore the

Doppler effect.) (2 pts)

b) We can tell the temperature difference between stars based on their colors. But why can we NOT

say that Neptune (a blue planet) is hotter than Mars (a red planet)? (2 pts)