終極物理題（２０點）

Consider the electromagnetic spectrum, the energy is higher on the blue end and lower on the red end. For monochromatic light, blue flame is releasing more energy than the red flame.

For "white" light, it is composed of light across the whole spectrum. It is a mixture of different light. So white hot flame is only achievable when the flame release energy at different energy level, generating light in different colour at the same time.

In most situation, this does not happen. If you burn a fuel / material, the energy release is governed by the atomic / electronic configuration of the fuel / material, and only light of one or a few energy level is released. As a result, you can only have flame in a monochromatic colour. As said before, blue end has higher energy and thus blue frame is hottest. White hot is still the hottest (the reaction happens at the SUN), but it is unlikely to have white hot flame in laboratory environment.

When a body is heated (due to combustion or just heated by a heat source), it radiates. The radiation spectrum is best described by thermal radiation or black body radiation. The following is a link describing this phenomenon:
http://en.wikipedia.org/wiki/Black_body

In the article above, there is a figure about the relationship between temperature and wavelength (you will have to click on it to make it larger). As you can see, when the temperature increases, the peak of the black-body radiation curve moves to shorter wavelengths. This explains why the hotter the object the whiter it is when temperature raises from 3000K to 4000K. Color changes from red to white (shorter wavelength got emitted more).

When the object is heated more (7500K), one should expect the emission peak will be around 470nm which is blue. Thus, if an object is emitting blue-white light, it is hotter than just white light.

For stars, the hotter they are, more bluish they will become:
http://www.enchantedlearning.com/subjects/astronomy/stars/startypes.shtml