Two stars appear different colours: Star A looks reddish and Star B looks bluish-white. (a) Which star is hotter, and why? (b) If Star B's surface temperature is about 12,000 K and Star A's is about 4,000 K, roughly how many times more energy does each square metre of Star B radiate compared to Star A? Solution: (a) Star B is hotter: hotter blackbodies peak at shorter (bluer) wavelengths by Wien's law, and emit more total power. (b) By Stefan-Boltzmann, power per area is proportional to T^4: ratio = (12000/4000)^4 = 3^4 = 81. Each square metre of Star B radiates about 81 times more energy.